Pest
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Community Development Home Page
Dr. John F. Baniecki
WVU Extension Service
Plant Pathology and Entomology Specialist - 3/2000
A plant disease is any disturbance that prevents a plant's normal
development and reduces its economic or aesthetic value.
- Symptoms and/or signs are associated with diseases.
Symptoms are the visible response of a plant to a causal agent
over time.
Signs are structures produced by the living causal agent of a disease that are more
specific than symptoms and are more useful in accurate disease diagnosis.
- Symptoms may be grouped according to how the plant responds or reacts to the
disease-causing agent. The symptoms may appear as:
Leaf spots
Blights
Wilts
Cankers
Root and/or crown rot
Trunk rot
Abnormal growth
Diseases are caused by living agents (biotic) and non-living
agents (abiotic).
Most living agents of diseases are fungi. Many fungi that produce
plant diseases are not visible and can be seen only with a microscope. Fungi lack
chlorophyl and therefore depend upon more advanced plants for survival. Fungi parasitize
susceptible plants and cause plant diseases under proper environmental conditions. They
produce structures that aid in their spread and survival. These structures may be visible.
- Other living agents (biotic) of disease are:
bacteria (enter the plant through natural openings or wounds;
produce visible oozing on the plant)
viruses/ mycoplasmas/ viroids/plasmids (cause yellowing, stunting, abnormal growth,
and reduced yield)
nematodes (microscopic worms; produce root decay, abnormal growth, stunting, and
reduced yield)
- Non-living agents (abiotic) of disease are:
Environmental (cold, frost, hail injury, lightning damage, wind
damage, heat damage, drought, excessive moisture)
Mechanical (cultivation damage, weed eater or lawn mower damage, pruning damage,
misuse of guide ropes or wires)
Cultural ( improper planting, overwatering, overmulching)
Pesticide misuse (herbicide, fungicide, insecticide damage)
A susceptible plant, a causal agent, and a favorable environment
occurring over a period of time must be present for a disease to occur.
Disease management practices are:
Proper seed selection
Proper plant selection
Proper planting
Rotation
Weed management
Roguing or removal
Cultivation
Use of fungicides
Integrated plant health management involves knowing a plant's
cultural characteristics, selecting a plant adapted to the area, planting properly in the
right location, pruning, fertilization and watering, regularly monitoring for any plant
problems during the growing season, and taking action to solve the problem when it occurs
with the available cultural or biological practices, with the last option being fungicide
use.
Chemical fungicides, if applied, must be used according to the
label directions.
The fungicide needs to cover the plant thoroughly, both upper and lower
leaves and stems. Fungicides, therefore, are best applied with a compressed air sprayer.
Most fungicides are protectants and are used before the disease-causing agent arrives.
Fungicides are applied on a five to seven day schedule, depending upon environmental
conditions.
Some commonly used chemical fungicides are:
wettable sulfur
copper sulfate or tri-basic copper or fixed copper
Bordeaux mixture
captan
mancozeb
chorothalonil or Daconil 2787
Banner or Immunex or propiconazole
Bayleton or triadimefon
Cleary's 3336
Funginex or triforine
Subdue or metalaxyl
Chipco 26019 or iprodione
Biological fungicides are microorganisms that compete with or
destroy disease-causing agents. An example is the fungus, Gliocladium virens.
This beneficial fungus can be added to the soil to protect seeds or plants against fungal
disease-causing agents that cause stem or root rot.
Some common major diseases are:
Vegetables
- Tomato
early blight (distinctive target board spots; warm, humid
weather)
late blight (distinctive blotches with mold present; 60o-70o
,humid weather)
Fusarium wilt (fungus lives in the soil indefinitely)
blossom-end rot (calcium deficiency; moisture)
mosaic virus (yellowing; stunting; carried by insects)
- Pepper
bacterial leaf spot (distinctive spots; heavy defoliation)
blossom-end rot (calcium deficiency; moisture)
- Potato
early blight (distinctive target board spots; warm, humid
weather)
late blight (distinctive blotches with mold present; 60o - 70o,
humid weather)
viruses (yellowing; stunting; carried by insects)
black leg (rotting of roots and underground stems; sporadic locations; problem
early in the season)
- Beans
anthracnose (sunken, crater-like spots on the pod; occurs during
July-August)
rust (rusty areas on the leaves; occurs during July-August)
root rot (dark brown or reddish brown sunken areas on underground stems or roots;
yellowing and stunting of plant; reduced yield)
viruses (yellowing of leaves; stunting; reduction in yield)
- Cucurbits
bacterial wilt (wilting of ends of vines; bacteria carried by the
spotted and striped cucumber beetle)
anthracnose (sunken, crater-like spots on the fruit)
- Sweet Corn
corn smut (abnormal growth on the leaf, stalk, tassel, and ear)
viruses (yellowing; reddening; stunting; barren stalks)
- Onion
blast (white spotting; death of tubular leaves)
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Ornamentals
- Juniper
tip blight (death of needle tips, extending inward; black
fruiting bodies evident with hand lens)
- Azalea
leaf and bud gall (fleshy white growth on the leaves and floral
parts)
- Maple
Phyllosticta leaf spot (distinctive circular spots with black
fruiting bodies evident with a hand lens)
bull's eye spot (translucent spots; heavy defoliation in August)
- Oak
iron chlorosis (yellowed leaves with green veins; eventual
browning, death of leaves)
- Dogwood
anthracnose (spotting, blotching of leaves, death of stems and
branches; eventual death of entire tree)
- Elm
Dutch elm disease (wilting of leaves, defoliation on one side of
the tree, then the entire tree; eventually death; present underneath the bark, brown
streaking in the stem)
- Maple, Elm, Catalpa
Verticillium wilt (wilting on one side of tree, leaves hanging
on, then the entire tree; eventually death)
- Taxus (Yew)
Phytophthora root rot (wilting of above ground parts; rotting of
the crown and roots; brick red coloration under the bark)
wet feet (poor soil drainage; excessive moisture; decayed roots)
- Euonymous
crown gall (abnormal growth [swellings] on leaves, stems, crown
and roots)
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Turf
Dreschlera leaf spot (distinctive circular leaf spot with dark
purple border)
dollar spot (straw-colored areas on leaf blade; ends of the leaf
blade shrivel with a bow-tie effect; areas the size of silver dollars on bent grass; tan,
irregular areas on bluegrass)
Rhizoctonia brown patch ( brown circular patch with smoke ring of
the fungus on the perimeter of the patch)
spring and summer patch (irregular yellow and brown spots on the
leaf; crescent or circular, straw-colored ring with green center (frog-eye appearance);
blackened crown and root system )
snow mold ( white, matted turf in circular or irregular patterns )
fairy ring (deep green, circular or irregular ring with light green
or brown center)
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