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Ten Common Plant Diseases / Disorders You Can Diagnose by Eye

Authors: Brian Hudelson, UW-Madison Plant Pathology
Last Revised: 04/30/2021
D-number: XHT1283

Vegetables such as squash and pumpkin are very susceptible to powdery mildew

Powdery Mildew

Hosts:    Herbaceous and woody ornamentals, fruits, vegetables, turf

Pathogens:    Miscellaneous powdery mildew fungi

Signs/Symptoms:  Powdery white growth on leaves

For more information see:       UW Garden Facts XHT1005a/b/c

Symptoms of tar spot on silver maple leaves.

Tar Spot

Hosts:   Maples

Pathogen:   Rhytisma spp.

Signs/Symptoms:  Tarry areas (either solid spots or clusters of small spots) on leaves

For more information see:       UW Garden Facts XHT1126

Leaf distortions and discoloration typical of peach leaf curl.

Peach Leaf Curl

Hosts:   Peach

Pathogen:   Taphrina deformans

Signs/Symptoms:  Light-green, yellow or purplish-red puckered areas on leaves

For more information see:       UW Garden Facts XHT1069

Sooty mold on pine needles

Sooty Mold

Hosts:   Any plant

Pathogen:   Miscellaneous sooty mold fungi

Signs/Symptoms:  Powdery black growth on leaves or needles

For more information see:       UW Bulletin A2637


Hosts:  Oak, red maple

Cause:   Iron or manganese deficiency, often induced by high soil pH

Signs/Symptoms:  Yellow leaves with dark green veins

For more information see:       UW Garden Facts XHT1002

Gymnosporangium Rusts

Gymnosporangium Rusts

Hosts:   Juniper, apple, crabapple, hawthorn, quince

Pathogen:   Gymnosporangium spp.

Signs/Symptoms:  Brown blobs with orange gelatinous masses (juniper); yellow/orange leaf spots (other hosts)

For more information see:       UW Garden Facts XHT1009

A typical older black knot gall with colonization by whitish secondary fungi.

Black Knot

Hosts:   Prunus spp. (plum and cherry)

Pathogen:   Apiosporina morbosa

Signs/Symptoms:  Black poop-like growths on branches

For more information see:       UW Garden Facts XHT1056

Elderberry Rust

Hosts:   Elderberry

Pathogen:   Puccinia sambuci

Signs/Symptoms:  Light yellow, powdery growths on branches

For more information see:       UW Garden Facts XHT1280

A yellow branch with orange spots typical of golden canker on pagoda dogwood.

Golden Canker

Hosts:   Pagoda dogwood

Pathogen:   Cryptodiaporthe corni

Signs/Symptoms:  Gold-colored branches with orange spots

For more information see:      UW Garden Facts XHT1125

A dog vomit slime mold is shown spreading across mulch and up the base of a shrub.

Dog Vomit Slime Mold

Hosts:   Any plant and on mulch

Cause:   Fuligo septica

Signs/Symptoms:  Scrambled egg-like masses on mulch or at the base of plants

For more information see:      UW Garden Facts XHT1091

For more information on common plant diseases:  See or contact your county Extension agent.

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