Plant Diseases

Terms• Abiotic plant disease—caused by unfavorable growing

conditions.• Biotic plant disease —caused by plant pathogens.• Pathogen- an infectious, biological agent such as a

virus, bacteria or fungus that causes disease to its host.

Introduction

• Plant disease—a harmful change in the growth of a plant.

Abiotic Plant Diseases

• Not caused by pathogens• Caused by unfavorable growing conditions

– Improper light– Temperature extremes– Mechanical injury

• Abiotic diseases lead to stress that can increase susceptibility to damage from insects or pathogenic diseases

• Most abiotic plant diseases are avoidable

Biotic Plant Diseases

• Plant disease triangle• Causal agents:

– Fungi– Bacteria– Viruses, etc.

Causal Agents (Pathogens)• Fungi

– Most turf and ornamental disease are caused by fungi

– Do not produce their own food– Reproduce by forming spores

• Spread– Some by water– Some by wind– Mechanical/animal– Infected plants

Causal Agents (Pathogens)• Bacteria

– Single celled microbes– Enter plants through injuries or natural

openings • Spread

– Some by water– Some by wind– Some in soil– Mechanical/animal– Infected plants

Causal Agents (Viruses)• Viruses

– Smaller than cells---multiply in cells– Often spread– Infected plants must be removed---no cures

• Spread– Mechanical---e.g. contaminated equipment– Contact between plants– Sucking insects

Susceptible Host Plants

• Ornamental plants vary in disease resistance• Susceptibility increases with stress• Symptoms of decline

– Late flowering or leaf emergence– Smaller leaf size– Less shoot growth– Excessive water spouts

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Important Plant Diseases-TurfBrown Patch• Symptoms

– Irregular brown areas inches to feet wide– Grass leaves turn olive green then wilt, turn

light brown, and die.– Stems and crowns can also be infected

• Causes– All grasses susceptible– Caused by Rhizoctonia solani, reside in soil

Important Plant Diseases-TurfBrown Patch

• Favorable conditions– Excess thatch– High temperatures (75-90 degrees)

• Control• Aerate • Water to 4-6 inches depth when needed• Avoid excess nitrogen

Important Plant Diseases-TurfDollar Spot• Symptoms

– Small, bleached out or straw-colored spots– Seldom larger than 2 inches in close-mowed

turf– On taller grass (1-3 inches) spots can be 3-6

inches in diameter – Spots can merge together– Does not kill the grass plants

• Causes– Caused by fungi – indicates Nitrogen deficiency

Important Plant Diseases-TurfLeaf Spot

• Favorable conditions– Nitrogen deficiency– Warm days with cool nights – High humidity

• Control– Adequate nitrogen and potassium– Water in the morning let turf dry out before

dusk– Preventative fungicides– Curative fungicides

Important Plant Diseases-TurfLeaf Spot• Symptoms

– Start as small, dark purple or black spots– Become oval spots with buff centers and purple

margins• Causes

– Primary cause--- fungus Bipolaris sorokinianum

Important Plant Diseases-TurfLeaf Spot

• Favorable conditions– Dry periods alternating with cloudy, wet

weather

• Control– Avoid over-lush turf– Mow so that no more than 1/3 of the leaf blades

are cut– Resistant cultivars

Important Plant Diseases-TurfMelting-Out• Symptoms

– Circular to elongated, purplish or brownish spots

– General e.g. spread throughout the grass• Causes

– Melting-out is the summer phase of leaf spot– Brent grass, fescues, ryegrass and

burmudagrass---fungus Bipolaris sorokinianum

Important Plant Diseases-TurfMelting-Out

• Control– Reduce shade– Mow at recommended heights– Improve soil aeration

Important Plant Diseases-TurfNecrotic Ring Spot• Symptoms

– Begins as scattered light green patches 2-6 inches in diameter

– Patches enlarge and fade to tan, may become sunken . Streaks or crescents 1-3 or more feet in diameter

– Patches can increase over the years• Causes

– Fungus Leptosphaeria korrae – Formerly called fusarium blight

Important Plant Diseases-TurfNecrotic Ring Spot• Favorable conditions

– Cool or mild periods followed by dry, hot conditions

– Widespread disease of Kentucky and annual bluegrass that is intensively managed---follows stress

• Control– Good cultural practices: aeration, drainage,

balance of fertilizers, water in the morning– Resistant cultivars– Systemic fungicides

Important Plant Diseases-TurfPink Snow Mold• Symptoms

– Appears first as snow melts – Round, water-soaked spots 1-3 inches in

diameter

• Causes– Fungi Fusarium nivale or Microdochium nivale– These fungi inactive in warm dry conditions

Important Plant Diseases-TurfPink Snow Mold

• Favorable conditions– High humidity and temperatures around 40

degrees• Control

– Adequate, balanced fertilizer– Reduce shade– Aeration– Drainage

Important Plant Diseases-TurfPowdery Mildew on Turf• Symptoms

– Thin, white powdery coating– Lives primarily on outer surfaces

• Causes– Fungi Fusarium nivale or Microdochium nivale– These fungi inactive in warm dry conditions

Important Plant Diseases-Turf Powdery Mildew on Turf• Favorable conditions

– Cool (55-70 degrees) cloudy weather– Important disease on Kentucky bluegrasses

during extended periods of low light, especially in areas with poor air circulation

• Control– Include shade tolerant cultivars in seed

mixtures– Prune trees and shrubs to permit light to reach

turf– Deep, infrequent watering

Important Plant Diseases-TurfRust• Symptoms

– Begins as light green or yellow flecks becomes– Reddish brown to orange (spores)

• Causes– Puccinia species

Important Plant Diseases-TurfRust

• Favorable conditions– Cool to warm, moist weather– Dew or condensed moisture on plants for 12 hours

• Control– Maintain healthy growth with fertilizer and irrigation– Remove infected portions of leaves with weekly

mowing– Curative fungicides

Important Plant Diseases-TurfSlime Molds

• Symptoms– Slimy masses 1 inch to 2 feet in diameter– Shades of grays and yellow– Easily removable

• Causes– Non parasitic on turf---feed on organic matter

Important Plant Diseases-TurfSlime Molds

• Favorable conditions– Warm, moist weather and high humidity– Spread by winds, water, equipment

• Control– Will disappear but can rake or hose-off

Important Plant Diseases-TurfStripe Smut• Symptoms

– Long, black stripes (pustules)– Grass leaves curl, become shredded and die– From a distance appears clumpy and patchy– Plants die when hot weather occurs

• Causes– Fungus Ustilago striformis often contaminates turf

seed– Can remain dormant for 3 years in soil

Important Plant Diseases-Turf Stripe Smut

• Favorable conditions– Moderate temperatures (50-65 degrees)– Prevalent in spring and fall– Occurs on other grasses but more common on

bluegrasses

• Control– Resistant cultivars of bluegrass– Seed treatment with fungicides– Dystemic curative fungicides

Important Plant Diseases-TurfFairy Ring• Symptoms

– Circular or semi-circular dark green band on turf (released nitrogen from breakdown of organic matter in soil)

– Mushrooms will often develop around edge after wet weather

• Causes– Several fungi– Occurs in areas with high levels of organic

material e.g. thatch

Important Plant Diseases-Turf Fairy Ring

• Favorable conditions– Presence of high concentrations of organic matter

e.g. tree stumps, roots, construction lumber

• Control– Remove excess organic matter– Antagonist strains for heavy infestations

Important Plant Diseases-TurfPythium Blight

• Symptoms– Large areas can die in 24-48 hours when

conditions favoring pythium blight occur– Round to irregular, dark and water-soaked,

greasy or slimy– Sunken patches 6-12 inches wide– Reddish brown areas fade to tan and die

• Causes– Several species of Pythium fungi

Important Plant Diseases-TurfPythium Blight

• Favorable conditions– Temperatures 80-90 degrees with 90% RH (relative

humidity) and warm nights (.70 degrees) for most Pythium but some thrive in cool weather

– Waterlogged soils with heavy thatch– Excess nitrogen

• Control– Improve drainage– Systemic fungicide prior to hot, humid weather

Important Plant Diseases-TurfSummer Patch• Symptoms

– Bluish-green patches become wilted then die– Patches become circular or crescent-shaped.

• Causes– Several species of Pythium fungi

Important Plant Diseases-TurfSummer Patch

• Favorable conditions– Usually occurs when a wet period follows hot,

dry weather – Frequent or excessive watering

• Control– Before planting new turf remove stumps and

construction material– Avoid mowing or walking on wet turf– Apply systemic fungicides when temperatures

reach 70 degrees

Important Plant Diseases-Ornamentals Anthracnose• Symptoms

– Leaf buds do not emerge in spring– Defoliation from progressive death of mature leaf

tissue along leaf midrib or veins• Causes

– Anthracnose in many common tree species– Transitional disease, intermediate between a leaf

and stem disease caused by fungi (genus Gnomonia)

Important Plant Diseases-OrnamentalsAnthracnose

• Favorable conditions– Disease begins in late winter or early spring during

periods of warm weather– Splashing of spores during warm. Wet weather spreads

the disease

• Control– Fungicide application on leaves and buds during

emergence– Cultural practices including avoiding planting species

that are especially susceptible, maintaining adequate irrigation and fertilization

Important Plant Diseases-Ornamentals Cytospora Canker• Symptoms

– Affects poplar and willow– Circular or irregular cankers first appear as

brown, sunken areas on younger trees– Cankers often start at wound openings

• Causes– Fungus Cytospora chrysosperma

Important Plant Diseases-OrnamentalsCytospora Canker• Favorable conditions

– Fungus is opportunistic, infects weakened or wounded tissues

– Spread by splashing rain, wind• Control

– Remove dead and dying branches– Clean pruning tools– Fertilize– Deep water (10-12 inches)– No fungicides are available

Important Plant Diseases-Ornamentals Dutch Elm Disease• Symptoms

– Leaves brown and curl---usually drop early– Large elms may die over 2 or more years

• Causes– Fungus Ceratocysitus ulmi – Infects elms and closely related plants– Transmitted by:

• Elm bark beetles • Root grafts with adjacent trees• Pruning tools

Important Plant Diseases-OrnamentalsDutch Elm Disease

• Control– Plant cultivars with superior resistance (Siberian and

Chinese) – Water and fertilize– Spray with appropriate insecticide in late winter/early

spring before the buds swell (requires special training and equipment)

– Systemic injections– Halt transmission through root grafts (see oak wilt

procedures)

Important Plant Diseases-Ornamentals Fire Blight• Symptoms

– Infected flowers first appear water-soaked– Leaves and stems then appear brown or black as though

scorched by fire

• Causes/favorable conditions– Bacterium Erwinia amylovora overwinters in plant

tissues and emerges when temps reach 65 degrees– Spread by insects, wind, water, hail

Important Plant Diseases-Ornamentals Fire Blight

• Control– Plant resistant varieties of apple, crabapple, and pear

where available– Good cultural practices (fertility, watering, drainage)– Prune in dormant season and disinfect tools between

cuts with 70% rubbing alcohol solution– 6 inches of mulch around susceptible tress reduces

chance of infection

Important Plant Diseases-Ornamentals Fungus Leaf Spot• Symptoms

– Difficult to diagnose from anthracnose– Variable spot colors and shapes

• Causes– Wide variety of fungi

Important Plant Diseases-OrnamentalsFungus Leaf Spot

• Favorable conditions– Cool weather, light rains, high humidity– Crowded plantings

• Control– Preventive measures usually not needed– Good culture– Remove fallen leaves and branches– Proper spacing

Important Plant Diseases-Ornamentals Juniper Twig Blight

• Symptoms– In spring tips of twigs and branches turn light green

then brown– Usually affects branches less than 1/3 inch diameter– Spores emerge during wet weather in spring and early

summer• Causes

– Fungus Phomopsis juniperova • Controls

– Cultural practices– Provide ventilation in shaded areas– Fungicides every 2 weeks early spring to fall

Important Plant Diseases-OrnamentalsOak Wilt• Causes

– Fungus Ceratocytis fagacearum– Spread by

• beetles from infected plants on their bodies f • Root grafts

Important Plant Diseases-OrnamentalsOak Wilt• Symptoms

– Impairs water conduction from roots to branches and leaves

– Trees wilt from the top down and leaves wilt from the tips to the base

– Fallen leaves have brown tips and green along the main vein

Important Plant Diseases-OrnamentalsOak Wilt• Favorable conditions• Control

– Avoid pruning wounding) from April 15 to July 1st if wound occurs during that time cover with tree wound dressing or latex paint---this is the critical period of beetle movement

– Isolate infected trees---preventing root grafts---trenching machine or vibratory plow (cut to 4.5-5.0 feet deep)

– Red and black oaks most susceptible– Reduce spore quantity---cover pruned wood, debark

branches that are 3 inches and over (fungus mats below bark)

Important Plant Diseases-Ornamentals Rust• Symptoms

– Bright orange, yellow, chocolate brown or black pustules

• Causes– Several fungi

• Control– Alternate hosts e.g. junipers and cedars one year,---

apple, crabapple, hawthorne, mountain ash the next– Disease free plants– Prune infested tissue

Important Plant Diseases-Ornamentals Scab• Symptoms

– Apple scab develops as circular lesions during leaf expansion in spring. Color progresses from dark green to black

– Fruit can be infected at any time during development

• Cause– Fungus Fusicladium saliciperdum

Important Plant Diseases-OrnamentalsScab

• Favorable conditions– Apple scab overwinters on leaf litter– Wet weather during bud break spreads spores– Cool, wet conditions

• Control– Plant resistant varieties when available– Prune out diseased tissue– 3-4 applications of fungicides at 10 day intervals

starting just before bud break

Important Plant Diseases-Ornamentals Sphaeropsis Blight (Diplodia Tip Blight)

• Symptoms– On new, emerging needles browning occurs near the

base of the needle and progress to the tip. Ooze forms– Infection spread to twigs and die back to the next whorl– Begins with lower branches and progresses up the tree

over successive years– In severe cases the tree dies

• Causes– Fungus Sphaeropsis sainea

Important Plant Diseases-OrnamentalsSphaeropsis Blight (Diplodia Tip Blight)

• Control– Removal of infected branches– Balanced fertilizers in spring– Adequate watering

Important Plant Diseases-Ornamentals Phytopthora• Symptoms

– Variable, may be slow decline or rapid death during dry weather

– Root and crown root– Symptoms may include chlorosis, sparse

foliage, reduced sized foliage in trees and shrubs

• Causes– Fungi from the genus Phytopthora soilborne

Important Plant Diseases-OrnamentalsPhytopthora

• Favorable conditions– High occurrence in areas with high soil moisture

and low fertility

• Control– Plant in well drained soil– Allow plants to dry out between irrigation– Systemic fungicides

Important Plant Diseases-Ornamentals Verticillium• Symptoms

– Especially common on maples but can occur on several landscape trees and shrubs

– Usually the leaves on the entire plant or all the leaves on one side of the plant suddenly wilt

– Dieback– May progress through several years

• Causes– Fungi from genus Verticillium– Soilborne and spread by contamination

Important Plant Diseases-Ornamentals Verticillium• Control

– Plant disease-free nursery stock– Avoid wounding and contaminated tools– Avoid excess nitrogen– Replace infected plant with resistant varieties

or immune species

Important Plant Diseases-OrnamentalsPowdery Mildew

• Symptoms– White, powdery spots that grow to cover the entire

surface– Young plants most vulnerable, especially in shade– Dwarfing, distortion, yellowing.– Leaves may drop

• Causes– Over 1000 species of fungi primarily Microsphaera and

Odium

Important Plant Diseases-OrnamentalsPowdery Mildew

• Favorable conditions– Shade– Areas with poor air circulation– Easily spread by splashing water

• Control– Prune affect parts– Select plants for resistance– Because powdery mildew are usually external they can

be controlled ---curative fungicides