Category Archives: Disease – Fruit Crop

Disease - Broad Leafed Woody Ornamental, Disease - Fruit Crop, Disease - Herbaceous Ornamental, Disease - Needled Woody Ornamental, Disease - Vegetable, Quick Reference Guide

Plant Problems to Watch for in 2026

January 8, 2026 ddlang

UW Plant Disease Facts

Authors: Brian Hudelson, UW-Madison Plant Pathology

Last Revised: 4/14/2026

D-number: D0137

Root and Crown Rots

Hosts: Any plant
Pathogens: Assorted root rot fungi/water molds
Signs/Symptoms: Poor growth, branch dieback, discolored and deteriorated roots
For more information see: UW Plant Disease Facts D0094

Planting-Related Issues/Decline

Hosts: Woody trees and shrubs
Pathogens: None (abiotic)
Signs/Symptoms: No visible root flare, girdling roots, frost cracks, canopy thinning, early fall color, branch dieback, tree/shrub decline and death

Scab (Apple and Pear)

Apple Scab

Hosts: Apple, crabapple, pear, mountain-ash
Pathogens: Venturia inaequalis, Venturia pirina
Signs/Symptoms: Feathery-edged spots on leaves and fruits often leading to leaf loss and tree defoliation
For more information see: UW Plant Disease Facts D0004

Septoria Leaf Spot of Lilac

Host: Lilac
Pathogen: Septoria sp.
Signs/Symptoms: Dead spots on leaves, potentially leading to complete leaf browning

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 Plant Disease Facts D0058/0131

Chlorosis

Hosts: Pin oak, red maple, birch, azalea, white pine, blueberry
Pathogen: None (Abiotic)
Signs/Symptoms: Yellow leaves with dark green veins
For more information see: UW Plant Disease Facts D0030

Septoria Leaf Spot and Early Blight

Host: Tomato
Pathogens: Septoria lycopersici and Alternaria solani
Signs/Symptoms: Spotting and eventual total death of leaves working from the bottom of the plant up
For more information see: UW Plant Disease Facts D0100/46

Powdery Mildew

Hosts: Herbaceous and deciduous woody ornamentals, fruits, vegetables, turf
Pathogens: Miscellaneous powdery mildew fungi
Signs/Symptoms: Powdery white growth on leaves
For more information see: UW Plant Disease Facts D0084/86/87

Rhizosphaera Needle Cast

Hosts: Colorado blue spruce, other spruces
Pathogen: Rhizosphaera kalkhoffii
Signs/Symptoms: Browning/purpling of interior needles of lower branches, followed by needle drop
For more information see: UW Plant Disease Facts D0093

Diplodia Shoot Blight and Canker

Hosts: Austrian pine, other pines
Pathogen: Diplodia spp.
Signs/Symptoms: Dieback of brand tips with dead needles showing uneven lengths
For more information see: UW Plant Disease Facts D0042

Tobacco Rattle

Hosts: Many herbaceous ornamentals and vegetables
Pathogens: Tobacco rattle virus
Signs/Symptoms: Blotchy leaf color; yellow lines, ring spots, or dead spots on leaves; distorted leaf growth
For more information see: UW Plant Disease Facts D0116

For more information on plant problems to watch for in 2026:

See the UW-Madison Plant Disease Diagnostics Clinic website (https://pddc.wisc.edu/) or contact the University of Wisconsin Plant Disease Diagnostics Clinic (PDDC) at 608-262-2863 or pddc@wisc.edu.

This Fact Sheet is also available in PDF format:

Full Color PDF

An EEO/Affirmative Action employer, University of Wisconsin-Madison Division of Extension provides equal opportunities in employment and programming, including Title IX and ADA requirements. This document can be provided in an alternative format by calling Brian Hudelson at 608-262-2863 (711 for Wisconsin Relay).

Thanks to Julie Hill, Denise Rocha and Janell Wehr for reviewing this document.

Thanks also to Marissa Wilmot (Septoria leaf spot of lilac), and Anette Phibbs (tobacco rattle) for use of their photos.

A complete inventory of UW Plant Disease Facts is available at the University of Wisconsin-Madison Plant Disease Diagnostics Clinic website. (https://pddc.wisc.edu)

Submit additional lawn, landscape, and gardening questions at the Ask a Gardening Question page (https://hort.extension.wisc.edu/ask-a-gardening-question/) of the University of Wisconsin-Madison Division of Extension Horticulture website.

Disease - Broad Leafed Woody Ornamental, Disease - Fruit Crop, Disease - Herbaceous Ornamental, Disease - Needled Woody Ornamental, Disease - Vegetable, Quick Reference Guide

Plant Problems to Watch for in 2025

April 21, 2025 ddlang

UW Plant Disease Facts

Authors: Brian Hudelson, UW-Madison Plant Pathology

Last Revised: 04/14/2026

D-number: D0136

Scab (Apple and Pear)

Apple Scab

Septoria Leaf Spot of Lilac

Host: Lilac
Pathogen: Septoria sp.
Signs/Symptoms: Dead spots on leaves, potentially leading to complete leaf browning

Aster Yellows

Hosts: Many herbaceous ornamentals and vegetables
Pathogens: Aster yellows phytoplasma
Signs/Symptoms: Yellow/orange/purple leaves, stunted and distorted growth, leafy flowers, brooming
For more information see: UW Garden Facts D0007

Gymnosporangium Rusts

Winter Injury/Winter Burn

Hosts: All conifers, particularly yew and juniper
Cause: Insufficient water
Signs/Symptoms: Needle browning/bleaching over winter or in spring as plants come out of dormancy
For more information see: UW Plant Disease Facts D0127

Bacterial Canker

Hosts: Stone fruits (cherry, peach, plum)
Pathogen: Pseudomonas syringae
Signs/Symptoms: Masses of sap on affected branches/trunks, branch die back, tree death
For more information see: UW Plant Disease Facts D0009

Powdery Mildew

Septoria Leaf Spot and Early Blight

Rhizosphaera Needle Cast

Diplodia Shoot Blight and Canker

Chlorosis

For more information on plant problems to watch for in 2025:

See the UW-Madison Plant Disease Diagnostics Clinic website (https://pddc.wisc.edu/) or contact the University of Wisconsin Plant Disease Diagnostics Clinic (PDDC) at 608-262-2863 or pddc@wisc.edu.

This Fact Sheet is also available in PDF format:

Full Color PDF

Thanks to Julie Hill, Kathy Johnson and Margaret Murphy for reviewing this document.

Thanks also to Marissa Wilmot (Septoria leaf spot of lilac) for use of their photo.

A complete inventory of UW Plant Disease Facts is available at the University of Wisconsin-Madison Plant Disease Diagnostics Clinic website. (https://pddc.wisc.edu)

Disease - Broad Leafed Woody Ornamental, Disease - Fruit Crop, Disease - Herbaceous Ornamental, Disease - Needled Woody Ornamental, Disease - Vegetable, Quick Reference Guide

Plant Problems to Watch for in 2024

April 30, 2024 ddlang

UW Plant Disease Facts

Authors: Brian Hudelson, UW-Madison Plant Pathology

Last Revised: 04/14/2026

D-number: D0135

Winter Injury/Winter Burn

Chlorosis

Blister Canker

Hosts: Apple, crabapple, serviceberry, mountain-ash
Pathogen: Biscogniauxia marginata
Signs/Symptoms: Dark, sunken, cracked areas on trunks with dark, round, flat pads of fungal growth
For more information see: UW Plant Disease Facts D0134

Golden Canker

Host: Pagoda dogwood
Pathogen: Cryptodiaporthe corni
Signs/Symptoms: Dead branches that are gold in color, often with orange spots
For more information see: UW Plant Disease Facts D0055

Diplodia Shoot Blight and Canker

Hosts: Austrian pine, other pines
Pathogen: Diplodia spp.
Signs/Symptoms: Dieback of brand tips with dead needles showing uneven lengths
For more information see: UW Plant Disease Facts D0042

Fire Blight

Hosts: Apple, crabapple, pear, mountain-ash
Pathogen: Erwinia amylovora
Signs/Symptoms: Branch dieback (often with the tip having a shepherd’s crook), eventual tree death
For more information see: UW Plant Disease Facts D0052

Cucumber Mosaic

Hosts: Herbaceous ornamentals and vegetables
Pathogens: Cucumber mosaic virus
Signs/Symptoms: Blotchy light and dark green leaf color, distorted leaf growth
For more information see: UW Plant Disease Facts D0036

Powdery Mildew

Hosts: Herbaceous and woody ornamentals, fruit, vegetables, turf
Pathogens: Miscellaneous powdery mildew fungi
Signs/Symptoms: Powdery white growth on leaves
For more information see: UW Plant Disease Facts D0084/86/87

Red Star Rust

Hosts: Chinese juniper, apple, crabapple
Pathogen: Gymnosporangium yamadae
Signs/Symptoms: Brown blobs with orange gelatinous masses (juniper), red leaf spots (apple, crabapple)
For more information see: UW Plant Disease Facts D0131

Boxwood Blight

Host: Boxwood
Pathogen: Calonectria pseudonaviculata
Signs/Symptoms: Circular, brown leaf spots followed by leaf drop and shrub death
For more information see: UW Plant Disease Facts D0023

Improper Planting

Hosts: Woody trees and shrubs
Pathogen: None
Signs/Symptoms: No root flare at the soil line, girdling roots, frost cracks, canopy thinning, early fall color, branch dieback, tree/shrub decline and death

For more information on plant problems to watch for in 2024:

See the UW-Madison Plant Disease Diagnostics Clinic website (https://pddc.wisc.edu/) or contact the University of Wisconsin Plant Disease Diagnostics Clinic (PDDC) at 608-262-2863 or pddc@wisc.edu.

This Fact Sheet is also available in PDF format:

Full Color PDF

References to pesticide products in this publication are for your convenience and are not an endorsement or criticism of one product over similar products. You are responsible for using pesticides according to the manufacturer’s current label directions. Follow directions exactly to protect the environment and people from pesticide exposure. Failure to do so violates the law.

Thanks to Julie Hill, Margaret Murphy and Denise Rocha for reviewing this document.

Thanks also to Serenella Linares (blister canker), Nancy Gregory-University of Maryland (lipstick rust), and David Clement (boxwood blight) for use of their photos.

A complete inventory of UW Plant Disease Facts is available at the University of Wisconsin-Madison Plant Disease Diagnostics Clinic website. (https://pddc.wisc.edu)

Disease - Broad Leafed Woody Ornamental, Disease - Fruit Crop, Disease - Needled Woody Ornamental

Red Star Rust

April 28, 2024 ddlang

UW Plant Disease Facts

Authors: Brian Hudelson, UW-Madison Plant Pathology

Last Revised: 04/14/2026

D-number: D0131

What is red star rust?

Red star rust (also known as Japanese apple rust or lipstick rust) is a type of Gymnosporangium rust (see UW Plant Disease Facts D0058, Gymnosporangium Rusts). The disease affects junipers [specifically Chinese junipers (Juniperus chinensis), flaky juniper (Juniperus squamata), Hollywood juniper (Juniperus chinensis var. kaizuka), Japanese garden juniper (Juniperus procumbens), Sargent juniper (Juniperus chinensis var. sargentii), and savin juniper (Juniperus sabina)], as well as apple and crabapple trees (Malus spp.). The disease is native to Asia (in particular China, Korea and Japan) and was first detected in the US in Delaware and Pennsylvania in 2008. Red star rust was observed in Wisconsin in 2021 and to date has been found in Dane, Kenosha, Milwaukee, Outagamie, Ozaukee, Portage, Racine, Sheboygan, Walworth, Waukesha, and Winnebago Counties. Red star rust is of concern because of its potential negative impact in commercial apple production.

What does red star rust look like?

On junipers, red star rust leads to branch swellings or small-diameter galls that sprout ¹/₈ to ³/₈ inch long, flattened, orange, gelatinous protrusions in late spring to early summer. The galls/swellings and gelatinous masses can be confused with those produced by cedar-hawthorn and cedar-quince rust (see UW Plant Disease Facts D0058, Gymnosporangium Rusts for details).

On apples and crabapples, red star rust typically leads to bright red, maroon, or fuchsia-colored leaf spots that become readily visible by mid-summer. Yellow or orange spots surrounded by a red, maroon, or fuchsia margin have also been reported for the disease. Beneath the colorful leaf spots, numerous spiny or tendril-like structures form. Red star rust leaf spots are similar to cedar-apple and cedar-hawthorn rust leaf spots except for their color; cedar-apple rust and cedar-hawthorn rust leaf spots tend to be solid yellow or orange.

In Asia, red start rust can cause severe defoliation of apple trees. Infections on apple fruits are rare.

If you see what you believe is red star rust, please contact the UW-Madison Plant Disease Diagnostics Clinic at 608-262-2863 or pddc@wisc.edu to make arrangements to submit a sample for a free diagnosis. Samples will be used to determine how widespread the disease is in the state.

Where does red star rust come from?

Red star rust is caused by the fungus Gymnosporangium yamadae. The fungus overwinters in infected branches and galls on junipers.

Spores produced in the gelatinous masses on infected branches and galls spread on air currents to apple and crabapple trees leading to leaf infections. Similarly, spores produced in the tube-like spines on apple and crabapple leaves drift to junipers leading to new branch infections and additional gall formation.

How do I save a tree or shrub with red star rust?

On junipers, prune infected branches to remove the disease. Cut four to six inches below visible galls or areas on a branch that appear to be infected. Be sure to decontaminate pruning tools between cuts by treating them for at least 30 seconds in 70% alcohol (e.g., rubbing alcohol or certain spray disinfectants) or 10% bleach.

Decontaminating tools will prevent movement of rust fungi from branch to branch or from plant to plant during pruning. If you use bleach, be sure to thoroughly rinse and oil your tools after pruning to prevent rusting. You can burn (where allowed) or bury pruned branches, or drop them off at your local yard waste disposal site.

Once infections occur on apple and crabapple leaves, there is no cure. Fortunately, cases of red star rust on apple and crabapple trees observed thus far in Wisconsin have not been severe. Damage to apple and crabapple trees has been primarily cosmetic, making trees less attractive but not causing significant leaf loss or yield loss (in fruit-bearing apple trees). No long-term detrimental effects due to the disease have been observed in Wisconsin at this time. All of this said, the negative impacts of red star rust may change as the disease becomes more established and widespread in the state, and during years with wetter weather.

How do I avoid problems with red star rust in the future?

The best way to avoid red star rust is to not grow junipers and apples or crabapples close to one another. In urban settings where yards are small however, keeping both hosts adequately separated may be impossible. Also, Chinese junipers have become increasingly common in many landscape settings because of their resistance to other Gymnosporangium rusts (i.e., cedar-apple rust, cedar-hawthorn rust, cedar-quince rust). If red star rust becomes a serious problem in your area, consider growing evergreens (e.g., pine, fir, spruce) and flowering trees and shrubs (e.g., cherry, plum, lilac) that are immune to the disease.

In Asia, commonly grown apple varieties differ in terms of their susceptibility to red star rust. Apple varieties grown in Wisconsin likely also differ in susceptibility. However, which apple varieties commonly grown in Wisconsin may be resistant (and to what extent) is not known. Also, while fungicides treatments are available to control cedar-apple rust, cedar-hawthorn rust, and cedar-quince rust (see UW Plant Disease Facts D0058, Gymnosporangium Rusts for details), how effective these treatments might be in controlling red star rust is also not known.

For more information on red star rust:

Contact the University of Wisconsin Plant Disease Diagnostics Clinic (PDDC) at 608-262-2863 or pddc@wisc.edu.

This Fact Sheet is also available in PDF format:

Full Color PDF

Thanks to Sam Fieweger, Leslie Holland, and Janell Wehr for reviewing this document.

A complete inventory of UW Plant Disease Facts is available at the University of Wisconsin-Madison Plant Disease Diagnostics Clinic website. (https://pddc.wisc.edu)

Disease - Broad Leafed Woody Ornamental, Disease - Fruit Crop

Blister Canker

March 8, 2024 ddlang

UW Plant Disease Facts

Authors: Ariana Abbrescia, Brian Hudelson, and Leslie Holland, UW-Madison Plant Pathology

Last Revised: 04/08/26

D-number: D0134

What is blister canker?

Blister canker (also known as nailhead canker) is a potentially lethal disease that primarily affects apple and crabapple trees. Other trees and shrubs grown in Wisconsin that can be affected by the disease include American elm, honey locust, hornbeam, magnolia, mountain-ash (rowan), oak, pear, and serviceberry.

Nondescript trunk blackening on a serviceberry due to blister canker (left). Stromata of the blister canker fungus (right). [Photos courtesy of Jake Kubisiak, Wachtel Tree Science (left) and Serenella Linares, mycologist and board member of the Mycological Association of Washington, D.C. (right)]

What does blister canker look like?

Symptoms of blister canker often appear as darkened, sunken areas (i.e., cankers) that extend from cracks or wounds on tree branches and trunks. These areas can be large, extending up to three feet in length. Initially, particularly in young trees, affected areas may have a mottled appearance due to a mix of diseased and healthy tissue. As the disease progresses, affected branches die and the bark peels away. Trunk infections can lead to tree death.

Within affected areas on branches and trunks, dark, round, flat structures (called stromata) form. These structures range from ¹/₈ to ³/₈ inches in diameter. Stromata resemble nailheads and often form in clusters, giving the bark a blistered appearance. In the early stages of symptom development, blister canker may be difficult to distinguish from other canker diseases [e.g., fire blight (see UW Plant Disease Facts D0052, Fire Blight), Nectria canker (see UW Plant Disease Facts D0074, Nectria Canker)]. However, the appearance of stromata helps distinguish blister canker from other diseases.

Where does blister canker come from?

Blister canker is caused by the fungus Biscogniauxia marginata, which survives on infected apple (or other) trees in both living and dead wood. Spores of the fungus (produced in the stromata) blow from tree to tree, and the fungus infects through wounds (e.g., pruning sites, broken branches). The fungus subsequently spreads under the bark.

How do I save a tree with blister canker?

Once a tree is infected by the blister canker fungus, it cannot be cured. Infected branches can be removed by pruning approximately six to eight inches below the canker. Decontaminate pruning tools after each cut by treating them for at least 30 seconds with bleach diluted to a final concentration of 0.5% sodium hypochlorite (the active ingredient) or (preferably due to its less corrosive properties) 70% alcohol (e.g., rubbing alcohol, certain spray disinfectants). If you use bleach, be sure to thoroughly rinse and oil your tools after pruning to prevent rusting. Trees with trunk infections may survive for several years, but you should remove them immediately to limit spread of the blister canker fungus. Burn (where allowed) or bury infected branches and trunk sections.

How do I avoid problems with blister canker in the future?

Blister canker often affects trees that are stressed due to drought, poor soil fertility, or (for fruit-bearing trees) excessive fruit production. Addressing these underlying issues can help prevent blister canker. Established deciduous trees (like the ones affected by blister canker) require approximately one inch of water per week from the time that they begin to leaf out in the spring, through the summer, and into the fall up until they begin to turn their normal fall color. If there is insufficient rain, apply supplemental water around the drip lines of trees (i.e., the edges of where the branches extend) using drip or soaker hoses. To address soil fertility issues, have a soil sample tested for nutrients, and fertilize based on the test results. The UW Soil and Forage Lab (https://uwlab.soils.wisc.edu/) can assist with this testing. Thinning fruits to prevent overbearing is another practice that reduces tree stress. Ideally, fruit thinning should be carried out at the earliest stage possible, typically within three to four weeks after bloom, when the fruits are still small.

Properly prune tree species susceptible to blister canker on a regular basis so that they grow properly and are less prone to storm damage. Pruning routinely also allows for removal of smaller branches, leaving wounds that are less likely to become infected by the blister canker fungus. Pruning larger branches from older trees can create large wounds that increase the risk of infection. For details on proper tree pruning, see University of Wisconsin Garden Facts XHT1014, Pruning Deciduous Trees.

For more information on blister canker:

Contact the University of Wisconsin Plant Disease Diagnostics Clinic (PDDC) at 608-262-2863 or pddc@wisc.edu.

This Fact Sheet is also available in PDF format:

Full Color PDF

Thanks to Lisa Johnson, Vijai Pandian, and Mary Kay Thompson for reviewing this document.

A complete inventory of UW Plant Disease Facts is available at the University of Wisconsin-Madison Plant Disease Diagnostics Clinic website. (https://pddc.wisc.edu)

Disease - Broad Leafed Woody Ornamental, Disease - Fruit Crop, Disease - Herbaceous Ornamental, Disease - Needled Woody Ornamental, Disease - Vegetable, Quick Reference Guide

Plant Problems to Watch for in 2023

November 13, 2022 ddlang

UW Plant Disease Facts

Authors: Brian Hudelson UW-Madison Plant Pathology

Last Revised: 04/13/2026

D-number: D0132

Lipstick Rust

Host: Chinese juniper, apple, crabapple
Pathogen: Gymnosporangium yamadae
Signs/Symptoms: Brown blobs with orange gelatinous masses (juniper), red leaf spots (apple, crabapple)

Boxwood Blight

Host: Boxwood
Pathogens: Calonectria pseudonaviculata
Signs/Symptoms: Circular, brown leaf spots followed by leaf drop and shrub death
For more information see: UW Plant Disease Facts D0023

Late Blight

Host: Tomato, potato
Pathogen: Phytophthora infestans
Signs/Symptoms: Water-soaked spots on leaves, leathery areas on tomato fruits, rapid plant death
For more information see: UW Plant Disease Facts D0068

Septoria Leaf Spot and Early Blight

Host: Tomato
Pathogens: Septoria lycopersici and Alternaria solani
Signs/Symptoms: Spotting and eventual total collapse of leaves, working from the bottom of the plant up
For more information see: UW Plant Disease Facts D0100/46

Septoria Leaf Spot of Lilac

Host: Lilac
Pathogen: Septoria sp.
Signs/Symptoms: Dead spots on leaves, potentially leading to complete leaf browning

Verticillium Wilt

Hosts: Woody and herbaceous ornamentals, vegetables
Pathogens: Verticillium sp.
Signs/Symptoms: Wilting, branch dieback, plant death
For more information see: UW Plant Disease Facts D0121/D0122

Powdery Mildew

Hosts: Herbaceous and woody ornamentals, fruit, vegetables, turf
Pathogens: Miscellaneous powdery mildew fungi
Signs/Symptoms: Powdery white growth on leaves
For more information see: UW Plant Disease Facts D0084/86/87

Rhizosphaera Needle Cast

Diplodia Shoot Blight and Canker

Chlorosis

Hosts: Pin oak, red maple, birch, azalea, white pine, blueberry
Pathogen: None
Signs/Symptoms: Yellow leaves with dark green veins
For more information see: UW Plant Disease Facts D0030

Improper Planting

For more information on plant problems to watch for:

See the UW-Madison Plant Disease Diagnostics Clinic website (https://pddc.wisc.edu/) or contact the University of Wisconsin Plant Disease Diagnostics Clinic (PDDC) at 608-262-2863 or pddc@wisc.edu.

This Fact Sheet is also available in PDF format:

Full Color PDF

Thanks to Diana Alfuth, Lisa Johnson and Carol Shirk for reviewing this document. Thanks also to David Clement (boxwood blight), Nancy Gregory-University of Maryland (lipstick rust), Amanda Gevens (late blight), and Marissa Wilmot (Septoria leaf spot of lilac) for use of their photos.

A complete inventory of UW Plant Disease Facts is available at the University of Wisconsin-Madison Plant Disease Diagnostics Clinic website. (https://pddc.wisc.edu)

Disease - Broad Leafed Woody Ornamental, Disease - Fruit Crop, Disease - Herbaceous Ornamental, Disease - Needled Woody Ornamental, Disease - Vegetable, Quick Reference Guide

Plant Problems to Watch for in 2022

April 28, 2022 ddlang

UW Plant Disease Facts

Authors: Brian Hudelson UW-Madison Plant Pathology

Last Revised: 04/13/2026

D-number: D0132

Boxwood Blight

Lipstick Rust

Host: Chinese juniper, apple, crabapple
Pathogen: Gymnosporangium yamadae
Signs/Symptoms: Brown blobs with orange gelatinous masses (juniper), red leaf spots (apple, crabapple)

Late Blight

Septoria Leaf Spot and Early Blight

Septoria Leaf Spot of Lilac

Host: Lilac
Pathogen: Septoria sp.
Signs/Symptoms: Dead spots on leaves, potentially leading to complete leaf browning

Wood Rots

Hosts: Woody trees and shrubs
Pathogens: Miscellaneous wood rot fungi
Signs/Symptoms: Shelf-like growths on trunks and branches

Canker Diseases

Hosts: Woody trees and shrubs
Pathogens: Miscellaneous canker fungi
Signs/Symptoms: Sunken areas on trunks/branches
For more information see: UW Plant Disease Facts D0027, D0037, D0042, D0055, D0074, D0114

Virus Disaeases

Hosts: All plants, particularly herbaceous ornamentals
Pathogen: Miscellaneous plant viruses
Signs/Symptoms: Blotchy leaf color, growth distortions
For more information see: UW Plant Disease Facts D0036, D0063, D0067, D0115, D0116, D0130

Rhizosphaera Needle Cast

Chlorosis

Improper Planting

For more information on plant problems to watch for:

See the UW-Madison Plant Disease Diagnostics Clinic website (https://pddc.wisc.edu/) or contact the University of Wisconsin Plant Disease Diagnostics Clinic (PDDC) at 608-262-2863 or pddc@wisc.edu.

This Fact Sheet is also available in PDF format:

Full Color PDF

Thanks to Julie Hill, Jeannie Manis and Carol Shirk for reviewing this document. Thanks also to David Clement (boxwood blight), Nancy Gregory-University of Maryland (lipstick rust), Amanda Gevens (late blight), Marissa Wilmot (Septoria leaf spot of lilac), and Diane Malchow (wood rots) for use of their photos.

A complete inventory of UW Plant Disease Facts is available at the University of Wisconsin-Madison Plant Disease Diagnostics Clinic website. (https://pddc.wisc.edu)

Disease - Broad Leafed Woody Ornamental, Disease - Fruit Crop, Disease - Needled Woody Ornamental, Misc

Lichens

May 28, 2021 hudelson

UW Plant Disease Facts

Authors: Ken Frost* and Brian Hudelson, UW-Madison Plant Pathology

Last Revised: 04/13/2026

D-number: D0072

There are many types of lichens. Crustose lichens (left) are crust-like and adhere tightly to the surface upon which they grow. Foliose lichens (right) are leaf-like and composed of flat sheets of tissue that are not tightly bound.

What are lichens?

Lichens are organisms that arise from mutually beneficial interactions between certain filamentous fungi, algae and yeasts. The filamentous fungi provide the physical structures of the lichens, as well as protection for the algae and yeasts. The algae produce food for the fungi and yeasts via photosynthesis. The yeasts are thought to produce compounds to fend off disease-causing organisms and insect pests.

What do lichens look like?

Lichens come in four basic growth forms. Crustose lichens are crust-like and adhere tightly to the surface upon which they grow. Foliose lichens are leaf-like and composed of flat sheets of tissue that are not tightly bound together. Squamulose lichens are composed of scale-like parts. Fruticose lichens are composed of free-standing branching tubes.

Where do lichens come from?

Lichens are everywhere. There are an estimated 13,500 to 17,000 species of lichens, and lichens can be found growing in tropical, temperate and polar regions throughout the world. Lichens will grow on almost any surface that is stable and reasonably well-lit. In temperate regions, lichens can often be found growing on the bark of trees or old fence posts. Others lichens grow in less hospitable places, such as bare rock surfaces or old headstones in graveyards, where they aid in the breakdown of rocks and the formation of soil.

How do I save a tree with lichens?

Do not panic! Lichens do not harm trees; they are not pathogens or parasites, and do not cause disease. Lichens are self-reliant, with the algal component of the lichen producing food for the organism via photosynthesis. Lichens absorb water and minerals from rainwater and the atmosphere, and because of this, they are extremely sensitive to air pollution. As a result, the presence or absence of certain lichen species can be used as an indicator of levels of atmospheric pollutants. Information on the abundance and species of lichens growing in an area can give a good indication of the local air quality.

For more information on lichens:

Contact the University of Wisconsin Plant Disease Diagnostics Clinic (PDDC) at 608-262-2863 or pddc@wisc.edu.

This Fact Sheet is also available in PDF format:

Full Color PDF

*Completed as partial fulfillment of the requirements for Plant Pathology 875 – Plant Disease Diagnostics Clinic Internship at the University of Wisconsin Madison.

Thanks to Lisa Johnson, Barb Larson and Mike Maddox for reviewing this document. Thanks also to the Wisconsin State Herbarium and Marie Trest for providing the photo.

A complete inventory of UW Plant Disease Facts is available at the University of Wisconsin-Madison Plant Disease Diagnostics Clinic website. (https://pddc.wisc.edu)

Disease - Broad Leafed Woody Ornamental, Disease - Fruit Crop, Disease - Herbaceous Ornamental, Disease - Needled Woody Ornamental, Disease - Vegetable, Quick Reference Guide

Plant Diseases to Watch For in 2021

May 11, 2021 ddlang

UW Plant Disease Facts

Authors: Brian Hudelson, UW-Madison Plant Pathology

Last Revised: 04/13/2026

D-number: D0081

Septoria Leaf Spot and Early Blight

Host: Tomato
Pathogens: Septoria lycopersici and Alternaria solani
Signs/Symptoms: Spotting and eventual total collapse of leaves working from the bottom of the plant up
For more information see: UW Plant Disease Facts D0100/46

Late Blight

Hosts: Tomato, potato
Pathogen: Phytophthora infestans
Signs/Symptoms: Water-soaked spots on leaves, leathery areas on tomato fruits, rapid plant death
For more information see: UW Plant Disease Facts D0068

Septoria Leaf Spot of Lilac

Host: Lilac
Pathogen: Septoria sp.
Signs/Symptoms: Dead spots on leaves, potentially leading to complete leaf browning

Rhizosphaera Needle Cast

Gymnosporangium Rusts

Elderberry Rust

Hosts: Elderberry
Pathogen: Puccinia sambuci
Signs/Symptoms: Light yellow, powdery growths on branches
For more information see: UW Plant Disease Facts D0049

Scab (Apple and Pear)

Bur Oak Blight

Host: Bur oak
Pathogen: Tubakia iowensis
Signs/Symptoms: Wedge-shaped dead areas on leaves leading to dead leaves that stay attached to trees

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 Plant Disease Facts D0084/86/87

Diplodia Shoot Blight and Canker

Hosts: Austrian pine, other pines
Pathogen: Diplodia spp.
Signs/Symptoms: Dieback of branch tips with dead needles showing uneven lengths
For more information see: UW Plant Disease Facts D0042

Boxwood Blight

For more information on plant diseases to watch for:

See the UW-Madison Plant Disease Diagnostics Clinic website (https://pddc.wisc.edu/) or contact the University of Wisconsin Plant Disease Diagnostics Clinic (PDDC) at 608-262-2863 or pddc@wisc.edu.

This Fact Sheet is also available in PDF format:

Full Color PDF

Thanks to Diana Alfuth, Margaret Murphy and Denise Worzalla-Rocha for reviewing this document. Thanks also to Amanda Gevens (late blight), Marissa Wilmot (Septoria leaf spot of lilac), Jenell Bindl (elderberry rust), Lina Rodriguez-Salamanca (bur oak blight) and David Clement (boxwood blight) for use of their photos.

A complete inventory of UW Plant Disease Facts is available at the University of Wisconsin-Madison Plant Disease Diagnostics Clinic website. (https://pddc.wisc.edu)

Disease - Broad Leafed Woody Ornamental, Disease - Fruit Crop, Disease - Herbaceous Ornamental, Disease - Needled Woody Ornamental, Disease - Vegetable, Misc, Quick Reference Guide

Ten Common Plant Diseases/Disorders You Can Diagnose by Eye

April 30, 2021 ddlang

UW Plant Disease Facts

Authors: Brian Hudelson, UW-Madison Plant Pathology

Last Revised: 04/16/2026

D-number: D0112

Powdery Mildew

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 Plant Disease Facts D0110

Peach Leaf Curl

Host: Peach
Pathogen: Taphrina deformans
Signs/Symptoms: Light-green, yellow or purplish-red puckered areas on leaves
For more information see: UW Plant Disease Facts D0076

Sooty Mold

Host: Any plant
Pathogen: Miscellaneous sooty mold fungi
Signs/Symptoms: Powdery black growth on leaves or needles
For more information see: UW Bulletin A2637

Chlorosis

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 Plant Disease Facts D0030

Gymnosporangium Rusts

Black Knot

Hosts: Prunus spp. (plum and cherry)
Pathogen: Apiosporina morbosa
Signs/Symptoms: Black poop-like growths on branches
For more information see: UW Plant Disease Facts D0018

Elderberry Rust

Host: Elderberry
Pathogen: Puccinia sambuci
Signs/Symptoms: Light yellow, powdery growths on branches
For more information see: UW Plant Disease Facts D0049

Golden Canker

Host: Pagoda dogwood
Pathogen: Cryptodiaporthe corni
Signs/Symptoms: Gold-colored branches with orange spots
For more information see: UW Plant Disease Facts D0055

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 Plant Disease Facts D0102

For more information on common plant diseases:

See the UW-Madison Plant Disease Diagnostics Clinic website (https://pddc.wisc.edu/) or contact the University of Wisconsin Plant Disease Diagnostics Clinic (PDDC) at 608-262-2863 or pddc@wisc.edu.

This Fact Sheet is also available in PDF format:

Full Color PDF

Thanks to Diana Alfuth, Mike Maddox and Ann Wied for reviewing this document.

A complete inventory of UW Plant Disease Facts is available at the University of Wisconsin-Madison Plant Disease Diagnostics Clinic website. (https://pddc.wisc.edu)