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Plant Pathology Diagnosing Problems

Lisa Sanderson

Extension Agent, ANR

Henrico County

Diagnosing Plant Diseases

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What You Need to Know

Biotic and abiotic causes of plant diseases

Organism responsible for problem

Conditions

Diagnosing plant damage

Signs and Symptoms

Control

Resource Materials

What is a Plant Disease?

May be any disturbance that prevents the normal development and reduces its economic or aesthetic value

Problems

Biotic – are caused by Living Factors

Abiotic – are caused by Non-living factors

Problem Causing Organisms

Biotic Fungi

Bacteria

Virus

Nematodes

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Fungi

Filamentous (threadlike) organism

Can grow inside or outside plant tissues

Reproduce, spread, persist via spores

Common Symptoms

Leaf spots and blights

Fruit, stem, root wood and seedling rots

Cankers

Vascular wilts

Galls

Mildew and rust diseases

Bacteria

Minute, one celled organism

Large masses visible as ooze or slime

Need a microscope to see

Can behave similarly to viruses

Common Symptoms

Leaf spots and blights

Stem and fruit rot

Cankers

Galls

Vascular wilt

Viruses

Infectious modules (clumps)

Take over a plants metabolism to reproduce

Symptoms

Poor growth

Mottling or mosaic

Leaf crinkling

Nematodes Microscopic, none-segmented round worms

Treated separate from insects and mites (accident of history)

Many live in soil and feed on roots, while some live inside leaves and shoots

Symptoms

Poor root development

Root galls

Swollen root tips

Abnormal root branching

Yellowing of foliage

Conditions

Too wet

Too dry

Humidity

Temperature

Pathogen

Host

Other

Disease Cycle

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The Disease Doughnut Symptoms vs. Signs

Symptoms – plant’s response to stress

Wilting, yellowing, die-back, rings, patches, etc.

Sign – evidence of the causal agent

Fruiting structures, people, packaging material, mold, etc.

Washington State University Extension

Symptoms

Color Changes-Chlorosis Chlorosis:

Yellowing of normal green tissue; PATTERN of discoloration helpful; Many causes

General

yellowing of leaf or plant; nutrient deficiency, root problems, nematodes

Interveinal

yellowing of leaf tissue between veins; poor root function, root rot, nematodes, nutrient deficiencies, improper pH, chemical injury, virus

Color Changes- Chlorosis

Along the vein

Yellow along the vein; virus, some herbicides

Marginal

yellowing of leaf edges; chemical injury, nutrient toxicity

Mosaic, Mottle

irregular light and dark green areas on leaves; distinct (mosaic) less distinct (mottle); commonly viruses, genetic abnormalities, nutrient deficiency (mottle)

Color Changes - Chlorosis

Ring spot

circular area of chlorosis and necrosis with green center; virus, cold water (African violet)

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Color Changes - I

Other

Breaking: Abnormal streaks of different color in colored areas (esp. flowers); virus, genetic

Purple, reddening: Abnormal purple or red color in normally green tissues; phosphorus, boron deficiency, some herbicides, other

Bronzing: gold or copper metallic appearance; insects, mites, cold injury

Other

Browning: turns brown, brittle and dry, tissue death; Many causes

Russeting: superficial roughening of skin due to cork formation; some fungus (powdery mildew); frost, some chemicals

Necrosis

Spot: necrotic areas on leaves, superficial lesions, round, angular, irregular, concentric rings or surrounded by purple ring or chlorotic halo; commonly fungus, bacteria, abiotic (paraquat drift), uncommonly virus or nematodes

Blight: general killing of plant parts; Many

Necrosis continued Blotch

Large, superficial discolored areas of irregular shapes- leaves, stems fruits, stems; fungi, bacteria, chemical injury, sun scald

Scorch, Marginal Necrosis

Burning of leaf margins; drought, excess salt, root problems, cankers, vascular fungi, bacteria

Rot

tissues discolor, disintegrate (decay) & soften esp. wood rot (fungal) & root rot (usually fungal); fungi, bacteria

Necrosis

Canker:

Necrosis area, surface may be smooth or rough, sunken with margins raised, raised margins, target shaped; usually fungi, some bacteria

Necrosis

Dampening off

seed, seedling rot or canker like girdling seedlings causes plants at ground-line to fall over and rot; usually fungi, insects, soil conditions

Shot hole (shotgun)

dead areas fall away leaving holes in leaves; fungi, bacteria, virus, insect feeding, hail

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Necrosis continued

Dieback

twigs, limbs, shoots dieback from tip; see blight

Anthracnose

small blister on the lesion surface; fungi

Water soak

translucent cells fill with water; bacteria, fungi, frost

Miscellaneous

Dwarfing/Stunting

part or all plant does not reach normal size

Gall, tumor, knot

localized enlargement of plant parts (esp. root gall, crown gall, leaf gall); some fungi, bacteria, virus, nematodes, many insects, some mites

Witches broom

dense, broom like clustering of branches; phytoplasms, fungi, some mites

Miscellaneous.

Leaf curl

irregular growth; virus, some fungi, herbicides, ethylene, aphids

Wilt

parts limp from lack of water; drought, root rot, root damage by nematodes, other root problems, vascular pathogens – fungi, bacteria

Leaf drop, abscission

dropping off of leaves, fruit, flowers; leaf spot pathogens, root pathogens, various abiotic conditions

Miscellaneous continued

Epinasty

bending down of leaves, abnormal petiole growth; vascular wilt, ethylene, some herbicides

Gummosis

production of thick, gummy liquid in response to injury or disease; insect , fungal, bacterial infection

Signs

Mildews

gray, whitish growth of fungus

powdery (most common)

downy

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Molds

Many colors, commonly gray, white, black, blue or green, similar to mildew but from a different group of fungi

Sooty Mold

black fungal growth, sooty mold from excretions of sucking insects- aphids, whiteflies, scale

Rust

Spore pustules may be yellow, red, brown or black

Smut

spore masses, brown or black and powdery

Mushrooms

Large fruiting bodies, few are pathogens

Conk

large, woody shelf like fruiting body

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Bacterial Ooze or Slime

Drops of sap containing bacteria

Bacterial wetwood/slime flux

What Problems will you see when you talk to home owners?

Problems in Ornamentals Symptoms, Signs and Pictures

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Rose Black Spot • Symptoms

• Small, Round spots 1/16”

to ½”diameter

• Upper sides of leaves

• Adjacent tissue turns

yellow

• Eventually whole leaf turns

yellow and prematurely

falls

• Yes, Even Knockouts!

Rose Rosette Disease

Elongation of

New Shoots

Witches

Broom

Red Pigmentation

Distorted Flowers Hyperprickliness

Boxwood: Phytophthora Root Rot

•Symptoms

•Poor growth and off color

foliage

•Leaves light green turn

yellow, bronze, straw-

colored

•Leaves turn upwards,

lateral leaf margins roll

inward

•May appear on a few

branches to entire plant

ACES.edu

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Boxwood Blight

Defoliation – usually starting at the base of the plant

Large, diffuse, dark brown leaf spots

Blackened twigs and stems

English Boxwood Decline

Foliage discolored and

dying back

Root balls: Fibrous roots due

to decline; Plant in middle

healthy

Boxwood: Nematodes

•Root damage caused

by

•Ring, lesion, spiral

nematode

•Feeding damages

roots

•Fungus can enter

boxwood through

damage

Rootknot nematodes

Meloidogyne spp.

Rootknot nematodes

Meloidogyne spp.

Boxwood: Foliar and Stem Diseases

Dieback from

Volutella Blight

Fruiting bodies of

Macrophoma Leaf

Spot

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Powdery Mildew

Powdery Mildew on

Roses

Powdery Mildew on Crape

Myrtle

Dogwood: Spot Anthracnose

•Symptoms

•Circular lesions

with purple borders

Spot Anthracnose on Leaves

Spot Anthracnose on bracts of

flowering dogwood

Dogwood: Discula destructiva

•Symptoms

•Fungus not only

causes leaf and

bracet damage

but also dieback

•Margins of

spots are

purplish in color

•Lower branches

die first

Trunk Canker

Lower Branch dieback

Dogwood: Septoria Leaf Spot

•Angular, dark spots

with purplish margins,

bordered by leaf veins

•Late season disease,

appears early July in

VA, become severe in

late summer

•Can be confused wit

Discula, but spots are

more uniform

Crown Gall

•Common in Euonymus

•Other species: cypress,

hibiscus, lilac,

flowering peach, privet,

viburnum, and willow

are susceptible

•Positioned at wound

site

•Interfere with water

and nutrient transfer

Fire Blight in Ornamentals

Crabapple

Bradford Pear

•Blossoms

and leaves

wilt suddenly

and turn dark

brown to

black, shrivel

and die

•Blackened

petiole stay

attached

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Trees: Shade and Fruit

Sudden Oak Death (Phytophthora ramorum)

New Disease in US, prompted USDA to quarantine counties in CA and OR

First described: Germany and Netherlands 2001

Was diagnosed in Virginia 2003

Hosts

Map: USDA- Forest Service

2011

July 21, 2011– 2 black walnut in Chesterfield with TCD

And VDACS imposes temporary quarantine

Quarantine prohibits the movement of all walnut plants, parts, logs, stumps, firewood, roots, branches, mulch and chips out of quarantined area

Thousand Canker Disease

Chesterfield, Fairfax, Goochland, Hanover, King and Queen, King William, New Kent, Powhatan and Prince William Cos., and cities of Colonial Heights, Fairfax, Falls Church, Manassas, Manassas Park and Richmond

Confirmed TCD

States Confirmed and Quarantined Thousand Cankers Disease

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TCD positive locations 8/12/11

Disease cycle & Symptoms

Localized death of phloem

Inoculation by walnut twig beetle

Yellowing and thinning of crown

Successively larger stems affected

Death

Cumulative effect

Within 3 years of crown symptoms

Thousand canker disease Black walnut and

Butternut

Lethal to black walnut

Arizona, English, California

Variable susceptibility

Circa 2003 (???)

Probably much older

Fungus-insect complex

Geosmithia morbida

Pityophthorous juglandis

(walnut twig beetle)

Pathways

Raw wood

Logs, packing material, firewood

Nursery stock

Natural dissemination

Cankers caused by Geosmitha morbida

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What can be done?

No effective insecticides

No effective fungicides

Quarantines only minimally effective

Sooty Mold •Fungus

•Most grow on

excrement on sucking

insect, aphids, scales,

mealy bugs and

whiteflies

•Does not penetrate

leaf or bark tissues

•Indirectly a problem

to plants as excludes

sunlight and therefore

interferes with

photosynthesis

Sooty mold on Walnut

Sooty Mold: Hardwood or Conifers

Sooty mold on beech Sooty mold on White pine

Verticillium Wilt on Shade Trees •Fungus enters through the

roots

•Spreads upwards

•Interferes with water

movement and other plant

functions

•Problems accentuated by

drought, inadequate

nutrition, poor drainage,

other conditions that

decrease tree vigor

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Problems with fruits and vegetables: Symptoms and pictures

Fire Blight

•Ornamentals and fruits

•Infected fruit becomes

leathery

•Cankers form on limbs,

shrunken, dark brown to

purplish in color

•Dark streaking of the

wood

•Shepherd’s crook

Cucurbits: Powdery Mildew

Powdery mildew on a

pumpkin leaf

•White powdery mass

on leaves, petioles,

young stems

•Can cover an entire leaf

in a few days

•Bad infections leaves

turn yellow to brown to

shrivel

•Reduce both number

and size of fruit

Cucurbits Include

Pumpkins

Cantaloupe

Squash

Cucumbers

Pickling

Slicers

Burpless

Corn Smut

•Obvious on ears, but may also

form on leaves, tassels, and

stalks,

•Galls are enclosed by a

silvery membrane

•Mature, membrane breaks and

black powdery spores are

exposed

•Galls can be several inches in

diameter

Bacterial Spot of Pepper

•Plants infected in seed bed have

small, irregular black or water-

soaked spots on edges of 1st leaves

•Older plants; small, pale green,

water-soaked lesions

•Spots surrounded by yellow halo

•Center may dry and tear, giving a

gunshot appearance

•Leaves may drop off while green

•Young seedling may lose topmost

leaves

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Blossom End Rot: Tomato

•Brown water soaked

end rot

•Discolored area

increases in size

•Shrunken, concave,

black, leathery spot

on blossom end

Tomato: Early Blight

•Dampening off in seedbed

•Usually small, irregular

brown spots form on older,

lower leaves

•Spots may enlarge to up to

½ inch

•Concentric rings or ridges

form a target pattern

•Can appear later in season

Tomato Spotted Wilt

No Cure

Thrips vector the Virus

Infected plant need to be removed

Thrips need to be controlled

Broccoli : Wire Stem

•Dark water soaked lesion

•Grow poorly, stunted

•Eventually die

•Those that do survive

perform poorly and rarely

have marketable head

Cabbage: Bottom Rot

•Lower side of the head

•Lesions sunken, black

sharply elliptical with

long side of lesion

parallel to mid rib

•Black decay at base of

head

Diagnosing Abiotic Disorders

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What are Abiotic Disorders?

Normal function of a plant that is affected by non-living stresses.

Environmental/temperature extremes

Soil properties/pH

Chemical or salt injury

Mechanical damage

What are Abiotic Disorders?

aka Physiological Disorders

Injury or symptoms we see is ultimately due to the cumulative effects of the causal factors on the physiological processes needed for plant growth and development

Cumulative

Subtle

Can be difficult to diagnose

Biotic Disease vs. Abiotic Disease

How do you tell the difference?

Ask a lot of questions

Observe environmental clues

Take note of site history

Biotic Diseases

Usually scattered or randomly distributed

Abiotic Diseases

Typically appear in patterns or display uniformity

Plant Diseases Top 10 Abiotic Factors

• Nutrient Deficiencies

• Planting Depth

• Girdling Roots

• Mechanical Damage

• Packaging Material

• Excessive Irrigation

• Summer Scorch

• Herbicide Damage

• Winter Damage

• Environmental Pollutants

Nutrient Deficiencies

Symptoms vary depending on nutrient

Older leaves vs. newer leaves

Interveinal chlorosis, leaf coloration, marginal burning, etc.

Causes:

High pH (alkaline), low pH (acidic), compacted soils, saturated soils, excessively dry soils

Identifying

No evidence of insect, disease signs or specific symptoms

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Symptoms of Nutrient Deficiencies

Iron chlorosis on maple blueberry

Manganese deficiency on maple

Younger leaves

Older Leaves

Potassium deficiency

Older leaves

Older Leaves

Phosphorus deficiency

Planting Depth

Symptoms:

No visible root flare at soil surface

The trunk descends straight into the soil with no flare

Leaves and annual growth are smaller than normal

Early fall color and leaf drop

Dieback in the crown of the tree

Sparse amount of foliage

Causes

Repotted nursery stock

Depth of planting hole is too deep

Buried root flare on B&B trees

Girdling Roots

Symptoms:

May be visible at the surface

Excavation of soil at trunk base may reveal girdling roots

Leaves smaller than normal

Early fall color and leaf drop

Dieback in the crown of the tree

Sparse amount of foliage

Causes:

Containers

Planting hold width to narrow

Root bound nursery stock

Mechanical Damage

Symptoms:

Slow growth rate

Dieback above damage

Sunken or sloughing bark

Signs of visible damage

Causes

Improper pruning techniques

Lawn mower/string trimmer damage

Animals

Heavy equipment

Vandalism

Packaging Material

Symptoms:

Slow growth rate

Gradual decline in plant health

Visible signs of packaging material

Death

Causes:

Nursery tags, wire baskets, twin, burlap, and containers in ground or plants rooted through pot to ground

Excessive Irrigation

Symptoms

Poor root establishment

Gradual decline of plant

Wilting

Iron chlorosis

Root rot problems

Weak or stunted growth

Can be mistaken for drought stress

Causes

Excessive irrigation

Compacted/clay soils

Poor drainage

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Drought and Heat

Symptoms

Mortality

Wilting

Curling of leaves and conifer leaders

Reduced growth (even before visible symptoms)

Imposed dormancy resulting in possible out-of-season flowering

Sunburned leaves

Scenario

Moderate stress leads to stomata closing

Stomata closing leads to reduced photosynthesis and reduced transpiration (less cooling effect)

Reduced transpiration can lead to leaf scorch

Reduced photosynthesis leads to reduced growth

Summer Scorch

Symptoms

Browning of leaf margins

Reduced vigor late season

Damage increases with heat

May cause brown inter-veinal spotting

Target rings

Causes

Dry, hot summer winds

Found more on larger leaves

Poorly watered trees

Herbicide Damage

Symptoms

Curling or cupping leaves

Prominent veins

Interveinal discoloration

Elongated stem growth

Causes

Broadleaf weed killer misuse

Temperature, wind, concentration

Soil sterilant damage

Types of Cold Damage

Low Temperature Injury (cold damage)

Sunscald and frost cracking

Winter desiccation and Freeze injury

Snow and ice

Cold Damage

Above Freezing, low enough to cause injury

Varies by species – houseplants vs. landscape

Symptoms

Wilting

Desiccation

Water-logged areas

Physical distortion of plant parts

Winter Desiccation/Freezing Injury

Plants subjected to sub-freezing temps

Symptoms

Discoloration or bronzing of foliage

Brown needles or leaves in the spring

Excessive needle drop

Blackening of leaves

Dieback of twigs and small branches

Causes

Dry winter winds

Inadequate moisture for evergreens

Young thin bark subject to freezing temp fluctuations

Pruning issues

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Sunscald and Frost Cracking

Result of interaction of light and temperature

Caused by thawing and freezing due to a rapid fluctuation in stem temps

Exposure to afternoon sun (from southwest) causes increase in stem temps and subsequent thawing of stem/trunk moisture

Sun sets and temps drop rapidly – below freezing will result in possible ice crystals rupturing internal tissue

Sunscald will result in a sunken area of bark

Frost cracking results in vertical splits

Environmental Pollutants

Salt Damage

Symptoms

Brown needles on new growth

Drought stress in appearance

Leaves scorch as they emerge

Causes

Excessive salts from de-icing materials

Over-fertilization

Containers – watering inadequately

Environmental Pollutants

Pesticides

Symptoms (depends on variety of variables)

(can be similar to nutrient disorders)

Leaf chlorosis

Marginal and/or spotted necrosis

Total leaf necrosis

Causes

Misuse of pesticide (concentration, application rate)

Temperature, wind, humidity

Plant species

Other Causes of Abiotic Disorders

Wildlife/Animal Problems

Deer scraping tree bark, eating plants, dog spraying, etc.

Use DGIF for Nuisance Wildlife link at website

Wind Damage/Hail

Plant structure, tree topping

Soil Compaction

From equipment, cars

Root damage

Installation of irrigation or drainage

Mulch volcanoes

Resources

Manual of Woody Landscape Plants by Michael Dirr

Abiotic Plant Disorders, Symptoms, Signs and Solutions: A Diagnostic Guide to Problem Solving by Schultzki and Cregg of Michigan State

Abiotic Disorders of Landscape Plants: A Diagnotic Guide, Costello, et al., Univ. of CA

“Top 10 Abiotic Diseases of Woody Ornamentals” by Gunnel and Goodspeed of Utah State (modified)

Virginia Pest Management Guide

Resources

Virginia Cooperative Extension website:

http://www.ext.vt.edu

http://www.ento.vt.edu/bughunt

http://www.vtpp.ext.vt.edu

Books in office

Pesticide Management Guide

Ortho Problem Solver

Publications

Experience of Agents and other VCE Master Gardeners

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Summary

Ask questions

Know basics of the plant and diseases

Use resources

Lab

Books

Agents

Other Master Gardeners

Thanks to:

Cynthia Gregg, Extension Agent, Brunswick County

Mike Likins, County Agent, Chesterfield County