Category Archives: Disease – Fruit Crop

Disease - Broad Leafed Woody Ornamental, Disease - Fruit Crop

Silver Leaf

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UW Plant Disease Facts

 

Authors:   Janet van Zoeren, UW-Madison Entomology, and Patricia McManus, UW-Madison Plant Pathology
Last Revised:   03/02/2024
D-number:   D0101

What is silver leaf?  

Silver leaf is a fungal disease that affects a wide range of deciduous trees.  The disease has its biggest impact in fruit trees such as apple, pear and cherry, but can also affect ornamental trees such as willow, poplar, maple, oak, and elm.  Silver leaf has traditionally been considered a disease of older trees that have been physically damaged or are in decline due to other diseases.  However, beginning in 2017, severe cases of silver leaf have been observed on young, healthy apple trees in commercial orchards in Wisconsin.

Young, vigorous high-density apple trees, with trees showing symptoms of silver leaf (on the right) adjacent to those that do not (on the left).
Young, vigorous high-density apple trees, with trees showing symptoms of silver leaf (on the right) adjacent to those that do not (on the left).

What does silver leaf look like? 

The first symptom of silver leaf is a silver sheen that appears on leaves of affected trees. The number of leaves affected can vary dramatically from tree to tree.  The silver sheen develops when the epidermis of a leaf (i.e., the surface layer of cells) separates from the rest of the leaf, altering the way that the leaf reflects light.  The silvery leaves may also have brown, dead patches.  Leaf symptoms may appear one year, but may be less severe or even nonexistent in subsequent years.

Note that other tree stresses (particularly environmental stresses) can cause leaf symptoms similar to those of silver leaf.  An additional symptom that can help in identifying silver leaf is dark staining just under the bark of branches with symptomatic leaves. This staining can extend several inches down a branch.  Eventually, white edged, purple-brown, shelf-like conks (reproductive structures of the fungus that causes the disease) will appear on branches and/or trunks of the diseased trees.

Where does silver leaf come from? 

Silver leaf is caused by the fungus Chondrostereum purpureum.  Spores of the fungus are released from conks during wet periods in the autumn and spring and infect trees at pruning scars or other open wounds (e.g., wounds from branches breaking during severe storms or due to heavy, wet spring snow).  The fungus lives in the xylem (i.e., the water-conducting tissue) of infected branches, and its presence in the xylem leads to the dark staining as described above.  A toxin released by the fungus moves up into the leaves causing the epidermis separation that leads to the silver sheen of the leaves.  Eventually, wood in infected branches begins to decay, at which point the fungus starts producing conks.

How do I save a tree with silver leaf?  

On trees with limited damage, prune out branches showing leaf symptoms.  Also watch for any conks, and immediately remove branches where these are present.  Removing conks limits production of spores that can lead to infections in other trees.  When pruning, cut branches at least four inches below where you can see staining under the bark or where conks are visible.  Decontaminate pruning tools after each cut by treating them for at least 30 seconds in 70% alcohol (e.g., rubbing alcohol, certain spray disinfectants), a commercial disinfectant or 10% bleach.  If you use bleach, be sure to thoroughly rinse and oil your tools after pruning to prevent rusting.

The silver sheen of leaves typical of silver leaf (left) and conks (i.e., reproductive structures) of the silver leaf fungus, Chondrostereum purpureum (right).
The silver sheen of leaves typical of silver leaf (left) and conks (i.e., reproductive structures) of the silver leaf fungus, Chondrostereum purpureum (right).

In plantings where silver leaf symptoms are widespread, pruning out all symptomatic branches may not be practical, and the loss of that many branches might cause more harm than good.  Also, trees sometimes show symptoms one year but then appear to recover in subsequent years.  Therefore, instead of pruning symptomatic branches, consider marking diseased trees.  Carefully watch the marked trees each year to see if symptoms reoccur or if the trees lose vigor.  If trees lose vigor and/or conks become visible, then the trees should be removed.  Because the silver leaf fungus limits water movement in infected branches, make sure that affected trees receive adequate water.  In general trees should receive approximately one inch of water per week during the growing season from natural rain and/or irrigation.  Eventually infected trees will likely decline to the point where they should be removed.  In some instances, monitoring trees may not be feasible.  In such situations, removing trees the first year that they show silver leaf symptoms may be the best management option.

Any branches or trunk sections removed from trees with silver leaf should be disposed of by burning (where allowed by local ordinance) or burying.

How do I avoid problems with silver leaf in the future? 

Whenever possible, prune trees during the winter during dry periods when temperatures are below 32°F.  If you must prune during the growing season, only prune during dry periods.  Pruning at these times will decrease the risk of infection by the silver leaf fungus through pruning wounds. DO NOT use pruning paints or sealants when pruning.  At this time, there are no fungicides for silver leaf control.

For more information on silver leaf: 

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:

© 2019-2024 the Board of Regents of the University of Wisconsin System doing business as University of Wisconsin-Madison Division of Extension.

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).

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 Lynn Adams, Annie Deutsch, and Bryan Jensen 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.

Submit additional lawn, landscape, and gardening questions at https://hort.extension.wisc.edu/ask-a-gardening-question/.

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

Gymnosporangium Rusts

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UW Plant Disease Facts

 

Authors:   Brian Hudelson, UW-Madison Plant Pathology
Last Revised:   03/01/2024
D-number:   D0058

What are Gymnosporangium rusts? 

Gymnosporangium rusts are a group of closely related diseases caused by fungi that infect both junipers (in particular red cedar) and woody plants in the rose family such as, but not limited to, apple, crabapple, hawthorn and quince.  These fungi must infect both types of plants to complete their life cycles.  The most common Gymnosporangium rusts found in Wisconsin are cedar-apple rust, cedar-hawthorn rust and cedar-quince rust.  The names of these diseases are somewhat misleading, given that all three diseases can affect multiple rosaceous hosts in addition to those referenced in their names.

Yellow cedar-apple rust spots on an apple leaf (left) and slimy, orange, gelatinous cedar-apple rust galls on a juniper branch (right).
Yellow cedar-apple rust spots on an apple leaf (left) and slimy, orange, gelatinous cedar-apple rust galls on a juniper branch (right).

What do Gymnosporangium rusts look like? 

On junipers, symptoms of Gymnosporagium rusts vary.  Cedar-hawthorn and cedar-apple rust fungi induce formation of irregularly-shaped brown galls, with cedar-hawthorn rust galls tending to be smaller in size (approximately 1/8 to 9/16 inch in diameter) than cedar-apple rust galls (approximately 1/4 to 2 inches in diameter).  Both types of galls produce distinctive slimy, orange, gelatinous appendages in the spring.  In contrast, the cedar-quince rust fungus causes juniper branch swellings.  Orange spores ooze from these swollen areas in the spring.

On rosaceous hosts, Gymnosporangium rust symptoms also vary.  Symptoms of cedar-hawthorn rust and cedar-apple rust appear in mid to late May, typically as circular, yellow-orange areas on leaves.  Eventually, tube-like structures (that have a fringe-like appearance) form on the undersides of leaves beneath the yellow spots.  Symptoms of cedar-quince rust typically become obvious later in the summer (most commonly on hawthorns) as swollen, spiny branches and/or fruits.

Where do Gymnosporangium rusts come from? 

Several fungi in the genus Gymnosporangium cause Gymnosporangium rusts. These include Gymnosporangium juniperi-virginianae (cedar-apple rust), Gymnosporangium globosum (cedar-hawthorn rust), and Gymnosporangium clavipes (cedar-quince rust).  These fungi overwinter in infected branches and galls on junipers.  Spores oozed from the infected branches or produced in the gelatinous gall appendages drift to rosaceous hosts leading to leaf and fruit infections.  Similarly, spores produced in the tube-like structures/spines on rosaceous leaves and fruits drift to junipers leading to new branch infections and additional gall formation.

How do I save a tree or shrub with Gymnosporangium rust? 

Gymnosporangium rusts are primarily cosmetic diseases that make susceptible plants unattractive, but rarely have long-term detrimental effects.  Gymnosporangium rusts on leaves can, for all practical purposes, be ignored.  Gymnosporangium rusts on juniper branches can be easily managed by pruning approximately four to six inches below swollen areas or galls.  Rosaceous hosts with infected branches can be pruned similarly.  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.

Cedar-quince rust on hawthorn fruit.
Cedar-quince rust on hawthorn fruit.

How do I avoid problems with Gymnosporangium rusts in the future? 

The best way to avoid Gymnosporangium rusts is to not grow junipers (particularly red cedar) and susceptible rosaceous hosts close to one another.  In urban settings where yards are small however, keeping both hosts adequately separated may be impossible.  Where Gymnosporangium rusts have consistently been a problem, consider using evergreens (e.g., pine, fir, spruce) and flowering trees and shrubs (e.g., cherry, plum, lilac) that are immune to these diseases.  If you decide that you want to mix junipers with apple, crabapple, hawthorn, and quince on your property, check at your local nursery for resistant varieties that will satisfy your landscaping needs.  In general, Chinese junipers (Juniperus chinensis) tend to be relatively resistant to Gymnosporangium rusts.

Fungicides treatments are also available to control Gymnosporangium rusts, although such treatments should be considered only as a last resort.  Among fungicides marketed for use by home gardeners, those containing chlorothalonil, copper, mancozeb, myclobutanil, propiconazole, and sulfur are labeled for use for Gymnosporangium rust control.  These products may be useful for controlling Gymnosporangium rusts on rosaceous hosts, but will likely not be effective if used on junipers.  For optimal control on rosaceous hosts, apply treatments when flower buds first show color, when half of the flowers are open, at petal-fall, seven to 10 days after petal fall and finally 10 to 14 days later.  Be sure to read and follow all label instructions of the fungicide(s) that you select to ensure that you use the products(s) in the safest and most effective manner possible.  In particular, be sure that you select appropriate products when treating trees and shrubs with edible fruit.  If you decide to use propiconazole or myclobutanil, alternate use of these active ingredients with use of at least one of the other active ingredients listed above (but DO NOT alternate propiconazole with myclobutanil) to help minimize potential problems with fungicide-resistant strains of Gymnosporangium rust fungi.

For more information on Gymnosporangium rusts: 

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:

© 1999-2024 the Board of Regents of the University of Wisconsin System doing business as University of Wisconsin-Madison Division of Extension.

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).

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 Laura Jull, Darrin Kimbler, Sharon Morrisey, Charlene Schmidt and Janet Van Zoeren 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.

Submit additional lawn, landscape, and gardening questions at https://hort.extension.wisc.edu/ask-a-gardening-question/.

Disease - Broad Leafed Woody Ornamental, Disease - Fruit Crop, Disease - Herbaceous Ornamental, Disease - Vegetable

Southern Blight

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UW Plant Disease Facts

 

Authors:   Ann Joy and Brian Hudelson, UW-Madison Plant Pathology
Last Revised:   03/02/2024
D-number:   D0103
 
The Southern blight fungus produces large numbers of spherical, light tan to dark red resting structures called sclerotia.
The Southern blight fungus produces large numbers of spherical, light tan to dark red resting structures called sclerotia.

What is Southern blight? 

Southern blight is a lethal fungal disease that is most common in the tropics and subtropics.  This disease causes damage in the southern United States and can even cause problems in temperate locations like Wisconsin during periods of warm, moist weather.  Southern blight has a wide host range, affecting over 500 plant species.  Vegetable and fruit hosts include tomato, pepper, onion, beet, rhubarb, strawberry, lettuce, cucumber, melon, carrot, asparagus and parsley.  Ornamental hosts include aster, black-eyed Susan, dahlia, daylily, gladiolus, hosta, impatiens, peony, petunia, rose, salvia, sedum and viola.  Small woody trees and shrubs can be affected as well.

What does Southern blight look like? 

Southern blight initially leads to a water-soaked appearance on lower leaves or water-soaked lesions (spots) on lower stems.  Any plant part that is near or in contact with the soil may become infected.  Infected plants yellow and wilt, often within days of infection, particularly when the weather is moist and warm (80 to 95°F).  Fruit rots, crown rots and root rots are also typical symptoms of the disease.  Thick mats of white fungal threads (called mycelia) may grow from infected tissue, radiating from the plant onto the soil surface.  Sclerotia (small spherical structures that are about the size of mustard seeds) develop on infected tissue and on the soil surface.  Sclerotia range in color from light tan to dark reddish-brown to black.

Where does Southern blight come from? 

Southern blight is caused by the fungus Athelia rolfsii (formerly Sclerotium rolfsii), which lives in soil, on plants (including weeds), and in plant debris.  The fungus can be spread through movement of infested soil and plant debris, on infected plants, in contaminated irrigation water, and through use of contaminated tools.  In Wisconsin, A. rolfsii most likely enters gardens on infected nursery stock or infested mulch.  Freezing temperatures will kill A. rolfsii mycelia, but sclerotia can survive temperatures as low as approximately 14ºF.

Southern blight can be a serious disease of vegetables, including tomatoes, leading to wilting and plant death. (Photo courtesy of Gary E. Vallad, University of Florida)
Southern blight can be a serious disease of vegetables, including tomatoes, leading to wilting and plant death. (Photo courtesy of Gary E. Vallad, University of Florida)

How can I save a plant with Southern blight? 

Identify the extent of an infestation based on visible dead/dying plants, fungal mycelia and sclerotia.  Remove all plants (including roots), as well as three inches of soil, from at least 12 inches beyond the infested area.  Start at the edge of the infested area and work toward the center.  Bag all plants and soil and dispose of these materials in a landfill.  Turn any remaining soil in the infested area eight to 12 inches to bury any sclerotia that you may have missed.  This will reduce the length of time that the sclerotia will survive.  Grow non-susceptible plants (e.g., larger woody ornamentals) in the affected area for two to three years to allow time for sclerotia to die naturally.  Fungicides containing azoles (e.g., propiconazole, tebuconazole), fludioxonil, flutolanil, mancozeb, PCNB, strobilurins (e.g., azoxystrobin, fluoxastrobin), thiophanate-methyl, and triadimefon are all labeled for Southern blight control, but may have varying levels of effectiveness.  All of these products will likely be more effective if applied as preventive treatments rather than in an attempt to “cure” existing disease.  If you decide to use fungicides, DO NOT use one active ingredient for all treatments.  Instead, alternate the use of two or more unrelated active ingredients to help minimize problems with fungicide-resistant strains of A. rolfsii.  DO NOT alternate active ingredients that are chemically related (e.g., propiconazole and tebuconazole, or azoxystrobin and fluoxastrobin).  Be sure to read and follow all label instructions of the fungicide(s) that you select to ensure that you use the product(s) in the safest and most effective manner possible.

How can I prevent Southern blight in the future? 

Inspect new plants for sclerotia and mycelia of A. rolfsii prior to transplanting.  Bag and dispose of diseased plants as described above.  Use high-quality mulches (e.g., shredded oak bark mulch, red cedar mulch) in your garden and avoid any mulches that you suspect might be contaminated with A. rolfsii.  A. rolfsii thrives under moist conditions.  Therefore, thin existing gardens or space plants farther apart in new gardens to improve airflow and promote more rapid drying of foliage and soil.

For more information on Southern blight: 

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:

© 2005-2024 the Board of Regents of the University of Wisconsin System doing business as University of Wisconsin-Madison Division of Extension.

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).

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 Lynn Adams, Bryan Jensen, Mark Kopecky, Kaitlyn Lance, Mike Maddox, and Ann Wied for reviewing this document. Thanks also to Mike Maddox for supplying 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.

Submit additional lawn, landscape, and gardening questions at https://hort.extension.wisc.edu/ask-a-gardening-question/.

Disease - Broad Leafed Woody Ornamental, Disease - Fruit Crop

Apple Scab

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UW Plant Disease Facts
 
Authors:   Brian Hudelson, UW-Madison Plant Pathology
Last Revised:   02/28/2024
D-number:   D0004

What is apple scab? 

Apple scab is a potentially serious fungal disease of ornamental and fruit trees in the rose family.  Trees that are most commonly and severely affected include crabapple, hawthorn, mountain-ash, apple and pear.  In ornamental trees, leaf loss caused by apple scab can make trees unsightly and aesthetically unappealing.  In fruit trees, leaf loss can reduce fruit yield.  In addition, the presence of apple scab on fruits can make the fruit difficult, if not impossible, to market.

Leaf spots typical of apple scab on apple. Typical apple scab leaf spots (left) and fruit lesions (right). The disease can cause total defoliation and make fruit unmarketable.
Leaf spots typical of apple scab on apple. Typical apple scab leaf spots (left) and fruit lesions (right). The disease can cause total defoliation and make fruit unmarketable.

What does apple scab look like? 

Apple scab lesions (diseased areas) are often first noticed on leaves, where they most commonly occur on the upper leaf surface.  Fruits are also very susceptible to infection.  Lesions on both leaves and fruits are roughly circular with feathery edges and have an olive green to black color.  Lesions can be as small as the size of a pinhead or as large a ½ inch in diameter.  When disease is severe, lesions can merge and cover a large portion of the leaf or fruit surface.  Defoliation of a tree (i.e., extensive leaf drop) often follows.

Where does apple scab come from? 

Scab is caused primarily by the fungus Venturia inaequalis.  Other species of Venturia can be involved in diseases similar to apple scab.  In particular, Venturia pirina causes a very similar disease (called pear scab) on pear.  Venturia inaequalis and its relatives survive the winter in leaf litter from infected trees.  Scab is most severe in cool, wet years.

How do I save a tree with apple scab? 

Apple scab is not a lethal disease, even when trees totally defoliate.  Once symptoms are visible, it is too late to treat a tree.  Proper long-term management of apple scab is important however.  If left unchecked, defoliation due to apple scab year after year can stress a tree and make it more susceptible to other, more serious and lethal diseases and insect pests.

How do I avoid problems with apple scab in the future?  

If your crabapple, apple or pear tree has a history of severe scab, consider replacing it with a resistant variety.  See UW Bulletins A2105 (Apple Cultivars for Wisconsin), A2488 (Home Fruit Cultivars for Northern Wisconsin), and A2582 (Home Fruit Cultivars for Southern Wisconsin) for recommendations.  These publications are all available at https://learningstore.extension.wisc.edu/.

If you have a susceptible tree that you want to maintain, carefully collect up and discard fallen, infected leaves each autumn.  These leaves are a major source of spores that can infect leaves the following growing season.  Also, be sure to routinely thin your trees to open up the canopy and allow better airflow.  Thinning will promote more rapid drying of leaves, which is less favorable for apple scab development.

Even with proper fall leaf clean up and thinning, you may have to consider applying fungicide treatments to susceptible trees, particularly when the weather is cool and wet.  Captan, chlorothalonil, mancozeb, myclobutanil, propiconazole, or thiophanate methyl, are available for apple scab control, although not all of these active ingredients can be used on trees where fruit will eventually be eaten.  Be sure to read and follow all label instructions of the fungicide(s) that you select to ensure that you use the correct fungicide(s) in the safest and most effective manner possible.  Typically, you will need to treat every seven to 14 days from bud break until wet weather subsides.  DO NOT use myclobutanil, propiconazole, or thiophanate methyl as the sole active ingredient for all treatments.  If you decide to use one of these active ingredients, alternate its use with at least one of the other active ingredients listed above to help minimize problems with fungicide-resistant strains of the apple and pear scab fungi.

For more information on apple scab: 

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:

© 1999-2024 the Board of Regents of the University of Wisconsin System doing business as University of Wisconsin-Madison Division of Extension.

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).

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 Paul Hartman, Laura Jull, Patti Nagai and Scott Reuss 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.

Submit additional lawn, landscape, and gardening questions at https://hort.extension.wisc.edu/ask-a-gardening-question/.

Disease - Fruit Crop, Disease - Herbaceous Ornamental, Disease - Vegetable

Cucumber Mosaic

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UW Plant Disease Facts

 

Authors:   Kymberly R. Draeger*, UW-Madison Plant Pathology
Last Revised:   02/29/2024
D-number:   D0036

What is cucumber mosaic? 

Cucumber mosaic is a viral disease of worldwide distribution that affects over 1200 plant species.  Hosts include a wide range of fruits, vegetables, herbaceous and woody ornamentals, and weeds.  The disease has perhaps its biggest impact in vegetable production where it can cause significant losses in yield and vegetable quality.

Cumber mosaic on pepper (left) showing yellowing and ring spots, and on broad bean (right) showing mosaic and puckering of leaf tissue. (Photos courtesy of Russ Groves)
Cucumber mosaic on pepper (left) showing yellowing and ring spots, and on broad bean (right) showing mosaic and puckering of leaf tissue. (Photos courtesy of Russ Groves)

What does cucumber mosaic look like? 

Symptoms of cucumber mosaic can vary widely depending on host species, host variety, and time of infection.  Typical symptoms include stunting of entire plants, mosaic or mottling (i.e., blotchy white, yellow, and light green areas) and ring spots (i.e., ring-like areas of discolored tissue) on leaves and fruits, and a variety of growth distortions such as cupping, puckering and strapping (i.e., elongation and thinning) of leaves as well as warts on fruits.  In extreme situations, parts of an affected plant or even an entire plant may die from the disease.

Where does cucumber mosaic come from? 

Cucumber mosaic is caused by Cucumber mosaic virus (CMV) which can overwinter in susceptible biennial or perennial weeds, as well as in perennial agricultural crops (e.g., alfalfa) and perennial herbaceous and woody ornamentals.  Seeds and even pollen from certain host plants can carry the virus, and thus the virus can be spread via these plant parts.  More commonly, CMV is spread by aphids [see the University of Wisconsin Garden Facts XHT1043 (Aphids) for details on these insect pests] which can pick up the virus from infected plants and transmit it to healthy plants as they feed.  Over 80 species of aphids can potentially transmit CMV.  The severity of cucumber mosaic oftentimes depends on the size and activity of aphid populations in an area, as well as on the number infected plants in an area serving as reservoirs for the virus.

Cumber mosaic on hibiscus (left) showing mosaic and puckered leaves, and on bluebell (right) showing mosaic and line patterns. (Photos courtesy of Brian Hudelson)
Cucumber mosaic on hibiscus (left) showing mosaic and puckered leaves, and on bluebell (right) showing mosaic and line patterns. (Photos courtesy of Brian Hudelson)

How do I save plants with cucumber mosaic? 

There is no known cure for cucumber mosaic.  Infected plants should be removed and destroyed to eliminate the plants as potential reservoirs for the virus (which can subsequently be spread to other nearby healthy plants).  Infected plants can be burned (where allowed by local ordinance), deep buried or hot composted.  Killing infected plants with herbicides can also be an effective management strategy.

How do I avoid problems with cucumber mosaic in the future? 

Buy certified, virus-free seeds and plants.  Consider using CMV-resistant varieties of lettuce, spinach, cucurbits (e.g., cucumber, melon and squash) and other vegetables where available.  Seed catalogs often contain information on CMV resistance that can be useful for variety selection.  Remove weed hosts whenever possible around your garden, and mulch vegetable and ornamental gardens to inhibit weed growth.  Consider using floating row covers where possible to prevent aphids from reaching susceptible plants.  DO NOT use insecticides to control aphids because such treatments are unlikely to act fast enough to prevent aphids from transmitting CMV, and may actually stimulate aphids to move and feed more widely, thus leading to increased spread of the virus.

For more information on cucumber mosaic: 

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:

* Completed as partial fulfillment of the requirements for Plant Pathology 558 at the University of Wisconsin Madison.

© 2016-2024 the Board of Regents of the University of Wisconsin System doing business as University of Wisconsin-Madison Division of Extension.

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).

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 Russ Groves, Krista Hamilton, Michelle Keller-Pearson, Tomas Rush, Adam Snippen and Trisha Wagner 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.

Submit additional lawn, landscape, and gardening questions at https://hort.extension.wisc.edu/ask-a-gardening-question/.

Disease - Fruit Crop

Common Leaf Spot of Strawberry

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UW Plant Disease Facts

 

Authors:   John Hammel*, UW-Madison Plant Pathology
Last Revised:   02/29/2024
D-number:   D0032

What is common leaf spot? 

Common leaf spot of strawberry (also known as Mycosphaerella leaf spot, Ramularia leaf spot, strawberry leaf spot, bird’s-eye spot, gray spotness, and white spot) is a common fungal leaf disease that affects both wild and cultivated strawberries throughout the world.  Common leaf spot was once the most economically important strawberry disease, but the use of resistant strawberry varieties/cultivars and improvements in methods for growing strawberries have been effective in managing the disease and reducing its impact.  Today, the disease is often a cosmetic problem and typically has little impact on yield or fruit quality.

Typical common leaf spot symptoms: tan to gray spots with reddish purple margins. (Photo courtesy of Dr. Patricia McManus, Department of Plant Pathology, University of Wisconsin)
Typical common leaf spot symptoms: tan to gray spots with reddish purple margins. (Photo courtesy of Dr. Patricia McManus, Department of Plant Pathology, University of Wisconsin)

What does common leaf spot look like? 

Symptoms of common leaf spot can occur on leaves, fruits, berry caps, petioles, and runners.  The most noticeable symptoms of the disease are small, round, necrotic (i.e., dead) spots on strawberry leaves.  Initially, these spots develop on the upper leaf surface and are deep purple to red in color.  The spots eventually develop tan, gray or almost white centers with distinct reddish-purple to brown borders.  During warm, humid weather, uniformly rusty-brown spots without purple margins or light colored centers may develop instead.  Spots can occur on the undersides of the leaves as well, but these spots tend to be less vibrant in color.  As the disease progresses, spots enlarge to ⅛ to ¼ inch in diameter and may merge together, in extreme cases leading to leaf death.  Spots on berry caps, petioles, and runners resemble those produced on upper leaf surfaces.  Shallow, black spots (¼ inch in diameter) may develop on infected fruits, and are often surrounded by brown or black, leathery tissue.

Where does common leaf spot come from? 

Common leaf spot is caused by the fungus Mycospharella fragariae, which can enter a garden on infected strawberry plants or via windblown spores from nearby strawberries.  Once introduced into a garden, the fungus is spread predominantly by splashing water from rain or sprinklers used for watering.  M. fragariae is most active when temperatures range from 65°F to 75°F, with periods of high rainfall and humidity.  M. fragariae survives the winter on dead strawberry leaves and other plant parts, and it is moved to new foliage in the spring by early season rains.

How do I save strawberry plants with common leaf spot? 

Once common leaf spot develops on strawberry plants, the plants cannot be cured.  If the disease is detected early, its development may be slowed using fungicides.  Keep in mind however, that common leaf spot is often merely a cosmetic issue and the use of fungicides may not be warranted.  If you decide that fungicide treatments are needed, select a product that is labeled for use on strawberries and that contains captan, myclobutanil or copper as the active ingredient.  Use copper-containing fungicides only prior to flowering.  If you decide to use a myclobutanil-containing product, alternate applications of this product with applications of a second fungicide containing another active ingredient.  This will help prevent selection of myclobutanil-resistant variants of the common leaf spot pathogen.  Be sure to read and follow all instructions on the label(s) of the fungicide(s) that you select to ensure that you use the product(s) in the safest and most effective manner possible.

How can I prevent common leaf spot in the future? 

When establishing your strawberry patch, consider planting resistant strawberry varieties and use certified, disease-free nursery stock.  Examples of resistant varieties include ‘Crimson King’, ‘Earliglow’, ‘Glooscap’, ‘Ogallala’, and ‘Ozark Beauty’.  Plant strawberries in full sunlight, in well-drained soils, and with proper spacing to optimize air circulation and create a drier environment that is less favorable for the common leaf spot pathogen.  See UW Bulletin A1597 (Growing Strawberries in Wisconsin), available at https://learningstore.extension.wisc.edu/, for additional details on proper planting.

Once plants are in the ground, avoid overhead watering (i.e., DO NOT use a sprinkler) as this will splash the common leaf spot pathogen from plant to plant, and provide a wet environment that is more favorable for the fungus to infect.  Instead, use a drip or soaker hose for watering.  For similar reasons, DO NOT work in your strawberry patch (e.g., weeding, thinning plants or harvesting fruit) when it is wet; wait until the patch is dry.

For June-bearing strawberries, bed renovation techniques (in particular mowing) can be useful in managing common leaf spot.  See UW Bulletin A1597 (mentioned above) for details on proper renovation techniques.  At the end of the growing season, remove strawberry plant debris to minimize sites where the fungus can survive the winter.  Deep bury, burn (where allowed by local ordinance) or hot compost this material.

For more information on common leaf spot of strawberry: 

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:

* Completed as partial fulfillment of the requirements for Plant Pathology 558 at the University of Wisconsin Madison.

© 2016-2024 the Board of Regents of the University of Wisconsin System doing business as University of Wisconsin-Madison Division of Extension.

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 Bill Hemp, Patty McManus, Michael Meade, Quinn Paszkiewicz, Scott Reuss and Trisha Wagner 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.

Submit additional lawn, landscape, and gardening questions at https://hort.extension.wisc.edu/ask-a-gardening-question/.

Disease - Fruit Crop

Cane Blight

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UW Plant Disease Facts

 

Authors:   David S. Jones*, UW-Madison Plant Pathology
Last Revised:   02/29/2024
D-number:   D0028

What is cane blight? 

Cane blight is a fungal disease that affects the health of canes (i.e., stems) of cultivated and wild Rubus species (e.g., raspberries and blackberries), wherever they are grown.  Black and purple raspberries appear to be more susceptible to cane blight than red raspberries, but all commonly cultivated raspberry cultivars can get the disease.  Although cane blight is not typically fatal, it may cause significant fruit yield losses if left unmanaged.

Cane death on a thornless blackberry caused by cane blight. Look for a dark brown infection line and dead/dying shoots above the point of infection. (Photo courtesy of Michael Ellis, The Ohio State University)
Cane death on a thornless blackberry caused by cane blight. Look for a dark brown infection line and dead/dying shoots above the point of infection. (Photo courtesy of Michael Ellis, The Ohio State University)

What does cane blight look like? 

Symptoms of cane blight usually first appear in early summer after blossoming and leaf emergence, and in association with wounds caused by pruning or harvesting of fruit.  Look for sudden death of side branches and tips of fruit-bearing canes, as well as dark brown or purple spots (called cankers) on the canes below the dieback.  In wet weather, cankers may produce a grey ooze.  In dry weather, the cankers may appear fuzzy or powdery.  Dead canes may become brittle and snap off in windy conditions.

Where does cane blight come from? 

Cane blight is caused by the fungus Leptosphaeria coniothyrium which survives the winter in infected canes.  During wet periods, the fungus produces windborne spores and infects through open wounds on canes caused by pruning, harvest damage, insect damage, or abrasions from canes rubbing together.  As the disease develops, the fungus produces additional spores that can spread to other wounds by wind and splashing water, leading to additional infections.

How do I save plants with cane blight?  

DO NOT prune infected canes during the growing season as pruning wounds will provide the cane blight fungus easy entry into healthy tissue.  Label symptomatic canes as you see them and prune the canes to the ground during the dormant season (i.e., mid- to late winter).  Also prune any older cane stubs at this time to remove them as a source of abrasion and wounding for newer canes.  Pruning when plants are dormant allows ample time for wounds to close at a time of year when spores of the cane blight fungus are not being produced.  Use only sharp tools for pruning, and disinfest pruning tools after each cut by treating them for at least 30 seconds with 10% bleach or preferably (due to its less corrosive properties) 70% alcohol (e.g., rubbing alcohol or certain spray disinfectants).  If you use bleach, be sure to thoroughly rinse and oil your tools after pruning to prevent rusting.  Dispose of any canes that you prune by burning them (where allowed by local ordinance), burying them, or arranging to have them hauled away through municipal brush collection.  Be patient as you attempt to get cane blight under control; it may take two or more years of pruning and good sanitation to reduce cane blight to negligible levels.

How can I prevent cane blight in the future? 

When establishing a new raspberry patch, choose a site that is well-drained and sunny, and make sure that the distance between rows is approximately 18 inches.  Also be sure to keep weeds under control.  Proper site selection, row spacing and weed control will promote good airflow and drainage, and will reduce excessive moisture that is favorable for spore production by the cane blight fungus.

In addition, maintain optimum soil fertility.  If you underfertilize plants, they will produce weak canes that are less able to fend off infections by the cane blight fungus.  If you overfertilize plants (particularly with nitrogen), they will produce succulent new growth that is more prone to breakage and more prone to wounding by certain insects that will use the new growth as food.  Remember that wounds of any kind can provide entry points for the cane blight fungus.  For details on properly fertilizing raspberries, see UW Bulletin A1610, Growing Raspberries in Wisconsin (available at https://learningstore.extension.wisc.edu/).

For more information on cane blight: 

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:

*Completed as partial fulfillment of the requirements for Plant Pathology 558 at the University of Wisconsin Madison.

© 2015-2024 the Board of Regents of the University of Wisconsin System doing business as University of Wisconsin-Madison Division of Extension.

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).

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 Anne Donnellan, Kris Gabert, Abigail Jones, Patty McManus, Craig Schreiner, and Lisa Schreiner 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.

Submit additional lawn, landscape, and gardening questions at https://hort.extension.wisc.edu/ask-a-gardening-question/.

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

Wood Mulch and Tree Health

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UW Plant Disease Facts

 

Authors:   Jane Cummings-Carlson and John Kyhl, Wisconsin Department of Natural Resources Gina Foreman and Brian Hudelson, UW-Madison Plant Pathology
Last Revised:  03/02/2024
D-Number:   D0128

What are the benefits of wood mulch? 

Wood mulch is typically available as chipped wood, or shredded or chunked bark, and can contribute to tree health in many ways.  When high quality, composted mulches are applied two to four inches deep in a ring three to six feet in diameter (or greater) from the trunk of a tree, mulch can help preserve moisture, control weeds, limit damage to the trunk from mowers and string trimmers and moderate the soil temperature.  Use four inches of mulch when soils are light and well-drained, and two inches of mulch on heavier, clay soils.

Use of properly composted mulches can be beneficial to trees and shrubs in the landscape.
Use of properly composted mulches can be beneficial to trees and shrubs in the landscape.

Can wood mulch harm trees? 

Use of improperly composted mulches (some-times called “sour mulches”), can lead to tree nutrient deficiencies.  Sour mulches can also produce gases like methane and ammonia that can be toxic to plants.  Foliage on trees surrounded by sour mulches may initially turn yellow, then brown, die and fall off.  If your mulch smells like vinegar, ammonia or sulfur, it is likely a sour mulch and should be removed.  Replace the sour mulch with a high quality, composted mulch and consult with your local Extension agent about testing the soil for nutrient deficiencies.  Fertilize appropriately based on the results of these tests.

Improper application of mulch can also lead to problems.  Piling wood mulch up against the trunk of a tree can keep the bark underneath excessively wet.  This wetness can contribute to bark decay.  In addition, use of thick mulch layers (greater than four inches) can lead to overly wet soils that are favorable for development of root rots (see UW Plant Disease Facts D0094, Root and Crown Rot).  To avoid these problems, make sure mulch is applied at least four inches away from the trunk of a tree and that the mulch layer is the appropriate thickness for the soil type in your landscape (see above).

Does woody mulch harbor or attract insects? 

Insects such as earwigs [see UW Bulletin A3640, Controlling Earwigs (available at https://learningstore.extension.wisc.edu/)], centipedes (see University of Wisconsin Garden Facts XHT1113, Centipedes), millipedes (see University of Wisconsin Garden Facts XHT1108, Millipedes) and sowbugs (see University of Wisconsin Garden Facts XHT1110, Sowbugs) can feed on decaying organic matter in mulches.  While these insects are often only nuisances, earwigs can feed on and cause damage to a variety of ornamentals, particularly to flowering plants.  If mulch is used near entrances to a home or around basement windows, these unwanted insects may get inside.  Termites ingest wood and can be attracted to wood mulch, but new termite colonies are not likely to become established due to use of wood mulches.  Typically, termites are not a problem in Wisconsin, and when colonies are found, they occur only in the southern half of the state.

Carpenter ants [see UW Bulletin A3641, Controlling Carpenter Ants (available at https://learningstore.extension.wisc.edu/)] and powderpost beetles (see University of Wisconsin Garden Facts XHT1053, Powderpost Beetles) are unlikely to utilize mulch as a food source because conditions required for their development would not be satisfied by wood mulch.  Carpenter ants do not ingest wood as a food source; instead, they chew non-living wood (in trees or landscape timbers, etc.) to excavate galleries in which they live and raise their young.  Since wood mulch is composed of small wooden pieces, it would not serve as a home.  To avoid potential insect problems, keep mulch as far away from the foundation of your home as possible and seal all holes and crevices that insects might use as entry points.  Also, periodically inspect landscape timbers and the house for termites.

Does woody mulch harbor tree pathogens? 

Wood mulch may come from many sources, including trees and shrubs that have died from a wide range of diseases.  To be harmful to your trees, disease-causing organisms (pathogens) would have to survive in mulch and these organisms would have to move from the mulch either directly, or through the soil, to their new host – your tree.  There is currently very little research on this topic.

Elm trees killed by Dutch elm disease (see UW Plant Disease Facts D0045, Dutch Elm Disease), can serve as breeding areas for native and European elm bark beetles.  Bark beetles that breed in logs or firewood from these trees can pick up the fungi that cause Dutch elm disease (Ophiostoma ulmi and Ophiostoma novo-ulmi) and carry these fungi from tree to tree.  Chipping infected elm trees creates an unfavorable environment for bark beetles yet there is no scientific literature that describes the level of risk of transmitting the Dutch elm disease fungi from wood chips or bark chunks to healthy elms.

Oak trees killed by oak wilt (see UW Plant Disease Facts D0075, Oak Wilt) can be attractive to several sap-feeding beetles that can potentially pick up the oak wilt fungus (Bretziella fagacearum) and move it in the landscape.  This process is affected by moisture and temperature and would likely be disrupted by the chipping and composting process yet there is no scientific literature that describes the level of risk of transmitting the oak wilt disease fungus from wood chips or bark chunks to healthy oaks.

Research at the University of Wisconsin-Madison suggests that wood chip mulches produced from trees suffering from Verticillium wilt (see UW Plant Disease Facts D0121, Verticillium Wilt of Trees and Shrubs) can serve as a source of the fungus (Verticillium dahliae) that causes the disease.  These studies show that Verticillium can survive for at least one year in mulch and that use of this contaminated mulch can lead to Verticillium wilt in both woody and herbaceous plants.  Therefore use of mulches produced from trees with Verticillium wilt should be avoided.

For more information on wood mulch and tree health:  

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:

© 2003-2024 the Board of Regents of the University of Wisconsin System doing business as University of Wisconsin-Madison Division of Extension.

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 Ann Joy, Laura Jull and Phil Pellitteri 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.

Submit additional lawn, landscape, and gardening questions at https://hort.extension.wisc.edu/ask-a-gardening-question/.

Disease - Fruit Crop, Disease - Needled Woody Ornamental

White Pine Blister Rust

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UW Plant Disease Facts

 

Authors:   Joseph Schwartz, UW-Madison Forest Ecology and Management and Glen R. Stanosz, Ph. D., UW Madison Plant Pathology
Last Revised:   03/02/2024
D-number:   D0126

What is white pine blister rust? 

White pine blister rust is a serious, tree-killing disease of eastern white pine and its close relatives (pines with needles in bundles of five).  This disease is caused by the exotic (non-native) fungus Cronartium ribicola, which was introduced into North America in the early 1900’s.  This pathogen is now found in most regions where pines grow in the United States, including Wisconsin.  White pine blister rust affects trees of all ages, but perhaps most frequently and severely damages seedlings and saplings.

White pine blister rust cankers on branches and stems often have a cracked or blistered appearance and may bear yellow to orange spores in fruiting bodies of Cronartium ribicola.
White pine blister rust cankers on branches and stems often have a cracked or blistered appearance and may bear yellow to orange spores in fruiting bodies of Cronartium ribicola.

Where does white pine blister rust come from? 

White pine blister rust is caused by the fungus Cronartium ribicola, which requires two different plant hosts to complete its complex life cycle.  Spores of the fungus produced on white pine are blown to alternate host plants in the genus Ribes (gooseberries and currants).  After infecting gooseberry and currant bushes the fungus multiplies rapidly throughout the summer, repeatedly producing spores that germinate to result in additional gooseberry and currant infections.  Spores produced in late summer on gooseberries and currants are spread by the wind to white pines, where needles that are moist from rain, fog, or dew are infected.

What does white pine blister rust look like? 

Infection of needles by Cronartium ribicola results in development of yellow to brown spots and bands.  The fungus slowly grows through pine needles and bark to eventually form cankers on twigs, branches, and trunks of trees.  A canker is a localized diseased area (either swollen or sunken) that is surrounded by healthy tissues.  A typical white pine blister rust canker has resinous margins, and may appear “blistered” before rupturing to expose fruiting bodies (reproductive structures) with yellow to bright orange spores of Cronartium ribicola.  As a canker expands to completely encircle stems, all parts of the plant beyond the canker are killed.  Dead white pine branches may temporarily retain their orange to red dead needles to form a bright “flag” that is a common symptom of this disease.

Can I save a tree affected by white pine blister rust? 

There is no “cure” for a tree with a white pine blister rust canker on its main trunk or stem.  As the canker expands to completely encircle the trunk, all parts above the canker will die.  Branch cankers that extend to within four inches of the trunk, probably indicate that the trunk already is colonized by the fungus and future development of a main stem canker is likely.  Pruning off branches on which cankers are located farther from the trunk, however, can prevent the fungus from growing into the trunk.  These branch cankers can be removed by pruning at least six inches beyond any visible symptoms on the trunk side of the cankered branch.  Removal of branch cankers and trees with main stem cankers prevents production of spores that are spread to gooseberries and currants.  The fungus quickly dies in discarded branches and trunks and presents no further danger to either of its hosts.

How do I avoid future problems from white pine blister rust? 

Infection of pine is most frequent near diseased gooseberries or currants.  Therefore, removing these plants (especially within 200 feet) can greatly reduce the incidence of white pine blister rust.  Gooseberries or currants can be killed by uprooting them or by application of herbicides in accordance with label directions.

Conditions that promote or prolong needle wetness should be avoided.  These include use of sprinklers that wet the needles of ornamental trees, dense plantings, growth of weeds surrounding young trees, or planting white pines in low-lying, chronically moist areas near water or sites that lack good air movement.  Growing white pines under a hardwood overstory that will intercept evening dew can help keep needles dry and reduce infection.

Regular inspection allows prompt detection and removal of cankered branches on young trees.  Also, because infection very often occurs on needles of low branches, pruning to remove healthy branches (if aesthetically acceptable) will reduce the likelihood of this disease.  When trees are five to seven years old, starting close to the ground, prune off all branches up to no more than one-third to one-half the height of the tree.  Branch pruning can be continued every other year until the lower eight to nine feet of trunk is free of branches.

Planting or measures to encourage natural regeneration of conifer species other than white pines (including red or jack pines, spruces, firs, arborvitae, hemlock, and junipers) might be considered, especially where gooseberries or currants are prevalent and where moisture conditions favor infection.

Application of protectant fungicides may be a useful management practice in white pine production nurseries.  Nursery stock should be carefully inspected, particularly just prior to sale, because the planting of diseased white pine seedlings is one way Cronartium ribicola has been spread into previously disease-free locations.

For more information on white pine blister 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:

© 2002-2024 Glen Stanosz All Rights Reserved.

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).

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 Ann Joy and Brian Hudelson 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.

Submit additional lawn, landscape, and gardening questions at https://hort.extension.wisc.edu/ask-a-gardening-question/.

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

Root and Crown Rots

Extension Logo

UW Plant Disease Facts

 

Authors:   Brian Hudelson, UW-Madison Plant Pathology and Laura Jull, UW-Madison Horticulture
Last Revised:   03/01/2024
D-number:   D0094
 
Discoloration of maple crown and roots typical of Phytophthora root/crown rot.
Discoloration of maple crown and roots typical of Phytophthora root/crown rot.

What is root/crown rot? 

Root/crown rot is a general term that describes any disease of woody ornamentals where the pathogen (causal organism) attacks and leads to the deterioration of a plant’s root system and/or lower trunk or branches near the soil line.  Root rots can be chronic diseases or, more commonly, are acute and can lead to the death of the plant.

What does root/crown rot look like? 

Gardeners often become aware root/crown rot when they see above ground symptoms.  Affected plants are often slow-growing or stunted and may show signs of wilting.  Often the canopy of an affected tree or shrub is thin, with foliage that is yellow or red, suggesting a nutrient deficiency.  Careful examination of the roots/crowns of these plants reveals tissue that is soft and brown.

Where does root/crown rot come from? 

Several soil-borne water molds (i.e., fungi-like organisms) and true fungi can cause root/crown rots, including (most frequently) Phytophthora spp. and Pythium spp. (both water molds), and Rhizoctonia solani and Fusarium spp. (both true fungi).  These organisms have wide host ranges, and prefer wet soil conditions.  Water mold root rot organisms such as Pythium and Phytophtora produce thick-walled spores (called oospores) that can survive for long periods (years to decades) in soil.

How do I save a plant with root/crown rot?  

REDUCE SOIL MOISTURE!  Provide enough water to fulfill a plant’s growth needs and prevent drought stress, but DO NOT over-water.  Remove excess mulch (greater than four inches) around trees and shrubs.  Excessive mulch can lead to overly wet soils.

A thinning canopy with red or yellow leaves can indicate a root/crown rot problem.
A thinning canopy with red or yellow leaves can indicate a root/crown rot problem.

Chemical fungicides (e.g., PCNB, mefenoxam, metalaxyl, etridiazole, thiophanate-methyl and propiconazole) and biological control agents (e.g., Gliocladium, Streptomyces, and Trichoderma) are labeled for root/crown rot control.  However, DO NOT use these products unless you know exactly which root/crown rot pathogen(s) is(are) affecting your trees and shrubs.  Contact your county Extension agent for details on obtaining an accurate root/crown rot diagnosis and for advice on which, if any, fungicides you should consider using.

How do I avoid problems with root/crown rots?  

Buy plants from a reputable source, and make sure they are root/crown rot-free prior to purchase.  Establish healthy plants in a well-drained site, and when planting, place the root collar just at the soil surface.  To moderate soil moisture, add organic material (e.g., leaf litter or compost) to heavy soils to increase soil drainage, and DO NOT over-water.  Also, DO NOT apply more than three inches of mulch around trees and shrubs, and keep mulch from directly contacting the base of trunks and stems.  Prevent physical damage (e.g., lawnmower injury) that can provide entry points for root/crown rot pathogens.  Finally, minimize movement of root/crown rot fungi in your garden.  DO NOT move soil or plants from areas where plants are having root/crown rot problems.  DO NOT water plants with water contaminated with soil (and thus potentially with root/crown rot organisms).  After working with plants with root/crown rot, decontaminate tools and footwear by treating for at least 30 seconds with a 10% bleach solution or 70% alcohol (e.g., rubbing alcohol, certain spray disinfectants).  If you use bleach to decontaminate metal tools, be sure to thoroughly rinse and oil your tools after you are done gardening to prevent rusting.

For more information on root/crown rots: 

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:

© 2000-2024 the Board of Regents of the University of Wisconsin System doing business as University of Wisconsin-Madison Division of Extension.

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).

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 Karen Delahaut, Ann Joy and Sharon Morrisey 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.

Submit additional lawn, landscape, and gardening questions at https://hort.extension.wisc.edu/ask-a-gardening-question/.