Plant Diseases

The Disease Triangle

Susceptible host plant

Pathogen

Favorable climate

Time

Host Plant

Time

Diseases and Pathogens

Can be spread or transmitted to other plants

Caused by biotic or living organisms called pathogens - fungi - bacteria - viruses - nematodes - phytoplasmas - parasitic plants

Vector Means by which a pathogen is spread

Vector can also be wind, rain splash, insects,

humans, soil, and pruning tools

Example: elm bark beetles spread the fungus that

causes Dutch elm disease, so the beetles are the

vector of the disease

Types of Disease Symptoms

Blight

Leaf spot

Mosaic

Stunting

Marginal necrosis

Distortion

Wilt

Canker

Gall

Rot

Witches Brooming

Blight Large dead areas on leaves, shoots or flowers

Leaf Spots Small, discolored areas on foliage

Mosaic Patches of yellow and green tissue on a leaf

Stunting

Abnormally small

size of plant or

plant part

http://www.mntca.org/resources/treeid/tree_dec_opp_ash_green.html

Marginal necrosis Brown, dead tissue around the edges of leaves

Distortion Twisting or abnormally shaped leaves and shoots

Wilt Flaccid, limp condition of leaves or shoots resulting

from water deficiency

Canker Localized, often sunken, dead area on a twig,

branch, or stem

Gall Abnormal swelling of a portion of a branch, leaf,

root or bud, a tumor

Rot Tissue breakdown or decay

Witches brooming Twig growth resulting from a lack of apical

dominance causing side shoots to elongate equally,

forming a dense cluster or broom-like mass of twigs

Fungal pathogens

Fungi (sing. fungus) Cause the majority of plant diseases

Rots, cankers, leaf spots, blights, blotches, and wilt diseases

Spread by insects, wind, rain, pruning tools, infested plant tissue, soil

Need water for disease development

Most fruiting bodies are tiny and look like black pepper to the

naked eye

Spores emerge from the fruiting bodies

Fungal Diseases

Apple scab

Powdery mildew

Black spot on rose

Leaf curl and leaf blister

Cedar rusts (apple, hawthorn, quince)

Anthracnose (sycamore, ash, oak)

Guignardia leaf blotch

Cankers (Thyronectria, Nectria, Botryosphaeria)

Black knot

Volutella

Apple scab Most common crabapple disease

Begins as olive-green leaf spots with fuzzy edges

Leaves eventually turn yellow and fall off

Also infects fruit

Very common in wet springs

Powdery mildew

Affects many woody and herbaceous plants, including vegetables

Common during periods of warm days and cool, wet nights

More severe when air circulation is poor

Look for white to gray powdery appearance on leaves

May affect winter hardiness

Black spot of rose

Affects rose species

Symptoms - black spots with fringed margins on

usually the upper leaf surface

Infected leaves turn yellow and drop from the plant

Leaf Curl or Leaf Blister (Taphrina)

Fungus that causes blister-like distortion of leaves

Hosts include:

Peach (Peach leaf curl)

Oaks (Oak leaf blister)

Plums (Plum pockets)

All caused by Taphrina fungus respective to their host

Peach leaf curl

Oak leaf blister

Plum pockets

Oak leaf blister

Leaves form blisters that are at first yellow to gray, become red or brown, leaves may drop prematurely

Not usually a significant problem

May lose leaves in serious infection

Fungus overwinters in bud scales

Spores germinate in spring as buds break

Three kinds of cedar rusts

Cedar apple rust

Junipers and crabapple or apple leaves

Cedar hawthorn rust

Junipers and hawthorn leaves

Cedar quince rust

Junipers and fruits and twigs of rose family plants, especially hawthorns

Cedar apple rust

Forms large galls on junipers which

grow orange tendrils in spring rains

Spores are released as tendrils dry

Infection causes spots on apple leaves

Spores are released from spots

Spores landing on juniper twigs can cause galls

Cedar Hawthorn Rust

Forms smaller galls on junipers which grow orange tendrils in spring rains

Cedar Quince Rust Does not form galls on junipers, but lives

in trunk and twigs

Forms gelatinous masses in spring rains which release spores

Anthracnose

A term used to designate diseases caused by

morphologically similar fungi

Some anthracnose diseases are only leaf diseases

and some also attack twigs

Common hosts are sycamores, maples, oaks, and ash

Sycamore Anthracnose

American sycamore is host

Common fungal disease in cool, wet weather

Kills leaves, buds, shoots and one year old twigs, causes

witches brooming

Trees leaf out a second time, but pay a price in energy

Ash Anthracnose

Symptoms: large irregular tan to

brown lesions that form on

expanding leaflets

Leaves become distorted

Damage is aesthetic

Disease overwinters in fallen leaves

In spring, spores are blown onto

newly emerging leaves

Oak Anthracnose

Can show three different patterns depending on when leaves were infected

When infected early, young leaves brown and shrivel

When infected later, large irregular dead areas on distorted leaves

When infected late, small necrotic spots on leaves

Guignardia leaf blotch

Common on horse chestnuts and buckeyes

Brown leaf blotches with a yellow border

Can see black pepper-like fruiting bodies

Canker Diseases

Causes dieback and wilting

Cankers form on infected bark and sapwood beneath

canker becomes discolored brown

Disease is more severe on stressed plants

Usually a chronic problem

Thyronectria canker

Most common canker disease on honey locusts

Cankers girdle branches and cause branches to die

Fruiting bodies form at the junction of living tissue and dead twigs

More common on trees suffering from drought stress

Nectria canker Perennial canker disease of

many hardwoods

Destroys cambium

No chemical control

Avoid wounding trees

Keep trees healthy

Botryosphaeria canker Common fungal canker disease on many trees and shrubs

Common hosts: apple, ash, crabapple, dogwood, elm, holly,

honeylocust, linden, maple, mountain ash, oak, pine,

redbud, rhododendron, and sycamore

Volutella on Boxwood

Fungal canker disease which causes stem dieback

Damage looks similar to winter injury

Black knot

Serious fungal disease of plums and cherries

Can stunt or kill tree

Causes dark fungal galls, which eventually girdle

branch, killing it

Vascular diseases (wilt diseases)

Verticillium wilt

Dutch elm disease

Oak wilt

Pine wilt

Verticillium wilt

Soilborne disease that affects over 300 plants

Acute or chronic problem

Fungus produces toxins that causes tyloses to plug up xylem

Streaking seen under bark

Usually not seen until middle of summer

Dutch elm disease

Begin seeing new infection symptoms starting in June

Initially flags are seen caused by production of tyloses

Flags enlarge and spread throughout the tree

Streaking under bark

Trees eventually die

Oak wilt

Fungal disease

Similar action to Dutch elm disease

Tree wards off fungus by forming tyloses which plugs xylem

Branches wilt

Streaking under the bark

Red oak group

Red oaks more susceptible than white oaks

Symptoms usually appear in late spring, early summer

Leaves turn bronze and fall off tree in summer

Kills red oaks within 4–6 weeks

Spread by beetles, root grafts

Leaves become straw

colored from the leaf tip,

but remain attached to

branches

Infection occurs in crown

Branch die back occurs

Usually takes several years

to tree to die

White oak group may

temporarily “wall-off” the

fungus

White oak group

Bacteria (sing. Bacterium)

Microscopic organisms

Require a vector and natural opening or

wound to penetrate plant tissue

Spread by wind, rain, insects, pruning tools

Cause leaf spots, rots, bacterial galls, wilts

and cankers

Bacterial Diseases

Bacterial blight of lilac

Crown gall

Wetwood and slime flux

Fireblight

Bacterial leaf scorch

Bacterial blight of lilac

Leaves develop brown

spots or blotches, often

with yellow haloes, which

coalesce and may

eventually turn the entire

leaf brown

Shoots and flowers may

turn brown and limp, flower

buds turn black

Prevalent in rainy seasons

or with frequent overhead

irrigation when tissues are

succulent

Crown gall

Bacterial disease that causes tumor-like growths on lower stem near the soil line

Gall can girdle stem and kill all growth above it

Many hosts, especially common on ground cover Euonymus

Bacteria survive in soil

Wetwood or slime flux

Common bacterial disease on many trees, especially elms and poplars

Liquid seeps out of the tree, frequently at tree crotches

There is no satisfactory control for wetwood

Fire blight

Bacterial disease of plants in the Rose family

Serious disease that can kill trees

Spread by pollinators, wind, rain, pruning tools

Leaves remain on the tree, look like they were scorched by fire

Sometimes see characteristic shepherd’s crook

Can see bacterial oozing in wet weather

Bacterial Leaf Scorch

Marginal necrosis, usually with yellow edge

Most hardwoods are effected

Has not been detected at TMA

Also looks like environmental scorch

Viruses (sing. Virus)

Sub-microscopic particles that cannot be seen with

standard microscopes

Must be in a host cell to reproduce

Cause stunting, curling, and twisting or odd coloring

Spread by insects, plant propagation, and seeds

Viruses Wide range of host plants, especially on roses and ash trees

Yellow mottling and yellow or white ringspots and line patterns are typical symptoms. Sometimes also see poor vigor, stunting or reduced leaf size.

Need serological testing to confirm viruses

Transmitted via aphids and leafhoppers

No cure

Nematodes

Microscopic roundworms

Found in soil and plant tissue

May be vectored by insects

Pinewood nematode is the

cause of pine wilt disease

Pine wilt Most common in Scots pine, but found in all pines

(extremely rare in white pines)

Caused by pine wilt nematode, which is vectored by the pine sawyer beetle

First symptom is gray green needles which progress rapidly to yellow, then brown

Trees die in less than one season

Occurs midsummer to late fall or late winter to spring

Foliar nematodes

Angular leaf spots

Columbine, begonia, Cyclamen, Gerbera, Hibiscus,

geranium, Brunnera, Hellebore

Phytoplasmas

Microscopic pathogens

Cause growth distortion,

stunting, witches brooming,

and yellowing

Cause “yellows” diseases

(Ash and elm)

Found only in phloem

Vectored by phloem feeding

insects such as leafhoppers

Electron microscopy of cross-section of gladiolus phloem tissue infected with

phytoplasma. Numerous phytoplasma bodies are apparent in the upper sieve

elements. The sieve element in the lower left-hand corner is apparently free of

phytoplasma bodies. x6000. Courtesy of Assunta Bertaccini.

Yellows Symptoms

Ash Elm

Common Conifer Diseases

Needle diseases more serious than leaf spots because evergreens cannot refoliate

Dothistroma needle blight

Rhizosphaera needle blight

Diplodia (i.e. Sphaeropsis) tip blight

Phomopsis

Cytospora Canker

Dothistroma needle blight

Affects Austrian and ponderosa pines

Red and Scots are resistant

Begins with brown spots and bands on needles, eventually needle turns brown with base remaining green

Fungal spores are released during wet weather any time during the growing season from May to October

Rhizosphaera Needle Cast

Common on stressed blue spruce

Likes warm, wet weather

Infects new growth, but they do not show symptoms until fall or early spring

One year old needles fall off, can see fruiting bodies in rows on needle

Stigmina

Identified in 2006

Similar symptoms as Rhizosphaera

Photos by Jim Walla Photos by Justin Knott

Diplodia tip blight (i.e. Sphaeropsis)

Common fungal disease of stressed pines

Common on Austrian, Scots, mugo, and red pines

Disfiguring disease that kills new shoots

New needles are stunted, twisted, and killed

Sometimes see resin exudation

Starts on lower branches

Phomopsis

Fungal disease of junipers

Causes tip blight of branches, only new

growth is effected

Initially starts as yellow spots, progresses

into shoots

Tips become light green and then reddish

brown, eventually gray

Cytospora canker

Common fungal disease of spruces, especially Colorado and Norway

Affects older trees (at least 15 years old and 20 feet tall)

In spring, needles turn purple, brown and fall off

Kills older branches and progresses upwards

More common on stressed trees

Root Rot Infections

Symptoms

Uniform canopy dieback, leaf drooping, almost like

drought stress, brown or black roots, sloughing roots

Fungal pathogens

Phytophthora, Pythium, Armillaria, and Rhizoctonia

Bacterial soft rot

Due to excess moisture

Soil will smell acrid and sour

Above-ground Root Rot Symptoms

Below-ground Root Rot Symptoms

Fungal decay

Mushrooms at the base of

the tree

Conks on the stem

Mycelial mats found

under sloughing bark

Sometimes sunken or

darkened areas on the

stem

conk : a shelf-like, typically hardened basidiocarp of a

wood decaying fungus, usually a polypore

(Ganoderma applanatum). www.APSnet.org

Bacterial Soft Rot

Found in areas with excessive moisture

Soil will smell acrid, pungent, and swampy

Bark of dying roots will easily slough off and dying

roots will be discolored tan-brown

Fine roots will be black (dead)

Canopy will have symptoms of water stress,

dieback, and wilt

Plants recover when water is removed

Armillaria root rot

Common landscape disease on

700 plant species

Kills roots and lower trunk

Above ground symptoms are

similar to other diseases – early

fall color leaf drop, stunted or

yellow leaves, dieback

Can infest fast or slow

Look for white, fan-shaped

mycelia under the bark on major

roots and at base of trunk

Information for Diagnosis

Need a good history of the problem

Host plant (name, accession number)

Weather conditions (day, week, or month)

Type of care given (herbicide use, mulching, pruning)

Condition of surrounding plants (turf, wet areas,

competing plants, surrounding symptomatic plants)

Soil moisture

Identify the plant

• Family, genus, species, and cultivar, if possible • What are the environmental

requirements for the plant?

• Are these needs being met?

• Each plant has its growing conditions that it thrives in • Water, nutrients, soil pH, sunlight

• Determine the age of

the tree

Identify the plant

• Each plant has its own set of common diseases, insect problems and cultural needs • Biotic: fungi, bacteria, viruses,

phytoplasmas, insects

• Abiotic: pollution, mechanical damage, poor planting space, environmental

• Establish what a healthy plant

would look like

• Seasonal variation, weather

Tilia platyphyllos 'Laciniata'

Herbicide damage

What is normal for the plant?

What is perfectly healthy for one species may be a symptom of a

serious problem for a different species

Resin on bristlecone pine Pine needle scale

What’s the plant’s history

• When was the plant put in the ground?

• Was it planted correctly?

• Has anything changed around the plant?

• What sort of care has it received in the past?

• Consider fertilization, mulching, pesticide spray program

• Check the depth and location of the mulch

Abnormal symptoms

Look for Patterns

• Are other plants in the vicinity affected?

• Non-uniform damage patterns

• Uniform damage patterns

Non-uniform patterns

• Random branches on a plant

• Usually caused by living factors

• Insects or pathogens

• Specific symptoms and signs

• Limited to a one or a few hosts

• Plants within a family or genera

• Initially do not see wide-spread damage

• Takes time to multiply and spread

Uniform damage patterns

• All leaves of a certain age affected

• Are there symptoms of dieback and re-sprouting?

• Damage seen over a wide area

• Neighborhood, city, street, yard

• Usually caused by nonliving factors

• Frost, drought, flooding, chemicals, construction

• Multiple plant species may be involved

What is the environmental history?

• Winter temperatures

• Late frost

• Hail storms

• Water

– Drowning and drought have

similar symptoms

• Sources of water

• Exposure to sun Cold damage on thin-leaved hostas

Environmental observations

• Includes soil characteristics – Construction fill, soil type, clay content, soil moisture, pH

• Construction history, grade changes

• Identify surrounding plants – Competition or allelopathies

• Growth restrictions – May have originated at planting

• Sources of abiotic stress – Salt, exhaust, mechanical damage, stem-girdling roots

Define the abnormality

• Try to determine the primary problem and plant part

where the initial damage occurred

• Sometimes there is more than one problem

Time Development of Damage Pattern

• Observe the development of the pattern

• Direction of dieback

• Damage caused by living organisms are progressive and

spread with time

• Damage caused by nonliving organisms are not

progressive and do not spread with time and usually

effect several species in the area