Category Archives: Disease – Fruit Crop

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

Dead Man’s Fingers

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UW Plant Disease Facts

 

Authors:   Ann Joy and Brian Hudelson, UW-Madison Plant Pathology
Last Revised:   02/29/2024
D-number:   D0040

What is dead man’s fingers? 

Dead man’s fingers are mushroom-like fungal growths that can be found at the base of dead or dying trees and shrubs, as well as at the base of wood objects (e.g., wood barrels) that are in contact with soil.  Some types of dead man’s fingers are produced by wood-decomposing fungi.  Others are produced by fungi that cause black root rot.  This disease typically is a problem on stressed trees or shrubs, including apple, crabapple, pear, cherry, plum, American elm, Norway maple and honeylocust.

Dead man’s fingers growing from mulch next to a barrel planter. (Photo courtesy of Dick Becker)
Dead man’s fingers growing from mulch next to a barrel planter. (Photo courtesy of Dick Becker)

What does dead man’s fingers look like? 

The most recognizable dead man’s fingers are those that are black and club-shaped with a white interior, appearing as solitary or clustered irregularly-shaped “fingers” that are approximately 1½ to 4 inches tall.  They form on or near dead or dying wood.  In the spring, as the “fingers” first form, they may be pale blue with white tips.  Disease-causing species of the fungus initially form a pale sheath around roots that later becomes black and crusty, hiding a lighter interior.  This sheath/crust is sometimes visible when soil is brushed away from tree/shrub roots.  Trees/shrubs with above-ground symptoms of infection may show decline, dieback, slowed growth, and basal cankers.  Infected apple trees may produce an abnormally large crop of smaller than normal-sized fruits.

Where does dead man’s fingers come from? 

Dead man’s fingers are sexual reproductive structures of fungi in the genus Xylaria.  Sexual spores (called ascospores) are produced inside each club-like “finger” and released through a tiny hole in the top.  The “fingers” can release these spores for several months or years.  In the spring, Xylaria can produce asexual spores (called conidia) anywhere on its surface.  Xylaria also produces threadlike structures (called hyphae) that grow through dead or dying wood.  Xylaria can survive as hyphae in roots for up to 10 years, and can spread from plant to plant via hyphae when plant roots come in contact with each other.

How can I save a tree with dead man’s fingers? 

In urban settings, dead man’s fingers may grow from wood mulch and may not be an indication of disease.  Simply remove and discard the “fingers” if you find them unsightly.  If dead man’s fingers form around or near the base of an apple, crabapple or other known susceptible host, the fungus may be infecting the tree, causing black root rot.  In this situation, by the time the characteristic “fingers” appear, the infection is well advanced.  An infected tree should be carefully removed, including the stump and as much of the root system as possible.  DO NOT use wood from Xylaria-infected trees for mulch.  There are no fungicides registered in Wisconsin for treatment of black root rot.

How can I prevent dead man’s fingers from being a problem in the future? 

DO NOT plant susceptible trees or shrubs in a site where dead man’s fingers has been observed.  In addition, make sure that susceptible trees/shrubs in other locations are well watered, fertilized, mulched and otherwise maintained to reduce stresses that might predispose them to infection by disease-causing species of Xylaria.  Unfortunately, Xylaria-resistant tree and shrub varieties/cultivars are not available.

For more information on dead man’s fingers: 

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:

© 2011-2024 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 Jane Anklam, Mark Kopecky and Judy Reith – Rozelle 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

Crown Gall

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UW Plant Disease Facts

 

Authors:   Ann Joy and Brian Hudelson, UW-Madison Plant Pathology
Last Revised:   02/29/2024
D-number:   D0035

What is a crown gall? 

Crown gall is the most widely distributed bacterial disease of plants in the world, affecting over 100 species of fruit crops, and woody and herbaceous ornamentals, including rose, euonymus, lilac, poplar, viburnum, willow, apple, pear, brambles, stone fruits and grapes.  Crown gall can cause severe damage on young plants, while mature woody plants with the disease may show no ill effects.

Crown gall leads to tumor-like growths that form at or near the soil line.
Crown gall leads to tumor-like growths that form at or near the soil line.

What does crown gall look like? 

Crown gall gets its name from the round or irregularly shaped tumor-like growths (i.e., galls) that usually form on plant crowns just above or below the soil line.  Galls can also form on roots, stems, trunks, or branches.  Galls can be pea-sized, or as large as several inches in diameter.  Galls interfere with water transport within the plant.  Therefore, affected plants may suffer from water or nutrient deficiencies, becoming stunted.  Flower and fruit production may also be reduced.

Where does crown gall come from? 

Crown gall is caused primarily by the bacteria Agrobacterium tumefaciens, as well as (on grape) by A. vitis.  Both bacteria survive in soil and in (or on) susceptible plant roots.  The bacterium is spread through movement of contaminated soil, water and infected plant material.  The bacterium enters plants through wounds (e.g., mechanical injuries including pruning cuts, freeze injury, and nematode feeding sites) or natural plant openings (e.g., lenticels) and stimulates plant cells to undergo unregulated growth, leading to gall formation.  Once galls begin to form, they can continue to enlarge even if the bacterium is no longer present.  Galls become visible anywhere from several weeks to one or more years after the time of infection.

How do I save a plant with crown gall? 

There is no cure for crown gall once galls begin to form.  Galls can be pruned away, but new galls may reform elsewhere on the plant.  To prevent spread of the crown gall bacterium, remove infected plants, surrounding soil, and as many of the infected plant’s roots as possible.

How do I avoid problems with crown gall in the future? 

Use disease-free, winter hardy stock from a reputable nursery and inspect the roots and crowns for galls before planting.  Avoid wounding plants during transplant and cultivation.  Decontaminate tools, equipment and shoes with 10% bleach or 70% alcohol for at least 30 seconds to prevent spread of the bacterium.  Use of a biological control product can prevent A. tumefaciens infections at the time of planting.  Current biological control products contain A. radiobacter, a close relative of the crown gall organism.  These products are not effective on all hosts, or against all variants of the pathogen.  In areas with infested soil, rotation to a non-susceptible plant (such as grass) for 3 years, may provide a good means of disease control.  In commercial settings, soil fumigation may provide limited control of the crown gall bacterium in soil.  However, fumigation does not kill the bacterium in roots that remain in the soil after removal of infected plants.  Fumigation is not recommended for homeowners.

For more information on crown gall: 

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 Diana Alfuth, Paul Hartman, and Patty McManus 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

Brown Rot

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UW Plant Disease Facts

 

Authors:   Ashley Ellinghuysen*, UW-Madison Plant Pathology
Last Revised:   02/28/2024
D-number:   D0024

What is brown rot? 

Brown rot is a destructive fungal disease of trees and shrubs in the genus Prunus, which includes peaches, plum, cherries, apricots and nectarines.  Brown rot is particularly a problem on the fruits of susceptible plants, with the potential to cause losses of 50% or more prior to harvest.  After harvest, additional losses due to the disease are possible if fruits are injured, bruised or stored at warm temperatures with moisture.

The presence of powdery gray masses on the surface of rotting fruit is characteristic of brown rot. (Photo courtesy of Wayne Griffiths)
The presence of powdery gray masses on the surface of rotting fruit is characteristic of brown rot. (Photo courtesy of Wayne Griffiths)

What does brown rot look like? 

Initial symptoms of brown rot often occur in the spring as brown spots on blossoms.  Affected blossoms eventually collapse completely, and can produce a gummy material that sticks to twigs leading to infections and subsequent twig dieback.  Fruits that develop from healthy flowers can become infected as they mature leading to a brown fruit rot that quickly encompasses an entire fruit.  Eventually, affected fruits will dry and shrivel to form “mummies”.  Characteristic powdery, gray masses of spores form on the surfaces of both rotting fruits and mummies.

Where does brown rot come from? 

Brown rot is caused by two fungi in the genus Monilinia (primarily M. fructicola and less commonly M. laxa).  These fungi may be introduced into a garden via airborne spores produced on nearby wild or volunteer Prunus trees and shrubs.  Insects such as sap beetles, vinegar flies and honeybees can also transport spores.  These insects are attracted to brown rotted fruit and can subsequently visit and drop off spores on otherwise healthy fruit.  Wounds due to insect feeding or hail can provide an entry point into fruits for brown rot fungi.  Further spread can occur when infected and healthy fruits touch.  Once introduced into a garden, brown rot fungi can overwinter on infected twigs and in mummified fruits that are hanging from trees or have fallen to the ground.  Initial infections each spring are typically due to spores that are blown or splashed from twigs or from the gray masses on mummified fruits.  More rarely, mummified fruits that are partially or shallowly buried in the ground will produce small (up to 1/16 inch diameter) mushroom-like structures called apothecia.  Apothecia produce a second type of spore that can cause infections.  Brown rot can occur under a wide range of temperatures (40 to 86°F), but tends to be more of a problem when the weather is warm (i.e., 68 to 77°F) and wet (i.e., with three or more hours of rain or dew formation).

How do I save a tree with brown rot? 

Brown rot is not a lethal disease, but once fruits are infected, there are no curative treatments.  To manage twig infections, prune four to six inches below sunken or dead tissue on each branch.  Dispose of these branches by burning (where allowed by local ordinance) or burying them.  To prevent spread of brown rot fungi on pruning tools, decontaminate tools between each cut by treating them for at least 30 seconds in a 10% bleach solution or preferably (due to its less corrosive properties) 70% alcohol.  Rubbing alcohol and many spray disinfectants contain approximately 70% alcohol and are easy to use.  If you use bleach, be sure to thoroughly rinse and oil tools after pruning to prevent rusting.

Fruits with brown rot eventually shrivel and dry forming a structure called a mummy. (Photo courtesy of Wayne Griffiths)
Fruits with brown rot eventually shrivel and dry forming a structure called a mummy. (Photo courtesy of Wayne Griffiths)

How do I avoid problems with brown rot in the future?  

Remove and destroy any wild or volunteer Prunus trees and shrubs on your property, as well as all rotting and mummified Prunus fruits, as these can be reservoirs for brown rot fungi.  Burn (where allowed by local ordinance) or deep bury these materials.  Thin your Prunus trees to increase air flow and promote more rapid drying of twigs and fruits.  For information on how and when to prune, see UW Bulletin A3629, Growing Apricots, Cherries, Peaches, and Plums in Wisconsin (available at https://learningstore.extension.wisc.edu/).  Be sure to decontaminate cutting tools after tree/shrub removal and pruning as described above.  Carefully handle fruits during harvest to minimize bruising, and store fruits in a cool, not overly wet environment.

If brown rot has been a chronic problem in your garden and the cultural methods discussed above have not proven successful for control, consider using fungicides.  Select products that are labeled for use on apricots, cherries, peaches and/or plums and that contain captan, myclobutanil or propiconazole.  To prevent blossom infections, make one application when approximately 10% of flowers are open.  To prevent fruit infections, begin making applications three weeks before anticipated harvest, particularly if there is wet weather.  When applying more than one application, DO NOT use myclobutanil and/or propiconazole for all treatments, as these active ingredients have similar modes of action; excessive use of these active ingredients can select for fungicide-resistant strains of brown rot fungi.  Instead, alternate use of these active ingredients with use of captan or only use captan (which is not known to have problems with resistance development) for control.  Also consider insecticide treatments to manage insects that can damage fruits and provide entry points for brown rot fungi.  See UW Bulletin A4104, Midwest Fruit Pest Management Guide (available at https://learningstore.extension.wisc.edu/), for suggested insecticides and timings of applications.  Be sure to read and follow all label instructions of any fungicides and insecticides that you select to ensure that you use these products in the safest and most effective manner possible.  Be especially careful not to apply any pesticides closer to harvest than is allowed on the label.

For more information on brown rot: 

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.

© 2013-2024 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 Nolan Bornowski , Rebecca Ellinghuysen , Patty McManus , Patti Nagai, Scott Reuss and Brittany Seidl 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 - Field and Forage, Disease - Fruit Crop, Disease - Herbaceous Ornamental, Disease - Needled Woody Ornamental, Disease - Vegetable, Misc

Black Walnut Toxicity

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UW Plant Disease Facts

 

Authors:   Ann Joy and Brian Hudelson, UW-Madison Plant Pathology; Laura Jull, UW-Madison Horticulture
Last Revised:   02/28/2024
D-number:   D0021

What is black walnut toxicity? 

Black walnut trees (Juglans nigra) produce a toxic substance (called juglone) that prevents many plants from growing under or near them.  Related trees like butternut (J. cinerea) and shagbark hickory (Carya ovata) also produce juglone, but in lower concentrations than black walnut.  Juglone occurs in all parts of black walnut trees, but especially in buds, nut hulls and roots.  The toxic effects of a mature black walnut tree can extend 50 to 80 feet from the trunk of the tree, with the greatest toxicity occurring within the tree’s dripline.  In this area, plants susceptible to juglone may wilt or die; plants tolerant to juglone will grow normally.  Vegetables such as tomato, potato, eggplant and pepper, and ornamentals such as lilac, peony, rhododendron and azalea are particularly sensitive to juglone.

Wilting of eggplant due to black walnut toxicity.
Wilting of eggplant due to black walnut toxicity.

What do the effects of black walnut toxicity look like? 

Plants sensitive to juglone may be stunted, have yellow or brown, twisted leaves, exhibit wilting of some or all plant parts, and die over time.  Often, the vascular (i.e., water-conducting) tissue of affected plants will be discolored.  Symptoms may occur rapidly, even within a few days after sensitive species are transplanted into a walnut tree’s root zone.  Alternatively, some plants may survive for years near a young walnut tree but then wilt and die as the tree increases in size.  Black walnut toxicity can be confused with wilts caused by bacterial and fungal pathogens (e.g., see UW Plant Disease Facts D0121, Verticillium Wilt of Trees and Shrubs, and D0122, Verticillium Wilt of Vegetables), herbicide injury (see UW Plant Disease Facts D0060, Herbicide Damage), or drought.

How do I avoid problems with black walnut toxicity? 

There is no cure for a plant affected by walnut toxicity.  Removing a walnut tree may not be practical, as the tree could be the focal point in a landscape.  In addition, even if a walnut tree is removed, juglones will not immediately be eliminated, because it is next to impossible to remove all root pieces from the soil and remaining pieces may continue to exude toxins for several years as they decay.

When establishing a garden around a walnut tree, try to plant species that are tolerant to juglone (see table on the reverse side).  If you are growing sensitive species near a walnut tree, transplant them elsewhere in your garden.  If you must grow sensitive plants near a black walnut, keep beds free of walnut leaves and hulls, and remove walnut seedlings as they appear.  Grow shallow rooted woody and herbaceous plants, and improve drainage to help diminish the effects of juglone.  Alternatively, consider building raised beds with wood, stone, or concrete barriers that limit root growth through and under the beds.

When disposing of bark and wood from a walnut tree, do not use these materials for mulch.

The information in the following table is intended to provide guidance in selecting plants to grow near walnut trees.  Inclusion of plants in this table is based on observation, not on formal testing.  In addition, the plant lists in this table are by no means exhaustive.  Oftentimes the juglone sensitivity or tolerance of specific plants has never been observed or documented.  Finally, sources often disagree on whether particular plants (e.g., columbine, lily, narcissus, tulip) are juglone sensitive or tolerant.  Some varieties may be susceptible while others may be tolerant.  Most plant species with conflicting information regarding their sensitivity or tolerance to juglone have not been included in the table.

  Sensitive to Juglone Tolerant of Juglone
Vegetables asparagus, cabbage, eggplant, pepper, potato, rhubarb, tomato beans, beet, carrot, corn, melon, onion, parsnip, squash
Flowers autumn crocus, chrysanthemum, forget-me-not, petunia, peonies aster, astilbe, bee balm, begonia, black-eyed Susan, bluebell, calendula, crocus, daylily, ferns, grape hyacinth, some hosta varieties, hollyhock, impatiens, Jack-in-the-pulpit, Jacob’s ladder, marigold, morning glory, pansy, phlox, Siberian iris, squill, sweet woodruff, trillium, zinnia
Trees alder, apple and crabapple, basswood, pine, spruce, silver maple, white birch black locust, catalpa, Eastern redbud, hackberry, Canadian hemlock, hickory, most maples, oaks, pagoda dogwood, poplar, red cedar
Shrubs and Vines azalea, blackberry (and most berries other than black raspberry), cotoneaster, hydrangea, lilac, mountain laurel, potentilla, privet, rhododendron, yew arborvitae, bittersweet, black raspberry, clematis, currant, forsythia, euonymus, greenbrier, most honeysuckle, pachysandra, rose-of-Sharon, sumac, most viburnum, Virginia creeper, wild grape, wild rose, willow, witch hazel
Field Crops and Grasses alfalfa, tobacco fescue, Kentucky bluegrass, orchard grass, soybean, timothy, wheat, white clover

For more information on black walnut toxicity: 

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).

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 Lisa Johnson, Mike Maddox and Patti Nagai 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

Black Knot

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UW Plant Disease Facts

 

Authors:   Brian Hudelson, UW-Madison Plant Pathology
Last Revised:   02/28/2024
D-number:   D0018
 
A typical older black knot gall with colonization by whitish secondary fungi. (Photo courtesy of Patricia McManus)
A typical older black knot gall with colonization by whitish secondary fungi. (Photo courtesy of Patricia McManus)

What is black knot? 

Black knot is a disfiguring and potentially lethal disease of trees and shrubs in the genus Prunus.  This genus includes stone fruits such as wild, fruit-bearing and ornamental plums and cherries.

What does black knot look like? 

During the first year of infection, black knot-infected trees develop greenish-brown to brown swellings on affected branches and trunks.  During the second year, these swellings enlarge into the ugly, black, erupting tumors (galls) characteristic of the disease.  These galls resemble animal feces attached to branches (affectionately referred to as “poop-on-a-stick”).  Older (greater than two years old) gall tissue often dies and then is colonized by fungi that give the gall a whitish or pinkish color.  Severe black knot infections may cause general tree decline or death if galls girdle large limbs, or tree trunks.

Where does black knot come from? 

Black knot is caused by the fungus Apiosporina morbosa, which survives in black knot galls on infected Prunus trees.  Spores of the fungus are released from these galls and infect new branches in late spring or early summer during periods of wet weather and mild temperatures (55°F to 75°F).

How do I save a tree with black knot? 

To manage existing black knot galls, simply remove the galls each winter from infected trees, then burn (where allowed by local ordinance) or bury them.  Prune branches six to eight inches below each gall.  On trunks, you can use a chisel to remove tissue from at least one inch beyond the infected area.  Keep in mind however, that doing this creates a wound that can provide an entry point for other serious Prunus pathogens such as the bacteria that cause bacterial canker (see UW Plant Disease Facts D0009, Bacterial Canker).  After removing galls, be sure to clean pruning tools between cuts by treating them for at least 30 seconds with 70% alcohol (e.g, rubbing alcohol or spray disinfectants) or a 10% bleach solution.  This will prevent accidental movement of the black knot fungus from branch to branch, or from tree to tree as galls are removed.  If you use bleach, be sure to thoroughly rinse your tools after you are done pruning and oil them to prevent rusting.

How do I avoid problems with black knot in the future?  

In established plantings, remove any volunteer or wild cherry or plum trees from within 500 feet of susceptible fruit-bearing or ornamental cherries or plums.  When purchasing new cherries and plums, carefully inspect trees prior to purchase to be sure they are free of black knot.  When available, buy black knot-resistant ornamental cherry or plum species or varieties such as Prunus ‘Accolade’, Prunus sargentii, and Prunus maackii.  DO NOT attempt to control this disease using fungicide treatments, as these treatments are expensive and not likely to be effective.

For more information on black knot: 

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).

Thanks to Patty McManus and Teryl Roper 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

Bitter Pit and Cork Spot

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UW Plant Disease Facts

 

Authors:   Teryl Roper, UW-Madison, Department of Horticulture
Last Revised:   02/28/2024
D-number:   D0017
 
An apple with symptoms of bitter pit.
An apple with symptoms of bitter pit.

Bitter pit and cork spot are common disorders of Wisconsin apples.  These two disorders do not occur every year, but when weather conditions are just right (or just wrong, depending on your perspective), they can be major problems.  This fact sheet describes the causes of the disorders and what can be done to reduce the severity of the problems.

The name bitter pit will be used for both cork spot and bitter pit since they are related disorders.  Bitter pit shows as brownish spots in the fruit flesh.  The spots actually occur in the outer layers of the fruit but are visible through the skin.  Bitter pit may develop before or after harvest.  Cork spot develops before harvest.  The flesh layers underlying the skin collapse, become brown and take on a “corky” appearance.  The exact underlying cause of these disorders is not known, however, they are both known to be calcium related.

Several factors are associated with development of bitter pit.

High tree vigor. 

Trees that are invigorated by heavy pruning, light crops, or nitrogen applications beyond sufficient ranges have a higher incidence of bitter pit.  Pruning is vegetatively invigorating and trees that are heavily pruned will also have light crops and substantial vegetative growth.  High nitrogen fertilizer doses also lead to substantial vegetative growth.  Vegetative growth competes with fruit for available calcium.  If calcium is shunted to vegetative growth, it is not available for fruit growth.  Further, since calcium moves with water in the transpiration stream, greater leaf area will shunt more water (and calcium) to the leaves rather than fruit.

Moisture stress.

Trees that have received either too much or too little water are more prone to bitter pit.  The typical situation is too little water.  Related to moisture stress is high temperature.  Hot days lead to greater evapotranspiration from leaves.  Calcium moves in the xylem along with the water that “feeds” the transpiration needs and ends up in leaves rather than fruit.  Once in the leaves, calcium is immobile and will not move back to fruit.

Large fruit. 

Large fruit are more generally more susceptible to bitter pit than smaller fruit.  A light crop will not only lead to vegetative vigor, but also to larger fruit and these fruit are more likely to show bitter pit.

To reduce the incidence of bitter pit in apples:

Provide irrigation. 

Most years, irrigation is not critical in Wisconsin and the cost of irrigation can be high.  During dry spells or drought years, however, irrigation is very valuable for maintaining soil moisture levels and reducing tree stress.

Apply calcium sprays. 

Since calcium is immobile in plants once it has reached its destination, it is good practice to apply calcium to a tree so that some will end up on the fruit and be absorbed through the cuticle.  You can use either calcium nitrate or calcium chloride, but calcium chloride is far cheaper.  An additional downside to calcium nitrate is that it aggravates the “too much vigor” problem.  Use four pounds of calcium chloride per acre, beginning with the first cover spray through late July.  Then increase the calcium chloride rate to 6 lbs. per acre.  For small orchards, this is equivalent to approximately 6 oz. of calcium chloride per gallon.  During prolonged dry weather, eliminate calcium chloride applications to prevent buildup on the foliage and fruit that may cause burning.  Do not make another application unless it has rained since the previous application.  There are different calcium chloride products.  Research in several states has shown that all products are equally effective if applied at the same rate of calcium, so use the cheapest option.

Include calcium in postharvest dip tanks or wash water. 

Michigan research has shown that a 4% dip or drench of calcium chloride will reduce the incidence or delay development of bitter pit in storage.  Mix the solution as 33 pounds of calcium chloride per 100 gallons of water.  After storage, fruits must be washed again to remove the calcium deposits.

Remember that calcium chloride is corrosive to metal so be sure to clean all equipment thoroughly after using calcium chloride.  Painting and upkeep becomes even more important if calcium chloride is used.

I wish there were some “magic bullet” that would reduce the incidence and severity of bitter pit.  Unfortunately, the problem is weather related and has several variables that determine its severity and incidence.  In general, techniques that keep vigor under control and maintain a reasonable crop load (to keep competing calcium sinks from accumulating calcium that should go to fruit), coupled with applying calcium in cover sprays and at post harvest will help reduce the problem.  There are no easy solutions.

For more information on bitter pit and cork spot: 

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).

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

Bacterial Canker

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UW Plant Disease Facts

 

Authors:   Mary Francis Heimann* and Brian Hudelson, UW-Madison Plant Pathology
Last Revised:   02/28/2024
D-number:   D0009

What is bacterial canker? 

Bacterial canker is a common and sometimes lethal disease of trees in the genus Prunus including cherry, plum and peach.  Bacterial canker is sometimes also referred to as “gummosis”, “blossom blast”, “dieback”, “spur blight” and “twig blight”.

Ooze (see arrow) on Prunus branches or trunks can indicate a bacterial canker problem.
Ooze (see arrow) on Prunus branches or trunks can indicate a bacterial canker problem.

What does bacterial canker look like? 

Often branch dieback is the first symptom of bacterial canker that homeowners notice.  However, other more subtle symptoms of flowers, leaves, fruits and branches typically precede this dieback.  Initially, infected trees are symptomless.  Infected flowers often open but then collapse.  Infected leaves become spotted and yellowed.  The centers of leaf spots often shothole (i.e., fall out).  If spotting is severe, leaves may fall off.  Infected fruits develop dead spots surrounded by water-soaked tissue.  Spots can eventually develop into a fruit rot.  Branch or trunk infections often occur at pruning sites and lead to cankers (i.e., sunken, dead areas).  Cankers often produce a gummy, resinous ooze.  Wood in the cankered area is typically discolored.  Flower, fruit and branch infections can become systemic, leading to twig dieback, death of larger branches, or even death of an entire tree.

Where does bacterial canker come from? 

Bacterial canker is caused by the bacteria Pseudomonas syringae pv. syringae (Pss) and P. syringae pv. mors-prunorum (Psm).  These bacteria overwinter in cankers, in asymptomatic, systemically infected branches, and in buds of susceptible trees.  Pss is also known to occur naturally on the leaves of many plants including many weed species.  Both Pss and Psm can subsequently be spread by rain splash, wind, or insects.  The bacteria can also be spread to healthy branches when contaminated pruning tools are used.  Infections most often occur during cool, wet conditions.

How do I save a plant with bacterial canker? 

Prune infected branches at least 12 inches below cankers or other dead tissue, and dispose of branches by burning (where allowed by local ordinance) or burying them.  Prune branches only during the winter (e.g., Jan. and Feb.) or during dry periods in late summer (e.g., Aug.).  DO NOT prune during the cool, wet periods (e.g., spring and fall).  Disinfest pruning tools after each cut by treating them for at least 30 seconds with a 10% bleach solution or preferably 70% alcohol (e.g., rubbing alcohol straight from the bottle or a spray disinfectant).  If you decide to use bleach, be sure to thoroughly rinse and oil your tools after pruning is complete to prevent rusting.

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

Healthy trees are better able to slow the development of bacterial canker.  Therefore make sure that trees are watered and fertilized properly.  Keep weeds and other plants that may harbor bacterial canker pathogens from around the base of susceptible trees.  Copper-containing sprays have been advocated for bacterial canker management.  However, in many areas, copper-resistant strains of Pss and Psm are present and therefore copper sprays are ineffective.

For more information on bacterial 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:

*Mary Francis Heimann is a Distinguished Outreach Specialist Emerita at the University of Wisconsin-Madison.

© 2004-2024 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 Diana Alfuth, Patty McManus 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.

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

Armillaria Root Disease

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UW Plant Disease Facts
 
Authors:   Michael Amman, UW-Madison Forest Ecology and Management and Glen R. Stanosz, Ph. D., UW-Madison Plant Pathology
Last Revised:   02/28/2024
D-number:   D0005

What is Armillaria root disease? 

Armillaria root disease, also known as shoestring root rot, is an often lethal disease of tree and shrubs.  It can affect almost any conifer or hardwood species, from seedling to maturity.  Herbaceous plants can also be affected.  Trees and shrubs stressed due to drought or defoliation can be particularly susceptible to Armillaria root disease.

White mats of fungal tissue called mycelial fans (arrow) may be present within and beneath the bark of stems and roots affected by Armillaria root disease.
White mats of fungal tissue called mycelial fans (arrow) may be present within and beneath the bark of stems and roots affected by Armillaria root disease.

Where does Armillaria root disease come from? 

Armillaria root disease results from colonization of trees and shrubs by fungi in the genus Armillaria.  These fungi produce tough, cord-like strands called “rhizomorphs” that grow from decaying stumps and roots through the soil.  Infection of other trees or shrubs can result from penetration of intact roots by rhizomorphs.  In late summer or early fall, honey-colored mushrooms of Armillaria fungi develop near the bases of colonized plants and produce spores that are distributed by wind.  Infection also can occur after these spores germinate in wounds on stems or roots.

What does Armillaria root disease look like? 

Above-ground symptoms of Armillaria root disease may include slow growth, yellowing and dwarfing of foliage, and thin crowns.  Dieback of twigs and branches also may occur as the disease progresses.  These symptoms may develop slowly and intensify over many years.  However, trees and shrubs also may be rapidly killed, with leaves or needles suddenly wilting or browning on a plant that appeared healthy just days or weeks earlier.  Bark on lower stems or roots may be killed and crack, with flow of resin common on conifers.  Thin white mats of fungal tissue called “mycelial fans” may be present within and beneath killed bark.  Stem and root tissue decayed by Armillaria fungi is often water-soaked, creamy to yellow in color, and spongy or stringy in texture.  Rhizomorphs are commonly seen on or beneath the bark and growing from decayed stumps and roots.

How do I save a tree affected by Armillaria root disease? 

There is no practical way to eliminate Armillaria from trees that are already colonized by the fungus.  The useful life of an affected tree might be prolonged however, by supplemental watering during dry periods and appropriate fertilization to improve overall host condition.  In very vigorous trees, the Armillaria fungi may be “walled off” and confined to just a portion of the root system or root collar.  There are no chemical treatments that can effectively target Armillaria fungi within diseased trees.

How do I avoid Armillaria root disease in the future? 

Practices that maintain trees in vigorous condition are the best means of preventing Armillaria root disease.  Watering and fertilization to avoid stress will help trees resist infection.  Because Armillaria root disease often develops in response to defoliation, suppression of both insect and leaf pathogen defoliators will indirectly reduce the occurrence and severity of Armillaria root disease.  Because stumps and root systems of previously colonized trees can serve as “food bases” supporting rhizomorph growth for many years, thorough removal of stumps and root systems will reduce the risk of infection of other trees.

For more information on Armillaria root disease: 

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 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 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 - Fruit Crop

Angular Leaf Spot of Strawberry

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UW Plant Disease Facts

 

Authors:   Isael Rubio*, UW-Madison Plant Pathology
Last Revised:   02/28/2024
D-number:   D0001

What is angular leaf spot? 

Angular leaf spot is a severe bacterial disease that can adversely affect strawberries wherever they are grown, potentially causing yield losses of up to 75%.  The disease appears to have originated in the U.S and was first described in Minnesota in 1960.

What does angular leaf spot look like? 

Symptoms of angular leaf spot develop on strawberry leaves and sepals (the green leaf-like parts of the strawberry fruit).  Initial symptoms are small, angular (i.e., straight-edged), water-soaked, translucent spots on lower leaf surfaces.  White, cream or yellow-colored ooze may appear on the spots when humidity is high.  This ooze can eventually dry and become crusty.  As the disease progresses, spots enlarge and merge, becoming visible on the upper surfaces of the leaves.  Severely affected leaves dry and turn reddish-brown.  Parts of the leaves can tear away.  Angular leaf spot can also cause sepals to darken, leading to so-called “black caps” that reduce the quality and salability of fruit.

Straight-edged spots on strawberry leaves with oozy masses in the center are typical of angular leaf spot. (Photo courtesy of Patricia McManus)
Straight-edged spots on strawberry leaves with oozy masses in the center are typical of angular leaf spot. (Photo courtesy of Patricia McManus)

Where does angular leaf spot come from? 

Angular leaf spot is caused by the bacterium Xanthamonas fragariae which is typically introduced into a garden on infected plants.  Once present in a garden, the bacterium can be spread by splashing water from rain or watering with a sprinkler.  Once established in a garden, X. fragariae overwinters on strawberry leaf debris.  Weather conditions that favor angular leaf spot are not well understood, although high humidity appears to play a role.

How do I save a plant with angular leaf spot?  

Once a plant is infected, it cannot be cured.  If angular leaf spot is detected early, use of copper-containing compounds labeled for use on strawberries may help limit disease development although the effectiveness of such sprays can be quite variable.  If you decide to use such a product, be sure to read and follow all label instructions for the product that you select to ensure that you use it in the safest and most effective manner possible.  Avoid using copper sprays once flowers have formed.

How do I avoid problems with angular leaf spot in the future?

Unfortunately, strawberry cultivars resistant to angular leaf spot are not available and some cultivars (e.g., ‘All Star’, ‘Annapolis’, ‘Cavendish’, ‘Honeoye’ and ‘Kent’) have been observed to develop particularly severe symptoms.  When first establishing a strawberry patch, be sure to purchase disease-free plants.  When watering, use a drip or soaker hose to reduce splash that can move X. fragariae from plant to plant.  Also, DO NOT handle strawberry plants or harvest berries when they are wet, as this can promote spread of the pathogen.  Prevent additional spread on gardening tools and other gardening items by dipping or treating them for at least 30 seconds with 10% bleach or preferably (because of its less corrosive properties) 70% alcohol.  Rubbing alcohol and many spray disinfectants typically contain approximately 70% alcohol and are easy to use.  If angular leaf spot occurs in your strawberry patch, remove infected leaves and any infested debris.  Dispose of this material by burning (if allowed by local ordinance) or burying it.

For more information on angular 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.

© 2014-2024 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 Diana Alfuth, Ricardo Garcia, Kristin Krokowski, Jenna Lind , Toby Lunt, Patricia McManus, Yoana Mendoza and Jaime Trejo 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/.