PLANT DISEASE CONCEPTS Guy J. Mussey Virginia Cooperative Extension
DEFINITION Disease: any deviation in normal functioning of a plant caused by some type of persistent agent
Damage Disease Disorder
Persistent agent ------------ Continuum -------------
More definitions Pathogen: any agent that causes disease 1. Biotic; fungus, bacteria, etc. 2. Abiotic; air pollution, environmental Symptom: characteristic signature of the pathogen and host resulting from the interaction of specific pathogens and hosts
And more definitions Signs - the actual causal agent of the disease such as fruiting bodies of fungal diseases Host specificity - pathogens that infect a limited number of related or unrelated hosts
Three Categories of Disease 1. Abiotic Plant Diseases 2. Biotic Plant Diseases 3. Decline Plant Diseases (Trees & Shrubs)
Comparison of Differences Symptoms Signs Host Specificity Spatial Distribution
Abiotic Diseases Symptoms - usually uniform in symptom expression and general distribution Signs - none Host Specificity - similar symptoms on two or more unrelated hosts Spatial Distribution - usually nonrandom
Biotic Diseases Symptoms - produced on specific plant parts, progressive invasion of tissues Signs - fruiting bodies and other structures Host Specificity - usually host-specific on related hosts Spatial Distribution - usually show a random but clumping distribution pattern
Decline Diseases Symptoms - progression of symptoms on individual plants & between plants, reduction of growth Signs - fruiting bodies and other structures Host Specificity - usually host-specific on related hosts Spatial Distribution - random distribution within a given location
Abiotic Agents 1. Nutrient deficiency and toxicity 2. Salt 3. Air pollution
Nutrient Deficiencies & Toxicities Imbalance of nutrients such as nitrogen (N), phosphorus (P), potassium (K), calcium (C), magnesium (Mg), sulfur (S) and micronutrients Symptoms range from interveinal chlorosis to discoloration of leaves to stunted growth
Mobile Nutrients Nitrogen Phosphorus Potassium Magnesium Zinc
Iron Chlorosis
Magnesium Deficiency
Nitrogen Deficiency
Salt 1. Chloride accumulation in leaves and growing points 2. Changes in soil structure due to sodium cations 3. Decrease in moisture availability
Salt Damage on Taxus
Biotic Agents Nematodes Viruses Bacteria Mycoplasmas Fungi
Nematodes Plant parasitic nematodes have spear-like mouthparts called a stylet. Stylet used to pierce root cells to suck cell contents Two groups of plant parasitic nematodes ectoparasites - feed outside root endoparasites - feed inside root
Nematode
Mode of Pathogenicity Direct injury due to feeding Produce wounds for infection Vector viruses Parasitize mycorrhizal fungi Contributing factor in decline diseases
Ectoparasitic Nematode
Symptoms of Nematode Disease Similar to nutrient deficiencies growth reduction sparse yellow foliage reduced size of foliage premature leaf drop abnormal wilting during hot, dry periods
Root Knot Nematodes endoparasites over 1,500 plant species parasitized produces galls on roots
Control of Nematodes Chemical nematocides soil sterilants Cultural crop rotation nonhost cover crops resistant varieties fallow periods between crops
Viruses Infectious agents composed of nucleic acid and protein obligate parasites seldom produce plant mortality enter host through slight injuries or injected by insect vectors naming based on the symptoms they produce, ex. Tobacco Mosaic Virus
Examples of Viral Diseases tobacco mosaic virus (TMV) tobacco ringspot virus (TRV) elm mosaic virus (EMV) apple mosaic virus (AMV) cherry leafroll virus (CLV) potato virus Y (PVY) oak mosaic virus (OMV)
Symptoms of Viral Diseases flower color streaks foliage ring spots and flecks necrotic lesions mosaic or mottling patterns veinal or marginal chlorosis line patterns curling and dwarfing witches brooms and rosettes
Tobacco Mosaic Virus on Tomato
Virus Disease - Veinal Chlorosis
Control of Viral Diseases Chemical none Cultural discard diseased plants prevention spray insecticides for vector virus free plants produced by tissue culture
Bacteria Produce disease by enzymatic maceration of cell walls, secretion of toxins, and interference with cell division and differentiation. Cannot actively penetrate and infect plants. Enter through wounds, natural openings and insect vectors.
Symptoms of Bacterial Disease Water soaking of leaves and stems followed by necrosis infection of vascular tissue produces wilting crown galls slime flux in trees
Water Soaking Symptom
Examples of Bacterial Diseases bacterial cankers leaf blights crown galls shoot blights, ie., fire blight wetwood vascular wilts
Crown Gall
Fire Blight
Bacterial Wetwood/Slimeflux
Control of Bacterial Diseases Chemical antibiotic sprays insecticidal sprays for insect vectors Cultural elimination of infected plant parts, ie., pruning quarantines soil aeration
Mycoplasmas prokaryotic microorganisms - no cell wall, very little known chlorosis of foliage (yellows diseases) aster yellows elm phloem necrosis produces clusters of short stems and chlorotic foliage (witches brooms) leafhoppers - vectors
Characteristics of Fungi mycelial growth form reproduce by spores Basic Requirements Lack of chlorophyll specific nutrient requirements oxygen free water temperature range: 32 0 to 104 0 F
Fungi Pathogen All three factors interact to produce disease. Manipulate any one factor to control disease. Plant Environment
Classification Saprophytes - utilize dead substrates Obligate parasites - parasitize only living hosts Facultative parasites - attack living hosts that are weakened by other stress factors
Types of Fungal Diseases Foliage Diseases Rust Diseases Canker Diseases Vascular Wilt Diseases Root Rot Diseases
Foliage Diseases wet, cool weather usually infect lower foliage most are both parasitic and saprophytic symptoms similar to other foliage disorders
Examples of Foliage Diseases Septoria leaf spot Anthracnose Powdery mildew Leaf blister Needle blight
Septoria Leaf Spot
Discula Anthracnose on Dogwood
Powdery Mildew
Oak Leaf Blister
Phomopsis Tip Blight
Control of Foliage Diseases Preventive fungicides Selection & breeding Site selection & manipulation Species selection
Rust Diseases Obligate parasites Require two different hosts Can infect foliage or succulent shoots Temperature & moisture sensitive
Examples of Rust Diseases Cedar Apple Rust White Pine Blister Rust
Cedar Apple Rust
White Pine Blister Rust
Control of Rust Diseases Eradication of alternate host Disease resistant varieties Fungicides Cultural controls
Canker Diseases Facultative parasites Infect stems and branches Annual cankers - invade wounds, usually last one year Perennial cankers - target cankers Diffuse cankers - produce toxins
Three Types of Cankers: Annual cankers are quickly overcome by host defenses Perennial target cankers show cycles of infection, host defense, etc. Diffuse cankers are characterized by poor to no host defense
Annual Cankers
Fusarium Canker
Perennial Target Cankers
Nectria Canker
Eutypella Canker
Diffuse Cankers
Chestnut Blight
Cytospora Canker
Hypoxylon Canker
Seiridium Canker
Control of Canker Diseases Keep trees healthy Sanitation - prune out limbs with canker
Vascular Wilt Diseases Organisms invade vessels and disrupt water movement causing wilting Invade plant through wound in stem or through roots Darkened streaks inside stems often present Rapid drooping of green leaves during hot weather
Examples of Wilt Diseases Dutch Elm Disease Oak wilt Verticillium wilt
Dutch Elm Disease
Oak Wilt
Verticillium Wilt on Maple
Control of Wilt Diseases Sanitation - remove and dispose of all diseased plants Do not replant in infected sight Control of insect vectors when necessary
Root Rot Diseases Two types: Structural root rots Absorptive (feeder) root rots Symptoms structural root rots Fruiting bodies at base of tree No obvious symptoms in canopy Trees easily windthrown
Examples of Root Rot Diseases Armillaria - structural Ganaderma - structural Phytophthora Pythium Fusarium
Ganaderma Root Rot
Ganaderma Root Rot on Willow
Phytophthora of Rhododendron
Pithium
Controls of Root Rot Diseases Maintain healthy plants Soil drenches for some types Fumigation in greenhouses and liner beds
Non Disease Fungal Problems Artillery Fungus (Sphaerobolus sp.) Dog Vomit Fungus
Artillery Fungus
Artillery Fungus Colonize dung or other organic matter such as wood mulch, wood benches and wood sheds Not pathogenic to plants or animals Propel sticky spore packages several feet towards light Usually a problem in Spring or Fall
Dog Vomit Fungus (Fuligo septica)
Dog Vomit Fungus Not a fungus actually a slime mold Not harmful to plants or animals Can appear overnight Colonizes decaying wood No controls necessary
Decline Disease Caused by the interaction of a number of interchangible, specifically ordered abiotic and biotic factors to produce a gradual general deterioration, often ending in the death of trees. Category not accepted by all forest pathologists
Complex declines involve three sets of factors: Predisposing factors Inciting factors Contributing factors
Factors of Decline Disease Predisposing factors Static, nonchanging climate, soil type, site, genetic potential result in permanent stress Inciting factors short in duration biological or physical insect defoliators, frost, drought, salt
Factors of Decline Disease (cont) Contributing factors produce noticeable symptoms on weakened host bark beetles, canker fungi, root rot, etc. persistent, often blamed for problem better understood as indicators of weakened host generally finishes off host
Death Spiral
An Example of a Decline Disease Maple Decline Predisposing factors roadside and urban conditions of soil compaction impeded drainage poor soil aeration heat air pollution
Maple Decline (cont) Inciting factors salt damage from deicing materials root damage from urban construction defoliating insects Contributing factors Armillariella mellea root rot Verticillium wilt