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Plant Disease Defined
Plant disease is the abnormal growth and development of a plant:
Growth and development of the plant does not live up to the normal expectations.
Some standard of “normal” must be assumed.
A
diseased plant is incapable of carrying out its normal physiological functions
to the best of its genetic potential.
Basic Concepts In Plant
Pathology
1. Many
different living and nonliving entities can have a negative affect on plants.
A.
Infectious Diseases (caused by biotic organisms):
a.
Fungi
b.
Prokaryotes:
i. Bacteria
ii. Mycoplasmas
c.
Viruses and viroids
d.
Nematodes
e.
Parasitic Higher Plants
B.
Non‑Infectious (caused by abiotic factors)
a. Temperature extremes
b. Moisture extremes
c. Light extremes
d. Nutrient extremes
e. Soil acidity or alkalinity (salt problems)
f. Pesticide toxicity
g. Air conditions: pollution, strong winds, etc.
h. Improper cultural practices
2. A
disease episode requires the interaction of three components which we call "The Disease
Triangle".
A. The host must be susceptible to the disease, is at the proper age and
physiological state for infection and development of disease to occur. Healthy,
strong growing non-stressed plants are less susceptible to disease than plants
under stress.
B.
The pathogen must be virulent (able to cause disease) not in a state of
dormancy and must be present at a certain minimum population level.
C.
The environment must be conducive (favorable) for the development of
disease:
temperature, moisture, nutrients, wind, etc. must all favor the pathogen.
D.
The degree to which these three components interact, relates to the severity of
the disease episode (e.g., if the host is highly susceptible, the pathogen
highly virulent and the environment highly conducive, then the disease will be
very severe).
E.
Successful disease control depends on the integrated use of available control
methods.
Symptoms and Signs
1. Symptoms are the response of the plant to attack by a disease causing
agent.
A.
Examples: leaf spots, wilting, stunting, chlorosis, necrosis, etc.
2.
Signs are the visual presence of some structure formed by the pathogen on
the host.
A.
Examples: mycelium, spores, fruiting bodies, bacterial ooze, etc.
Organisms Associated
with Diseased Tissue
1. Primary organism - the organism is directly responsible for the disease.
2. Secondary organism ‑ the organism(s) is(are) taking advantage of weakened
tissue.
3.
Disease complexes and organism succession.
A. Disease complex refers to the situation where the disease is caused by
more than one organism. Disease complexes are especially common in turf.
B.
Organism succession refers to the fact that plants are colonized over
time‑by many different organisms. For example, when plants are healthy, they are
colonized by nonpathogenic symbionts When the plants become diseased, they are
first colonized by primary pathogens, then by secondary organisms, and
eventually by other saprophytes. Saprophytic organisms are in association with
healthy plants. The primary disease causing agent is only operating by itself
for a short period of time. Secondary organisms may be weak‑pathogens or
pathogens. Weak pathogens are organisms that are not aggressive and (typically)
do not cause disease by themselves.
C.
Disease complexes add to difficulty in disease management as identification and
control of one organism may accelerate activity of another organism in the
complex.
D.
Organism succession makes primary pathogen identification difficult. The
diseased specimen must be examined relatively quickly after disease symptoms
begin; otherwise, secondary pathogens or saprophytes are all that can be found.
Accurate diagnosis of the causal agent is required for effective use of chemical
control measures. Use of an inappropriate chemical will not only be ineffective
against the disease agent, but can also lead to additional disease problems by
killing beneficial microorganisms in the environment.
Steps In Diagnosis of
Plant Disease
Questions to ask grower
or manager.
1. Identify the plant species affected: genus, species, and cultivar (whenever
possible).
A. In a mixed turf stand, identify the species and which are diseased or
not diseased.
B. In the landscape, note if (and what) other types of plants are
affected.
2. Observe the symptoms:
What plant parts are affected?
Is the disease spreading or localized? On the plant and to other plants.
Try to determine when the symptoms first appeared.
3. Determine the environmental conditions prior to and during symptom
development.
Temperature ‑ day and night.
Moisture ‑ air (humidity) and soil.
Wind, hail, dust, blowing sand, etc.
4. Determine the growing conditions:
A. Determine soil type ‑ sand, silt, clay, drainage, etc.
B. Where is plant growing? Lawn, home garden, organic garden, landscape,
courtyard, greenhouse, golf course, commercial nursery, indoors, etc.
C. What is the exposure ‑ sun, shade, etc.
D. What is the proximity of the plant to structures such as, other plants,
buildings, sidewalks, roads, walls, etc..
E. What is the irrigation history? How, how much, how often, time applied,
etc.
F. What is the fertilization history? What, how, how much, how often, etc.
G. What is the history of chemical use? what, how, how much, how often,
etc.
5. Additional Questions for;
A. Home gardens:
Size of garden.
Crop rotation.
Proximity to other structures.
B. For turfgrass:
-
Presence of other plants in or nearby the turf area.
-
Terrain (slope).
-
Age of turf and how it was established (seed, sod, plug).
-
Maintenance practices: mowing, aeration, de-thatching, raking,
etc.
C. For commercial fields:
-
Acreage.
-
Crop Rotation.
-
Past problems in the field.
-
Percent of plants affected and distribution of diseased plants in the
field (scattered, one side, in one area, etc.)
-
Proximity to structures.
-
Crops growing nearby.
6. Microscopic examination of the tissue ‑ dissecting, light and electron
microscopy.
7. Isolation and identification of associated microorganisms.
A. Plate sample on culture media.
-
General ; water agar, PDA, nutrient media, etc.
-
Organism specific ‑ bacteria, actinomycetes, fungi, etc.
-
Selective - type (e.g., group of fungi, such as oomycetes, etc.),
genus, etc.
B. Moist chambers.
C. Diagnostic kits:
8. Diagnostic resources:
9. After diagnosis ‑ what next?
A. Determine the major contributing factors for disease development.
B. Identify the means by which the organism ‑operates and survives (overwinters).
C.
Ask
D. Make recommendations:
-
Cultural practices
-
Variety selection
-
Chemicals
Principles of Plant
Disease Control
1. Avoidance ‑ Avoiding disease by planting at a time when, or in areas
where the pathogen is
ineffective, rare or absent.
2. Exclusion ‑ Reducing, inactivating, eliminating or destroying the
pathogen at the source.
3. Protection ‑ Preventing infection by use of a toxicant or other barrier
between the host and the pathogen.
4. Disease Resistance ‑ Use of plant genetic resistance or tolerance.
5. Therapy
‑ Reducing the effect of the pathogen in an already infected plant.
6. Trap Crop ‑ Establish plants attractive ‑to insect vectors on the borders
or the main crop, then destroy the trap crop and the vector.
Major Plant Disease
Control Methods
1. Cultural Control ‑ examples:
A. Sanitation ‑ pruning, removal of debris, removal of diseased plants,
sterilizing tools, washing hands, etc.
B. Use of disease‑free planting material.
C. Choice of planting location.
D. Time of planting.
E. Choice of irrigation method and schedule.
F. Choice of fertilizer: type, timing, application method, schedule.
G. Crop rotation.
H. Use of green manures or cover crops.
2. Biological Control ‑ the use of living organisms that are antagonistic to
pathogens.
A. Stimulate beneficial organisms in the environment with soil amendments
or other cultural practices.
B. Add beneficial organisms to the soil or plant environment.
3. Resistance or Tolerance ‑ host plant genetic control.
A. Generally the most effective means of control when available.
B. Must be continually monitored as pathogens will develop virulence to
tolerant plant material.
4. Chemical Control ‑ Act to eliminate, reduce or remove the pathogen at the
source (eradication); to prevent disease (protection), or to cure disease
(therapy).
A. Examples: fungicides, bactericides, nematicides, soil fumigants
B. Chemical used must be less toxic to the plant than to the organism(s)
they are designed to control.
C. Most fungicides are actually fungistats, which means that the chemical
limits the activity of the fungus, but doesn’t kill it. The disease will return
when the chemical is no longer active and the conducive environment for fungal
activity reoccurs, Thus, management of diseases with fungicides often requires
repeat applications.
D. Effective chemical use depends on:
a. Choosing the right chemical.
b. Applying the chemical in the right way, at the right time, and in
the right concentration.
c. Reading and following the label directions very carefully.
General Life Cycle
1. FUNGI
Fungi are the largest group of plant pathogens. They can be thought of as plants
which lack chlorophyll; but they are not plants, they are organisms in their own
kingdom. Fungi obtain food from other living organisms or from decaying organic
matter. They produce microscopic spores which can be compared to seeds of higher
plants. The spores develop into threads (hyphae) which grow and branch into
mycelium or other specialized structures (fruiting bodies).
Fungi enter plants through wounds, natural openings, or by direct penetration
through the surface of the plant. The fungal mycelium grows through the plant
and eventually produces more spores. These spores can then spread the disease to
other susceptible plants. Some fungi have complicated life cycles which require
more than one type of spore and/or more than one type of host plant to complete
the life cycle.
Fungi are disseminated (spread) by airborne spores (wind currents), soil, water
(move in irrigation water or rain splashes), seed, or by vectors. Vectors are
agents that transmit diseases from one plant to another. Examples of vectors
are: man, other animals, insects such as bark beetles that carry the blue stain
fungus, tools, other microorganisms (fungi, nematodes,
etc.), etc.
A.
Ornamental Diseases Caused By Fungi (examples):
1.
Powdery Mildew
Powdery mildew is the common name for the disease caused by several different
fungi which produce a whitish, powdery growth on the surface of infected plants.
A wide range of plants can be affected by powdery mildew, however each
individual fungal species has a somewhat limited host range.
In
general, powdery mildew fungi are favored by high humidity in the plant canopy
(but are inhibited by water on the leaf surface) and warm temperatures. Cultural
practices which increase air flow around plants, thus reducing the humidity in
the plant canopy can help control powdery mildew. Additionally, protective
fungicides are available for most plants, however timing of application is
important in effective control.
a.
Powdery Mildew of Rose
Powdery mildew of rose, caused by Sphaerotheca pannosa, is extremely
common worldwide. The fungus attacks young, succulent foliage. The symptoms
begin as slightly raised, blister‑like, red areas on leaves. Eventually all
infected above ground plant parts will develop a white powdery fungal growth.
S. pannosa
overwinters in infected canes or buds and in fallen leaves. In spring, new
shoots become infected from old mycelium from conidia (asexual spores) or from
ascospores (sexual spores). Conidia and ascospores are disseminated to other
susceptible hosts by air currents. The conidia and ascospores germinate and
directly penetrate the plant.
The
disease is favored by night temperatures between 58-62°F and day temperatures
between 65-78°F. The fungal spores cannot germinate in free water, but
germinate readily when the relative humidity in the plant canopy is high
(97-99% at night and 40‑70% during the day).
Powdery mildew is managed with good sanitation practices. Prune out all infected
canes, remove fallen leaves, and destroy all infected plant material. Protective
fungicide sprays can be used when weather conditions favor disease development.
Plants already infected with the fungus can be treated with systemic fungicides
which should help to reduce the spread and activity of the fungus. Be sure to
check for registered materials and read and follow the label directions very
carefully. When powdery mildew is known to be a severe problem, it is best to
plant tolerant varieties.
b.
Other common ornamental hosts of powdery mildew fungi: Euonymus, Photinia,
Lilac, Pecan, Verbena, Crepe-Myrtle, Sunflower, Catalpa, Cotoneaster, Holly,
Locust, Mesquite, Mulberry, Privet, Apple, Pear, Phlox, Zinnia, and Stone
Fruits.
2.
Verticillium and Fusarium Wilt Diseases
Verticillium spp.
and some Fusarium spp. are common soil-borne fungi which cause wilt
diseases in a wide variety of plants. While they are two distinctly different
fungi, the types of diseases they cause and the plants they infect are similar.
Plants infected with these fungi are wilted and chlorotic: In the case of
Verticillium wilt, the plants may exhibit wilt or chlorosis on only one side of
the plant. Infected plants also may have root rot. A helpful diagnostic symptom
is the presence of vascular discoloration.
The
fungi survive in soil and crop debris. They are spread by soil, water, wind
blown dust, seed and infected plant material. Verticillium spp. generally do
better at cooler temperatures than Fusarium spp.
Both
of the fungi exist in many different genetic strains which vary in their
aggressiveness and host range. Thus, a plant can be infected with a mild strain
and exhibit chronic mild to moderate symptoms over a long period. Or, a plant
can be infected with a severe strain and be killed within one growing season.
If
the plant is infected with a severe strain, there is little that can be done to
save the plant. If infected with a mild strain, good water and fertilizer
management can help to slow disease development and reduce symptoms. However,
the plant is still diseased and will eventually become weakened and should be
removed and destroyed.
When
replanting an area where a plant has died from this disease, it is best to
choose plants that are either non‑hosts, or are known to be tolerant to the
disease. Always replant using disease‑free seed and planting material. After
planting, avoid injury to roots and crown and maintain good strong growth
through proper water and fertilizer management.
a. Fusarium Wilt of Mimosa
Fusarium is a
soil-borne fungus that invades trees through the roots. The fungus typically
enters through wounds, but may also enter through direct penetration of the
roots or plants weakened by abiotic stress. In addition to the typical symptoms
listed above, this fungus causes infected Mimosa trees to ooze a frothy liquid
from cracks and growth sprouts on trunks. Fungal spores produced on the exterior
of the tree are easily washed off by rain or irrigation and can be moved long
distance in surface water runoff. Control is dependent on sound cultural
practices which maintains strong healthy trees. Mimosa trees should not be
planted where trees have died from this disease. Some trees tolerant of this
fungus include redbud, honey locust, and New Mexico locust.
3.
Phymatotrichum Root Rot
Phymatotrichum root rot (also known as Texas root rot or cotton root rot) is
caused by the soil-borne fungus Phymatotrichopsis omnivorum (PO). The
fungus has an extremely wide host range affecting over 2,300 species of
dicotyledonous plants (monocots are not affected, although the fungus has been
found to grow and reproduce on some monocots without causing any disease). PO is
limited geographically to parts of the United States (parts of Arizona, New
Mexico, and Texas) and Mexico. Even within its geographical boundaries, the
fungus is spotty in occurrence. The fungus is found in soils which are high in
alkalinity and low in organic matter. Spread of the fungus is limited as it does
not produce any viable spores, but spreads instead through root grafts between
nearby plants.
Symptoms first appear during the summer when air and soil temperatures are high.
The first evidence of the disease is a slight yellowing of the leaves. Quickly
the leaves turn to a bronze color and begin to wilt. Permanent wilting can occur
very rapidly; as little as two weeks from the first expression of disease.
Plants infected with PO die rapidly with the leaves remaining firmly attached.
In some cases, the tree wilts so quickly that there is little color change,
though they become dry and brittle. A reddish lesion develops around the crown
of trees killed by this fungus.
The
fungus also produces signs on or near infected plants. Strands of fungal hyphae
are produced on the surface of infected roots. These strands are usually visible
with a good hand lens. When strands are viewed under a light microscope,
cruciform (cross‑shaped) hyphae unique to this fungus can be seen. Another sign
is the formation of a white to tan colored spore mat on the surface of the soil
around infected plants. Spore mats develop during periods of high moisture and
are not always produced in New Mexico. Spores produced in spore mats have never
been germinated, and are considered to have no function in the survival or
infection by this pathogen. Therefore, spore mats do not spread disease, but are
merely evidence of the presence of the fungus.
This
disease is very difficult to control. The fungus survives over 12 feet deep in
the soil reducing the effectiveness of soil treatments, such as solarization and
fumigation. The fungus can be kept inactive by altering the soil environment
(reducing alkalinity and increasing organic matter). Kept in mind that the
following treatment must be applied very quickly after the first sign of
disease. Additionally, the treatment will need to be applied every year in order
to prevent reoccurrence of the disease. This treatment is no guarantee of
control and is expensive and labor intensive.
Steps to control PO:
1.
Heavily prune back infected trees or shrubs.
2.
Loosen soil underneath the plant out to the drip line.
3.
Cover ground with 2" of composted manure.
4.
Cover manure with ammonium sulfate (1 lb to 10 sq. ft.).
5.
Cover ammonium sulfate with soil sulfur (l lb to 10 sq. ft.).
6.
Soak area with water till water penetrates the soil to a depth of 3‑4 ft.
7.
Also treat any adjacent susceptible trees or shrubs even if there are no
symptoms.
Avoiding areas known to be infested with the pathogen is the best control
measure.
4.
Rust Diseases
Rust
is the common name for the disease caused by several different fungi which
produce dark‑colored spore pustules on the surface of infected plants. A wide
range of plants can be infected by rust fungi, but ‑most individual rust fungi
have a very limited host range. Rust fungi may have up to five different spore
stages in their life cycle. Rusts may be autoecious (having only one host) or
heteroecious (having two hosts). Heteroecious rusts need both hosts to complete
its sexual life cycle, although inoculum can build on one host through asexual
reproduction. When two hosts are required to complete the life cycle, one is
often considered the “economic host” (the desirable plant) and one is called the
“alternate host”. In some cases, such as apple‑cedar rust, both hosts are
“economic hosts”.
The
presence of spore pustules is a ten‑tale sign of rust infection. Spores produced
in rust pustules can be carried by wind currents up to several hundred miles.
Spores are also moved short distances by wind, insects, rain, and animals.
Additional symptoms include, leaf and stem cankers, stunting, yellowing, galls,
and a general unsightly appearance of the infected plant.
Rust
fungi may be controlled by the integrated use of several different management
practices. Rusts with two host may be reduced by eliminating the alternate host
(if one of the hosts is undesirable). Removing infected bedding plants or other
annuals will help to reduce spread in the garden. Depending on the host,
tolerant varieties may be available. Lastly, contact and systemic fungicides,
such as triforine, maneb, sulfur, ferbam and zineb, may be registered for use on
several host plants. Check chemical labels for appropriate use of these
materials.
5.
Black Spot of Rose
Black spot of rose, is a fungal disease caused by Diplocarpon rosea. The
disease develops in moderate temperatures when moisture is present on the leaf
surface. General leaf chlorosis and circular, well defined, black spots on
leaves are the most common symptoms. The disease is similar to powdery mildew in
that it overwinters in canes and in fallen leaf debris. The fungus germinates in
favorable conditions and is spread to susceptible hosts by splashing water or
airborne spores. Sanitation is important in the control of black spot. Rake and
destroy fallen leaves and prune out infected canes. Look for tolerant varieties
if black spot is a common problem in your area. Also some fungicides, such as
captan, chlorothalonil, and triforine, are registered for control of powdery
mildew. Check fungicide labels for complete instructions before using any
chemicals.
Click here: yardandgarden/ygbriefs/p414anthracnosemaple.html
6.
Sooty Mold
Sooty mold is a term used to describe the black sooty fungal growth on many
trees and shrubs. Several different fungi can cause sooty mold. These fungi are
generally not parasitic to the plants they grow on, but are growing on honeydew
produced by insects (aphids, scale, and mealy bugs). Sooty mold is common in
warm, humid weather. The fungi appear on leaves, stems or fruits as a
superficial, black growth. The fungi do not penetrate the host tissue and can be
wiped off with a damp cloth. Although sooty mold fungi are not pathogenic, they
do create a problem when: the growth of the fungi, becomes dense reducing the
amount of light which reaches the green leaves. This reduced light limits
carbohydrate production by the plant and weakens the growth of plant. The most
effective means of controlling sooty mold is managing honeydew producing
insects.
B. Foliar
Turfgrass Diseases Caused by Fungi (examples):
1.
Dollar Spot (causal agent: Sclerotinia homeocarpa)
Most
all turfgrass species are susceptible to dollar spot. The disease results in
circular, sunken patches (up to 6 cm in diameter) in the turf. Chlorotic,
water‑soaked lesions which become white with age occur on individual plants.
These lesions may be surrounded by a tan to reddish brown margin. The lesions
may be circular or hourglass in shape or may enlarge to cover the entire leaf
surface. The tips of infected grass blades dieback. With heavy dew, a white
cottony, fungal growth occurs over the surface of the infected grass.
The
fungus, S. homeocarpa, survives as mycelium in infected plants or plant
debris. It spreads from one area to another by movement of infected plant
material by equipment, people, animals, water or wind.
The
disease occurs from late spring through fall, during periods of high humidity in
the leaf canopy. Warm (60‑86F), humid days and cool nights which result in heavy
dew favor disease development.
2.
Downy Mildew or Yellow Tuft (caused by: Sclerophthora macrospora)
All
turfgrass species are susceptible to downy mildew. Turf affected by downy mildew
exhibits an overall spotty appearance. The diseased patches are small (1‑10 cm
in diameter) and chlorotic. Individual plants are stunted and thickened or have
broadened leaves and shortened roots. The whole plant becomes yellow over time.
A white, downy growth may appear on leaf surfaces during cool, wet periods.
The
fungus, S. macrospora, survives as mycelium or oospores (sexual resting
spores) in infected plants or plant debris. It is spread by movement of infected
plant material by equipment, people, animals, water, or wind. Swimming spores
produced by the fungus can move short distances in water and contribute to the
enlargement of diseased areas.
The
disease occurs from late spring through fall, when the leaf surface is wet. The
disease is favored by poorly drained soils and heavily watered areas.
3.
Powdery Mildew (caused by: Erysiphe graminis)
The
most common turf species affected by powdery mildew are bluegrass and fescue .
The disease results in large areas of the turf appearing as if they were dusted
with powder. The individual plants are infected with isolated colonies of
whitish fungal growth, which rapidly enlarges to cover the entire leaf surface.
Infected leaves eventually turn yellow then tan to brown as they die. Older
leaves are more susceptible to attack than the young succulent growth. Dark
fungal fruiting bodies called cleistothecia may form in mycelial mats on the
surface of leaves.
The
fungus, E. graminis, survives as mycelium in living, infected plants or
as cleistothecia embedded in plants or plant debris. The fungus is spread by
airborne spores which can move great distances in air currents.
The
disease occurs in the spring and fall. It is favored by temperatures between 60
and 72° F, high humidity, and cloudy periods. The disease is most severe in
shaded areas with poor air circulation.
4.
Rust (caused by several species of rust fungi)
All
turfgrass species are susceptible to rust fungi. Turfgrass patches infected with
rust are thin and weak, and are tinted red, brown or yellow in color. The
individual plants exhibits light yellow flecks on infected leaf blades and
stems. The yellowish area of infected leaves enlarge and elongate parallel with
the leaf or stem axis. Rust pustules on the surface of infected plants expose
colored spores.
Rust
fungi survive as mycelium and spores in infected plants or plant debris. The
fungi are spread by airborne spores which can move short and long distances in
air currents.
The
disease occurs from spring through fall on grass which is growing slowly under
stressed conditions. Disease development is favored by warm (68-86° F)
temperatures, low light intensity, and moist leaf surfaces. After infection,
disease spreads rapidly with high light intensity, dry leaf surfaces and high
temperatures (above 85° F).
5.
Copper Spot or Zonate Leaf Spot (caused by: Gloeocercospora sorghii)
Most
turfgrass species are susceptible to copper spot, however, the disease is most
severe on bentgrass. Affected turf areas will exhibit scattered, roughly
circular diseased patches, approximately 2‑7 cm in size, that are
salmon-to-copper colored. Individual plants have small red to brown lesions with
may coalesce to blight the entire leaf. During warm, wet weather, infected
leaves are covered with mycelium and fruiting bodies containing salmon‑colored
spores in a gelatinous matrix.
The
fungus, G. sorghii, survives in infected leaf debris. It is spread by the
movement of spores by equipment, people, animals, or water.
The
disease occurs from late spring through fall and is favored by warm, wet
weather. Grass growing under excessive nitrogen fertilization or on acidic soil
is more susceptible to the disease.
6.
Snow Molds (caused by several different fungi)
All
turfgrass species are susceptible to at least one type of snow mold. The
symptoms vary somewhat depending on the grass species and the fungus, but
typical symptoms include; spots to large patches which are yellow, white, gray,
brown, or pink in color.
Snow
mold fungi survive as mycelium or fruiting bodies in infected plants or plant
debris. They are spread by leaf to leaf contact, and usually require wet leaf
surfaces for infection.
The
disease occurs from winter to early spring. Snow molds are associated with low
temperatures, frozen soil, snow cover or several freeze/thaw cycles during the
winter. Snow molds are also favored by moisture from snow, frost, rain, and dew.
Control of snow mold is usually accomplished by application of fungicides
(labeled for the disease) before the first lasting snow and during periods of
snow melt.
C. Foliar
and/or Root Turfgrass Diseases Caused by Fungi (examples):
1.
Anthracnose (caused by: Colletotrichum graminicola)
All
grass species are susceptible to anthracnose, however the disease is most severe
on bluegrass and bentgrass. Symptom development is highly dependant on the
environment, but scattered chlorosis or irregularly shaped chlorotic patches
ranging from a few centimeters to a few meters in size, is characteristic of
infected turf. Individual plants have water‑soaked lesions on the leaves or
stems. The lesion eventually turns reddish brown and the entire leaf turns
yellow, then tan to brown as the leaf dies. Tiny black fruiting bodies may
appear on infected stems and leaves.
The
fungus, C. graminicola, survives as mycelium in plant debris. It is
spread by movement of spores by equipment, people, animals, water, and wind.
The
diseases occurs any time of the year, but is most severe during the summer
months. Disease development is favored by high humidity and leaf wetness. Grass
which is under stress, particularly from high temperatures and drought, are
particularly susceptible to this disease.
2.
Helminthosporium‑like Diseases (melting out diseases)
Melting‑out diseases, caused by several different species in the genera,
Bipolaris, Curvularia, Drechslera, and Exserohilum, are among the most
common turf diseases which occur in New Mexico. All turfgrass species are
susceptible to one or more of these pathogens. Symptoms of this disease are
variable depending on the fungal species causing the disease, however, some
general symptoms include overall thinning or decline of turf followed by
irregular patches 5 centimeters to 1 meter in size. The individual turf plants
are hard to find in infected areas as it appears as if the turf has “melted”
away. The appearance on individual plants can be in the form of brownish‑green
to black lesions, dappled yellow and green patterns on the leaves, or elongated
water‑soaked lesions with a yellow halo.
The
fungi survive in infected plants or plant debris and are spread by the movement
of spores by equipment, people, animals, water, and wind.
The
diseases occurs from spring through fall. Diseases caused by Bipolaris
are favored by either wet or dry conditions and temperatures between 68-86° F. Curvularia
diseases are favored by wet conditions and high temperatures (above 86° F).
Drechslera diseases are favored by wet conditions and cool temperatures
between 55‑65° F.
3.
Fusarium diseases (caused by: Fusarium spp.)
All
turfgrass species are susceptible to disease caused by Fusarium spp.
Diseased turf develops sunken, circular to irregular shaped patches between 2
and 30 centimeters in size. Patches may develop a “frog’s eye” symptom ‑ dead
circles with live grass in the center. Individual plants will have black to
brown “dry rot”; of the roots, ‘crowns, rhizomes, and stolens. Infected leaves
start out light green in color and rapidly fade to tan. White to pink mycelium
develops on infected grass during periods of high temperature and moisture.
The fungi survive as mycelium in infected plants or plant debris. They are
spread by movement of spores by equipment, people, animals, water and wind.
The
disease occurs from late spring through summer and is favored by high
temperatures and drought stress. Susceptibility increases in grass with
excessive nitrogen or unbalanced fertilizer applications.
4.
Pythium Diseases (caused by: Pythium spp.)
All
turfgrass species are susceptible to Pythium diseases. While the symptoms are
somewhat variable, the disease are typified by an overall decline in the turf
area. This decline may be gradual or rapid, depending on the environmental
conditions. Small areas of declining turf may coalesce to cover large areas.
Individual plants have dark, water-soaked lesions. The leaves turn yellow, then
tan. Roots of infected plants rot. In the early morning when dew is present, a
white, cottony growth may appear on the grass surface.
The
fungi survive as oospores in infected plants or plant debris. They are spread by
movement of infected plant material by equipment, people, animals, and water.
Swimming spores move short distances in water and contribute to the enlargement
of individual areas.
The
disease occurs anytime during the growing season. Disease development is favored
by hot (86-95° F), wet weather with night temperature above 68° F.
Susceptibility increases in dense turf and in turf growing in alkaline
conditions.
5.
Brown Patch (caused by: Rhizoctonia solani)
This
is one of the most common turf diseases in New Mexico. All turfgrass species are
susceptible to brown patch. Although the symptoms are highly variable, the
disease is characterized by small to large rings or patches of dead grass.
Individual plants have small to large, irregularly shaped lesions with a
distinct dark brown margin. Infected leaves become chlorotic, then brown with
age. Dark brown sclerotia, (masses of mycelium with a hard shell) may develop at
the base of infected plants.
The
fungus, R. solani, survives in soil and in infected plants and plant
debris. It is spread by leaf to leaf contact, and by movement of infected plant
material by equipment, people, animals, water and wind.
The
disease occurs from spring through fall and is favored‑by warm (70‑90° F), wet
conditions. Dense, highly fertilized, frequently watered grass is more
susceptible to the disease.
D.
Root Turfgrass Diseases Caused by Fungi (example):
1.
Summer Patch (caused by Magnaporthe poae)
Summer patch is common on bluegrass and fescue, but also has been found on
ryegrass and bentgrass. It is a common disease on golf greens and is often
identified when bluegrass is killed and seemingly unaffected bentgrass grows
into diseased patches. Before the causal agent, M. poae, was identified,
the disease was thought to be part of the Fusarium blight complex.
The
symptoms first appear as small (3‑8 cm), circular patches of slow‑growing,
thinned, or‑wilted turf. The diseased area may increase in size to 30‑60 cm in
diameter. Affected leaves rapidly fade from grayish green to a light straw color
during sustained hot weather. Infected roots, rhizomes, and crowns turn dark
brown as they are killed. Microscopic examination of these tissues reveals a
network of sparse, dark brown to black hyphae.
The
fungus survives as mycelium in plant debris or perennial host tissue and is
spread by aerification and dethatching equipment as well as by transport of
infected sod.
Infection occurs in the spring when soil temperatures stabilize between 65 and
68° F. Symptoms develop during hot (86‑95° F) rainy, weather or when high
temperatures follow periods of heavy rainfall. The disease is more severe when
turfgrass is maintained under conditions of low mowing height and frequent,
light irrigation.
E.
Fairy Rings and Slime Molds:
1.
Fairy
Rings are characterized by circles, rings or arcs of dark green, fast
growing turf.
Fairy Ring
Fungi are several species of basidiomycetes) Marasmius oreades (edible),
Chlorophyllum molybdites (greenish spores), Agaricus species (many edible
none
deadly),
Coprinus species (inky caps) etc. Unlike Fusarium fungus these fungi do not
infect the sod plants but survive in organic matter which they decompose.
The dark green growth is caused by the fungus making nutrients from the organic
matter available to the growing plants. If enough nitrogen is made available the
grass may be killed and
the diseased areas die‑out. Rings can vary in
size during the year and can appear or disappear throughout the growing season
depending on the availability of water. The dark green areas may be surrounded
by mushrooms, toadstools, or puffballs which are fruiting bodies of the fungi
which cause the disease. These fruiting bodies are excellent signs of the fungi
and are diagnostic.
The
fungi which cause fairy rings live in the soil on buried organic matter and on decomposing thatch layer.
They are spread from one area to another by the movement of infected plant
material or infested soil, by equipment and by wind blown spores.
The
diseases occurs from spring to early summer and into fall. The fruiting bodies
generally appear in the late summer (during the summer rainy period).
Fairy rings are difficult to control. Fungicides are rarely effective as well as
extremely expensive as the mycelium (the actual organism of the fungi) is often
perennial and may be deep within the soil. The affected turf is sometimes removed including a significant amount of soil. The
area must then be prepared and resodded. However, if the conditions for the
fungus to grow remain they will usually return. Removing fruiting bodies before they
mature can sometimes help to reduce the severity of the disease. Maturation
of the spores (seeds) of the fungus occurs very early in the mushrooms growth.
In lawns, the best control method is careful watering. Waiting until the turf
shows footprints before watering deprives the fungus of the abundant water it
requires.
2.
Slime Molds (caused by slime mold fungi)
Slime molds are not disease causing agents as they do not penetrate and infect
plants. However, they do cause harm to plants by covering the leaf surface and
reducing the amount of light reaching the plant surface. Slime molds are
pinhead‑sized white, gray, or purplish‑brown fungi, which are slimy in
appearance and texture. They appear in patches 2 to 60 cm in diameter) during
periods of warm, wet weather. They usually disappear within 2 weeks after the
favorable conditions are gone. Heavily thatched turf is more susceptible to
disease. Slime molds can be washed off of plants with a hard stream of water, or
swept off with a broom.
2. BACTERIA
Bacteria are single cell microorganisms that lack chlorophyll. They obtain food
from living or dead organic matter. The average size of a bacterium is less than
5 microns. Bacteria reproduce by division of the cell into two equal parts.
Bacteria can divide very rapidly: Under the proper environmental conditions one
bacterium can produce over 1 million identical cells in less than 1 hour.
Bacteria enter plants only through wounds or natural openings on the plant
surface. Once inside the plant, bacteria multiply rapidly and break down the
plant tissue creating a watery mass. Bacteria are only apparent to the naked eye
if clumped together to form a colony (bacterial ooze).
Bacteria are spread by splashing rain (waterborne), in soil, in windblown dust,
by vectors (man, tools, equipment, insects, etc.), and infected planting
materials (seed, transplants, cuttings, etc.). Diseases caused by bacteria are
generally favored by high air and soil moisture. Because of this, bacterial
diseases are few and somewhat rare in arid regions.
A.
Diseases of Ornamentals and Vegetables Caused by Bacteria (examples):
1.
Bacterial Spots and Blights
Bacterial spot‑ and blight diseases affect all above ground parts of plants:
leaves, stems, blossoms, and fruit. A wide variety of ornamentals and vegetables
are susceptible to spot and blight diseases. Spot diseases are characterized by
necrotic, circular or angular shaped spots. In some diseases, the spots are
surrounded by a yellow halo and/or dead leaf tissue may fall out of spot leaving
holes in the leaf (known as a “shot hole”). Bacterial blights are characterized
by a continuous, rapidly advancing death of infected organs.
The
bacteria overwinter in infected and healthy tissue, in or on seeds, in plant
debris, and in soil. The organism is spread by rain, leaf to leaf contact,
insects, cultivation, and tools. Water soaking of tissue caused by heavy rains
predisposes plants to infection.
Bacterial spots and blights are managed with good sanitation practices; removing
and destroying infected plant parts and frequent cleaning of tools and other
equipment. Copper fungicides and antibiotics can be useful for high value crops,
such as in orchard or nurseries. In vegetable crops, look for tolerant
varieties, and rotate crops around the garden.
1.
Fire Blight
Fire
blight is a bacterial disease caused by Erwinia amylovora. It affects
plants in the rosaceae family, most notably apple, pear, rose, pryracantha,
photinia and cotoneaster. Infected plants exhibit blighted branch tips which are
black and look like they have been scorched by fire. Another symptom is the
development of a shepherd’s‑crook on young, vegetative shoots. The bacterium
overwinters as cankers and symptomless infections in leaf and flower buds. In
the spring, bacteria oozes from infected cankers and is spread by water, pruning
tools and insects (primarily bees) to nearby blossoms. Ideal conditions for
infection and development of disease are rain (or high humidity) and
temperatures between 75-85° F.
Cultural practices can be highly effective in the management of fire blight.
Infected plant parts should be pruned, cutting at least 6” below the disease
margin (margin between healthy and diseased tissue), and destroyed. When pruning
infected trees, it is advisable to dip pruning shears in a 10% bleach solution
or in 70% alcohol in between cuts. Avoid over fertilization (especially with
nitrogen). Provide adequate, but not excessive water. Copper fungicides and
antibiotics can be effective sprays, however timing is critical and improper use
can lead to phytotoxicity (from the copper chemicals) or development of
resistance in the bacterial population.
2.
Bacterial Canker
Bacterial canker (also referred to as “gummosis”) is caused by several different
species of bacteria. It affects stems, branches, twigs, leaves, buds, flowers,
and fruit. The disease occurs on a wide range of hosts, including stone fruits,
apple, pear, lilac, rose, tomatoes and small grains. Symptoms include: splits in
trunks or stems, necrotic areas in the woody tissue, and sunken cankers which
may be soft, leathery, or scabby in appearance. Some cankers exude a slimy or
gummy substance. The bacteria overwinter in perennial cankers, in buds, plant
debris and in or on seed. The disease is spread by rain, runoff water,
cultivation, tools and infected plant material.
Bacterial canker is best controlled by good sanitation practices. This includes
pruning and destroying infected plant parts and maintaining strong, but not
excessively vigorous growth. Avoid planting any material which is suspicious in
appearance. Copper fungicides or antibiotics may help in reducing spread when
many similar plants are in close proximity (e.g. home orchards). Caution should
be taken in using copper fungicides as improper use may lead to plant injury and
effective use is dependent on proper timing of applications.
3.
Crown Gall
Crown gall, caused by the soilborne bacterium Agrobacterium tumefaciens,
has a wide host range among woody and herbaceous plants. The bacterium enters
roots or stems near ground through wounds created by cultural practices,
insects, etc. The bacterium can also infect plants above the crown by pruning
with infested cutting shears.
Once
inside susceptible hosts, the bacterium stimulates host cells to enlarge causing
tumorlike galls. The galls impede the movement of water and nutrients in the
plant. Reduced transport of water and nutrients causes chlorosis, stunting, slow
growth, and a general decline in the health of the plant. Some plants infected
with crown gall will continue to grow seemingly unaffected while others decline
over time until they have to be removed.
Once
infected, there is little that can be done to help the plant other than
providing adequate water and nutrients. Well managed trees are less likely to go
into a rapid decline. When planting susceptible hosts, use disease‑free nursery
stock. Avoid injury to roots and crown at planting and during cultivation of
turf or other plants nearby.
3. VIRUSES AND VIROIDS
Viruses consist of nucleic acid (RNA or DNA) surrounded by protein (coat
protein). Viroids consist only of nucleic acid (RNA only). Both are so tiny they
can only be seen with an electron microscope. They do not carry out respiration,
digestion, or other metabolic functions, therefore, they are not living
organisms. Viruses and viroids cannot grow or multiply outside the host cell,
but cause the plant to transform host plant components into more virus or viroid
particles.
Viruses and viroids are transmitted from one plant to another by vectors (so
they are biotic disease agents). Common vectors include man, insects, budding,
grafting, nematodes, fungi, seed and/or pollen. The virus or viroid particles
enter plants through wounds created by the vector.
Symptoms of these types of diseases are nonspecific and can look like symptoms
caused by many other disease agents. General symptoms include stunting,
yellowing, curling or twisting of leaves and stems, distorted leaves and fruit,
mosaics, mottles, and ring spots.
A.
Diseases of Ornamentals and Vegetables Caused by Viruses (examples):
1.
Rose Mosaic Virus
Rose
mosaic virus occurs worldwide. The symptoms are highly variable depending on the
variety and the environment, however chlorotic bands or rings, vein clearing,
and general mosaics are common. Symptom development on only a portion of a plant
is common. Infected plants have decreased vigor, poor flower production, and are
more susceptible to winterkill. The virus is transmitted through vegetative
propagation and pollen. There is no control for a plant infected with the virus.
Infected plants should be removed and destroyed.
2.
Beet Curly Top Virus
Beet
curly top virus (BCTV) is common in arid and semiarid regions on a wide host
range (affects over 300 plant species). The virus is transmitted (vectored) by
the beet leafhopper (Circulifer tenellus). Some of the more common hosts
include tomatoes, peppers, cucurbits, potatoes, beans, spinach, geranium,
nasturtium, petunia, stock, and zinnia. Weeds are important survival hosts for
the virus and the vector. Weed hosts include; Russian thistle, sow thistle,
London rocket, pigweed, purslane, knotweed, and lamb's-quarter.
Symptoms vary somewhat depending on virus strain and host plant, yet there are
some common characteristics among infected plants: Overall chlorosis, upward
curling of leaves, thickening of leaves and stems, stunting, deformed fruit, and
reduced fruit production. On some hosts, such as tomato and pepper, leaf veins
on the underside of the leaves may turn purple. Although viruses usually do not
kill their host plants, young seedlings attacked by BCTV may die.
The
only means of disease transmission is by leafhoppers. Leafhoppers feeding on
infected plants will rapidly (1 minute) acquire the virus. The virus then
circulates through the insect and can be transmitted to a susceptible plant
after as little as 4 hours. The leafhopper then remains infective for the
remainder of its life, although the effectiveness of transmission is decreased
when the insects do not continually feed on infected plants. The virus is not
passed on to progeny, however, young leafhoppers which develop on infected hosts
will quickly become carriers of the virus. As far as is known, the virus has no
negative or positive affect on the insect. Leafhoppers overwinter in winter
weeds. Mild winters and large populations of winter weeds are two important
factors in BCTV epidemics.
There are no chemicals available for the control of viruses, but several
cultural practices can help to reduce or eliminate infections. Good sanitation
practices, including weed and insect control and removing infected or suspect
plants, are essential in limiting the occurrence of the disease. Home gardeners
may also consider planting susceptible hosts, such as tomatoes and peppers, in a
slightly shaded part of the garden, as leafhoppers prefer to feed in sunny
locations. However even shaded plants will become infected when leafhopper
populations are high. Placing netted cages over susceptible hosts (particularly
when young) may help to prevent infection. The netted material should be small
enough to prevent leafhoppers from getting through the material. It is also
important that the cages be large enough that the plants do not touch the
netting. When plants mature, the cages should be removed. At this stage, the
plants is less susceptible to the virus. Although there is little tolerance
known to BCTV in commercial varieties, this is an active area of research and
new varieties may soon be released which possess a level of resistance to the
virus or the leafhopper.
3.
Tomato Spotted Wilt Virus
Tomato spotted wilt virus (TSWV) is an important disease of many vegetables and
ornamentals in temperate and subtropical regions of the world. In New Mexico, it
is particularly troublesome in greenhouses, but can easily move to gardens on
infected plant material or carried by its vector (thrips). The virus has a
tremendously wide host range including tomatoes, peppers, celery, lettuce,
spinach, potatoes, peanuts, begonias, geranium, nasturtium, impatiens, petunia,
snapdragons, verbena, stock, and statice. Like curly top, TSWV has many weed
hosts which help it survive from one season to the next. Common weed hosts
include curly dock, field bindweed, lamb's-quarters, pigweed, morning glory,
jimsonweed, and nightshade.
Symptoms of TSWV are numerous and varied. Some fairly characteristic traits of
infected plants are bronzing and yellowing of leaves, leaf spots, leaves become
distorted, and petioles curl downward creating a wilt‑like symptom although the
plants retains its turgor pressure. Other symptoms include dieback of the
growing tips and dark streaking of the terminal stems. Infected plants may
develop a one‑sided growth habit or may be completely stunted. Infected plants
produce little or no fruit. Fruit that is produced exhibits symptoms, such as
necrotic streaking, raised bumps, chlorotic spots or ring spots, uneven
ripening, and deformation.
TSWV
is transmitted from infected to healthy plants by at least 9 species of thrips.
Thrips transmit the virus in a persistent manner, which means that once the
insect acquires the virus it can transmit the virus for the remainder of its
life. The virus is not passed from adult to egg; however, progeny that develop
on infected plants will quickly pick up the virus and become effective disease
vectors.
Controlling the disease is difficult. The wide host range, which includes
perennial ornamentals and weeds, enables the virus to successfully overwinter
from year to year. In landscape situations, controlling thrips does not always
translate into reduced disease. This is probably due to the fact that large
populations of thrips may fly or be blown into treated areas from non‑treated
area nearby. Controlling thrips, is somewhat more effective as a disease control
measure in greenhouses. Control of thrips may be obtained with pyrethroids,
carbamates, chlorinated hydrocarbons, organophosphates, and insecticidal soaps.
However great care should be taken to avoid repeated use of any one type of
chemical as thrips in the treated population may rapidly build resistance to the
material. Rotating the insecticide class is the best approach to insect control.
Pesticide registrations are constantly changing so it is important to read the
label for legal uses and follow all label instructions carefully. In
greenhouses, thrips populations may be reduced by covering all openings (doors,
vents, etc.) with a fine mesh (400 mesh) screen.
While elimination of disease may not be possible, the incidence and severity of
the disease may be reduced by several cultural practices. It is .important to
start with virus‑free plants. Do not purchase or plant any plant which exhibits
symptoms such as described above. Remove any infected or suspect plants from the
greenhouse, garden and landscape. Control weeds. Efforts are underway to breed
cultivars with good horticultural characteristics that also exhibit tolerance to
the virus.
4. NEMATODES
Nematodes are microscopic, non‑segmented worms. They are filamentous in shape,
however, females in some species may become swollen at maturity and exhibit
round or pear shaped bodies. Plant parasitic nematodes have a hollow stylet
which penetrates plants cells. This stylet is visible under a light microscope
and sometimes under a dissecting scope. Nematodes have well developed digestive
and reproductive systems.
Nematodes are placed into different classification based on how they live their
lives. For example, ectoparasitic nematodes live freely in soil, do not
enter plant tissue and feed superficially on roots. Migratory endoparasitic
nematodes live freely in sod, enter plants and move through plant tissue feeding
internally. And sedentary endoparasitic nematodes have a free living
juvenile stage, but as adults they attach to the root and feed in one location.
All nematodes live at
least part of their life cycle in soil. Nematodes can move slowly in moisture
film surrounding roots and soil particles, but long distance spread is achieved
by the movement of soil. Thus, soil in irrigation water, on animals, on
equipment, and plant material can spread nematodes from one location to another.
A. Diseases
of Ornamentals, Vegetables and Turfgrass Caused by Nematodes (examples):
1.
Root‑Knot Nematode
Root‑knot nematodes (Meloidogyne incognita) can be a serious pest in home
gardens and field grown crops. The pest is most serious in warm, light soils.
Root‑knot nematodes have an extremely wide host range (over 2,000 species) which
includes most vegetables, many cereals and field crops, some trees, and weeds.
The
severity of symptoms depends on the host, the nematode and the age at which the
plant was infected. Younger plants are more severely damaged than mature plants.
Above ground symptoms are not unique, and in fact may be reminiscent of diseases
caused by many other pathogens and abiotic disorders. Common above ground
symptoms include stunting, chlorosis, wilting (particularly during the heat of
the day), and reduced yield. Roots of infected plants exhibit characteristic
galls, and may be distorted. These galls are usually visible with the unaided
eye, and can clearly be seen with a hand lens or dissecting microscope. The size
and number of galls is dependent on the type of host, the age of infection and
the nematode population. Because nematodes are sensitive to sod type, damage may
be “spotty” rather than uniform across a field or garden.
Root‑knot nematodes live in soil as eggs and juveniles, and in plants as adults.
Juvenile nematodes invade roots of susceptible hosts. Once inside the host root,
the nematodes swell and become pear‑shaped, disrupting the developing root
tissue. As the nematodes feed on root cells, the cells enlarge creating what is
called “giant cells”. It is the formation of giant cells and the presence of the
nematodes in the roots which creates the characteristic root galls. Female
nematodes lay their eggs (200‑500 per female) in a gelatinous matrix on the
outside of the root. Eggs hatch in the soils and new juvenile nematodes invade
other roots. During favorable conditions, the life cycle of this pest is
completed in about one month, so there may be up to four or five generations per
season. Therefore, populations can build up in soils very rapidly.
Several management practices can be used to reduce nematode populations. Never
plant transplants which exhibit nematode galling (remember that legumes form
root nodules for nitrogen fixations which may look similar to small nematode
galls). Avoid areas known to be heavily infested with nematodes. Crop rotation
may be helpful, but the wide host range of the pest limits its effectiveness.
There are chemicals registered with can be used to control nematodes, however
most are soil fumigants which can only be used prior to planting and they are
“Registered Use Materials” which means that they need to be purchased and
applied by a licensed pesticide applicator. Some success controlling nematodes
has been had in relatively small areas with the incorporation of soil amendments
containing crushed seashells. The seashells are made of chitin which is the same
protein which makes up the nematodes exoskeleton. The addition of chitin to the
soil stimulates beneficial soil microbes which can degrade chitin. This increase
in beneficial organisms will help to reduce the nematode population by
destroying the nematodes cell wall.
2.
Turfgrass Nematodes
Plant parasitic and saprophytic nematodes are components of every turfgrass
ecosystem. The importance of these microscopic worms in the overall health of
the turf varies depending on the type of nematode, the nematode population, and
the environment. While some nematodes can be beneficial in controlling some
pests, others can cause serious diseases in turf. Nematodes are more likely to
cause disease in warm temperate or subtropical regions, although disease can
occur in cooler regions as well.
All
turfgrass species are susceptible to nematodes. Several different species of
nematodes can cause damage in turf. Some of the common species in warm climate
species are:
-
a. Meloidogyne spp. - root-knot nematode
-
b. Trichodorus spp. and Paratrichodorus spp. ‑ stubby root nematode
-
c. Belonolaimus spp. - sting nematode
-
d. Dolichodorus spp. ‑ awl nematode
-
e. Helicotylewhus spp. ‑ spiral nematode
-
f. Hoplolaimus spp. ‑ lance nematode
Some of the common species in cool climates are:
-
a. Xiphinema spp. - dagger nematode
-
b. Pratylenchus spp. - lesion nematode
-
c. Macroposthonia spp. - ring nematode
-
d. Tylenchorhynchus spp. - stylet nematode
e.
From species above; a, b, e and f.
Nematode injury might appear as areas of low fertility, even when adequate
fertilizers have been applied. Symptoms will be slight to severe chlorosis,
declining growth, gradual thinning, wilting, and in severe cases, death. Turf
which is under stress, particularly from high heat, drought, low fertility, or
excessive thatch, is more susceptible to nematode injury.
Chemical treatment with a nematicide is available to professional sod growers,
nursery operators, and professional pesticide applicators. Retail nematicides
are not available to homeowners.
5. PARASITIC HIGHER PLANTS
Parasitic higher plants are flowering plants which live off other plants. True
and dwarf mistletoes and dodder are examples of parasitic plants.
Mistletoes have chlorophyll, but no roots and thus, rely on host plants for
water and nutrients. True mistletoes parasitize hardwood trees, such as
cottonwoods, elms, oaks, and locusts. This pathogen is disseminated by birds
which feed on the seed‑baring mistletoe berries. Dwarf mistletoes attack
conifers and are an important pathogen in conifer forests. This pathogen
forcibly discharges its seeds and is disseminated by wind currents.
Dodder has no chlorophyll and no true roots. It depends on its host for water,
nutrients and carbohydrates. Dodder is a soilborne vine‑like plant which twines
around its host. It is mostly a problem in agricultural fields, but can be
troublesome in home gardens and landscapes, particularly in newly developed
areas which were once used for agriculture.
Non‑infectious
disease causing agents (abiotics):
1. NUTRIENT DEFICIENCIES
Nutrient deficiencies occur when essential elements are not available in the
required amount. The effect on plants is dependent on the host plant and
element(s) that is deficient. Some general symptoms include, stunting, chlorosis,
small leaves, malformed leaves, poor root growth, weak plant growth, poor
turfgrass stand establishment, etc.
Common nutrient deficiency descriptions:
-
Nitrogen: slow growth, stunted plants, chlorosis (particularly older leaves).
-
Phosphorus: slow growth, stunted plants, purplish coloration of foliage on
some plants, dark green coloration with tips of leaves drying, delayed
maturity, poor fruit or seed development.
-
Potassium: leaf tips and margins “burn” starting with the older leaves, weak
stalks, small fruit, slow growth.
-
Iron: interveinal chlorosis of young leaves (veins remain green except in
severe cases), twig dieback.
-
Zinc: decrease in stem length, rosetteing of terminal leaves, reduced bud
formation, interveinal chlorosis, dieback of twigs (if deficiency lasts more
than one year).
-
Magnesium: interveinal chlorosis in older leaves, curling of leaves upward
along margins, marginal yellowing with green “Christmas tree” area along
midrib of leaf.
-
Calcium: death of growing points (terminal buds and root tips), abnormal dark
green, premature shedding of blossoms and buds, and weak stems.
-
Sulfur: light green color of (mostly) young leaves, small and spindly plants,
slow growth, delayed maturity.
-
Manganese: interveinal chlorosis of young leaves ‑ gradation of pale green
coloration with darker color next to veins. No sharp distinction between veins
and interveinal areas as with iron deficiency.
-
Boron: death to terminal buds, thickened, curled, wilted and chlorotic leaves,
reduced flowering and improper fertilization.
Soil availability of nutrients is influenced by soil characteristics. The pH of
the soil has a profound effect on nutrient availability. For example, iron,
though plentiful in the soil, is mostly unavailable to plants in alkaline soils
(pH above 7.5). Likewise, phosphorus, manganese, copper and zinc are also less
available in alkaline soils. Boron, which is needed by plants in very small
amounts in almost completely unavailable at pH between 7.5 and 8.5. A soil pH
between 6.5 and 7.5 gives a maximum availability of the primary nutrients
(nitrogen, phosphorus and potassium), and a ‘relatively high degree of
availability of the other essential elements. Unfortunately, much of the soil in
the southwest is alkaline.
Some
elements such as nitrogen are readily leached through the soil and therefore
need more frequent application to provide the plants with an adequate amount of
the element. Additionally, the relative amounts of different elements effect
nutrient availability. The excesses of certain nutrients may result in the
plants inability to take up another essential element.
Soil tests are needed to determine the base nutrient content and other
important soil characteristics. The results of soil test will help to determine
the type and amount of fertilizers needed for different plants. Nutrient
toxicities can occur with over fertilization or with improper application of
fertilizers. In most cases, application of a balanced fertilizer with essential
micronutrients is beneficial to plant growth. In some areas, additional foliar
applications of some microelements such as iron might be needed to keep plants
green.
2. PESTICIDE INJURY
All
pesticides, if used inappropriately, can be toxic to plants. In most cases,
damage results from improper application or from pesticide drift. Failure to
thoroughly clean spray equipment can also result in injury to nontarget plants.
Common symptoms of pesticide injury include: leaf burn, leaf distortion,
chlorosis, flattened or enlarged stems and roots, plant death, etc. Symptoms
type and severity are dependent on the type of pesticide and the concentration
of the chemical. In turf situations, damage appears in patterns associated with
application of the chemical.
When any pesticide is used, it is imperative that the material be applied
carefully and in accordance with the pesticide Label. It is also important to
avoid spraying on windy and/or hot days.
3. TEMPERATURE EXTREMES
Temperature extremes, both high and low, can cause injury to plants. High
temperature Results in excess transpiration, wilting, heat stress and sunscald.
The plants are unable to cool themselves by evapotranspiration. In turf, heat
stress is intensified by objects covering blades, high humidity, dry soil, and
lightning strikes.
Low
temperature injury causes leaf epidermal cells to separate from underlying
tissue giving the affected tissue a silvery appearance. The affected herbaceous
tissue will wilt and turn black. On trees, frost or freeze damage results in
splits and cracks in trunks branches and twigs, eventually causing cankers to
develop. These cankers become entry sites for secondary organisms such as fungi
and bacteria.
4. SALT INJURY
Salt injury occurs when excessive salts from either the soil or the irrigation
water are taken up by the plant. Damage results from a loss of feeder roots.
Symptoms include marginal necrosis, leaf, stem and twig necrosis. Salt injury is
often seen in association with heat stress.
5. LIGHT EXTREMES
Light affects germination, growth and shape of plants. Lack of light causes
etoliation (elongation) between nodes and results in poor color. Excess light
can result in sunburned foliage or fruit.
6. WATER EXCESS
Excess soil moisture results from excess irrigation, rainfall or poor soil
drainage. These sods have reduced oxygen levels which inhibits plant growth.
Plants may be chlorotic, have small or thin foliage, and have numerous dead or
dying roots. Roots may die from a lack of oxygen or from soilborne fungi which
are favored by high soil moisture. The final result may be plant death.
Chronically wet soils may become black in appearance and have a foul odor.
Water soaked above ground plant parts are predisposed to many diseases.
Additionally, excessive moisture on the foliage favors many foliar diseases
which require either free water or high humidity for germination and infection.
7. DROUGHT INJURY
Drought injury results from a chronic lack of water. Affected grasses turn
bluish in color and the leaves curl before turning brown. Shrubs and trees wilt
in the afternoon and recover at night until they wilt permanently. New foliage
is small and pale in color. Plant growth is restricted and plants are more
susceptible to heat stress.
8. WIND AND SAND INJURY
Wind Injury results from excess air movement. Damage is more severe if
temperatures are high. Plants become desiccated, and may become radically
altered in shape due to directional force of wind. Leaves become tattered either
from the force of the wind whipping the foliage around, or from wind blown sand.
Wind may lead to problems associated with wind blown pathogens.
9. HAIL DAMAGE
Hail
causes necrotic spots on foliage and fruit. Severe hail may cause holes in
leaves or leaves to become tattered. Hail striking the crown of young plants can
cause plants to fall over and die.
10. AIR POLLUTION
Air
pollution which results from a lack of sufficient air currents can cause
problems on many different types of plants. Combustion of fuels, auto exhaust,
coal burning, interaction of sunlight and nitrogen oxides make up different air
pollutants. The common major pollutants are: nitrogen oxides, ozone,
hydrocarbons,. peroxyacetyl‑nitrate (PAN), and sulfur dioxide.
Symptoms of air pollution vary somewhat depending on the type of pollutant,
however common symptoms include, flecking of upper leaf surface, bronzing of the
lower leaf surface, interveinal bleaching, or damage may be invisible.
11. IMPROPER CULTURAL PRACTICES:
A.
Management Practices that Impact the Health of Ornamentals
a.
Planting:
i.
Choice of location.
ii.
Soil preparation
iii.
Root preparation.
iv.
Planting depth.
b.
Irrigation:
i.
Timing.
ii.
Amount applied.
iii.
Frequency of irrigation.
iv.
Application method (coverage).
c.
Fertilization:
i.
Selection:
ii.
Timing.
iii.
Frequency of application.
iv.
Application method:
-
‑
water soluble
-
‑
slow release
-
-
foliar sprays
-
‑
injection
d.
Cultural Practices:
i.
Pruning; timing and method.
ii.
Sanitation; removal of debris, clean tools, weed and insect control
iii.
Staking
iv.
Mulching/Cultivation
B.
Management of Turfgrass
Turf diseases are best controlled with good management of the turfgrass. For
maximum disease control, turfgrass should be maintained at moderate rate of
growth. Turf which is very lush or under stress is more susceptible to disease
than turf which is grown at a. moderate pace. Management, practices include,
irrigation, fertilization, mowing, and de-thatching (aeration).
a.
Water
Turfgrass is best watered in the early morning. This allows for efficient use
of the water and for the grass blades to dry before nightfall. Water thoroughly
so that water penetrates several inches into the soil. It is better to water
less frequently and for longer periods than to water frequently for short
periods of time.
The
amount of water needed will vary depending on the time of year and the weather
conditions. You can determine the amount of water you are applying with each
irrigation by placing a few empty food cans in various locations on the lawn.
Turn on the sprinklers for a designated amount of time. After irrigation,
measure the amount of water which has accumulated in the cans. This will tell
you how much water in inches that you are applying in that specified amount of
time. You can then adjust the duration of irrigation to provide the desired
amount of water.
b.
Fertilizer
The
use of slow release nitrogen fertilizers can help to reduce the rapid flush of
lush growth after fertilization. Slow release fertilizers releases nitrogen
slowly over a long period of time. This avoids disease problems which may by
associated with large amounts of nitrogen available all at one time.
Fertilization is best applied four times a year.
c.
Mowing
Mow
the lawn as high as is practical for the use of the turf. Low mowed or scalped
grass plants are more susceptible to diseases. Be sure that your mower blades
are sharp and that they are making nice clean cuts of the grass blades. Dull
mowers tear or shred leaf blades, leaving jagged tips of injured leaves straw
colored. Overall appearance of turf is ragged and grayish in color. In addition
to poor appearance, the jagged cuts made by dull blades are more attractive to
many disease organisms. Scalping injury (mowing grass so short that yellow or
brown stem tissue is exposed) is caused by infrequent mowing, weedy grass areas,
and uneven areas. Scalping weakens turf plants making them more susceptible to
diseases.
If
you are mowing your lawn appropriately, you can leave the clippings on the
surface of the lawn and this will help with the overall balance of nutrients and
microorganisms in die thatch and soil. If you mow infrequently you do not want
to leave the clippings because the large amount of clippings will mat on the
surface of the lawn reducing the amount of water and air penetration to the
soil.|
d.
Dethatching and Aerification
Thatch is the layer just below the grass blades and above the soil. It is made
up of decomposing grass plants ‑ leaves, shoots, rhizomes and roots. A small
amount of thatch is desirable, however if thatch accumulates over 2 cm it
impedes water penetration and can cause detrimental affects to the grass. Roots
tend to grow in the thatch layer instead of the soil, increasing risk of drought
or high temperature damage. Excessive thatch is caused by keeping turf too wet
or too dry, high soil acidity or alkalinity, high nitrogen fertilization, and
repeated pesticide applications.
Thatch accumulation is controlled with periodic aeration of the grass. Aeration
is achieved by poking holes in the lawn, by vertical mulching, slicing or power
raking. You should aerate your lawn at least once a year.
Benefits of aeration:
-
Exchange of air and water
-
Reduce water runoff
-
Water penetration/retention
-
Increase root/shoot growth
-
Improve drainage/reduce thatch
-
Disease and insect control
e.
Fungicides for Turf Disease Control
Several broad spectrum and disease specific fungicides are available to help
control turf diseases. It is important that the disease be identified as best
possible to help in selecting the proper chemical. Only use registered
fungicides and use them only when absolutely necessary as repeated fungicide
application will reduce the effectiveness of the chemical due to the development
of resistance in the fungal population. Keep in mind that fungicides typically
do not kill the fungus, they stop the activity of the fungus and allow for the
turf to get a head start on filling in the diseased areas. However, if
conditions which initiated the disease problem reoccur, repeat treatment may be
necessary.
SUMMARY
-
Many living and non‑living factors cause abnormalities in plants.
-
The host, the pathogen and the environment must all work together to cause
disease.
-
Strong, healthy, well-managed plants are less susceptible to disease than
plants under stress.
-
Effective control programs disrupt the interactions between at least two of
the interacting components
-
The best disease management programs utilize an integrated approach to
control.
For additional help with diagnosing plant problems especially for people in
the Eastern parts of USA go to the Maryland site at
HGIC's
Plant Diagnostic Website
For the latest pdf file on Texas Root Rot.
http://www.cahe.nmsu.edu/pubs/_a/a-229.pdf
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