Category Archives: Disease – Broad Leafed Woody Ornamental

Dutch Elm Disease

July 23, 2015 hudelson

UW Plant Disease Facts

Authors: Jim Olis* and Brian Hudelson, UW-Madison Plant Pathology

Last Revised: 02/29/2024

D-number: D0045

What is Dutch elm disease?

Dutch elm disease (DED) is a lethal fungal disease of native North American elms. The fungi that cause DED entered the United States early in the 1900’s on elm logs imported from Europe. DED now occurs throughout the United States and has led to the loss of the American elm as the premier street tree.

Dutch elm disease has led to the loss of the American elm as a street tree.

What is Dutch elm disease?

Where does Dutch elm disease come from?

DED is caused by two fungi, Ophiostoma ulmi and Ophiostoma novo-ulmi. These fungi are thought to have originated in Asia and were spread as elm trees and wood were moved from location to location. Once these fungi are introduced into an area, they are moved to healthy trees by two species of elm bark beetles (one native, one of European origin). These beetles breed in stressed trees (which include those trees suffering from DED), then carry spores of the DED fungi to healthy trees and inoculate the trees as they feed. Once DED fungi gain entry into an elm, they can move to other nearby elms underground via root grafts. Human activities such as pruning can lead to spread of these fungi as well.

How do I save a tree with Dutch elm disease?

Elms suffering from DED should be removed. Prior to removal, disrupt root grafts between the infected elm and other nearby healthy elms using a mechanical trencher or vibratory plow. If you do not disrupt root grafts, removal of the infected tree may accelerate movement of DED fungi through root grafts to adjacent trees. Destroy wood from diseased elms by burning (where allowed by local ordinance) or burying it. If you decide to keep the wood, remove the bark, then pile the wood and cover it with a heavy tarp, burying the tarp edges with soil, until the wood is used. Covering the wood limits access by elm bark beetles that potentially could pick up DED fungi from the cut wood and move them to other trees.

Discoloration of wood just under the bark of elm branches can indicate the presence of one of the DED fungi.

If you decide not to remove an infected elm, at least remove, then bury or burn, dead or dying branches. Make cuts six feet below any area showing symptoms. Disinfect cutting tools between each cut by treating them for at least 30 seconds with a 10% bleach solution 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.

How do I avoid problems with Dutch elm disease in the future?

Fungicide injections of either propiconazole or thiabendazole can be useful for protecting high value, healthy American elm trees. Such injections are of limited use if trees are already infected. For best results, trees should be treated every two years by a certified arborist with additional training in making injection treatments.

If you decide to plant an elm, use non-native hybrid elms, such as “Accolade”, “New Horizon”, “Regal” and “Frontier”, that are resistant to DED. DED resistant American elm varieties are also currently available. Look for varieties such as “American Liberty”, “Independence”, “Princeton”, “New Harmony” and “Valley Forge” at your local nursery or garden center. Keep in mind however that resistance is not immunity, and even DED-resistant elm varieties can become infected. Typically however, in resistant varieties, DED does not develop as rapidly, providing time and opportunity to attempt to manage the disease using other strategies (e.g., pruning, fungicide injections).

For more information on Dutch elm 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:

Full Color PDF

*Completed as partial fulfillment of the requirements for a BS in Plant Pathology at the University of Wisconsin Madison

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 Lis Friemoth, Ann Joy and Laura Jull 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 - Specialty Crop, Disease - Vegetable

Downy Mildew

July 21, 2015 hudelson

UW Plant Disease Facts

Authors: Jim Olis* and Brian Hudelson, UW-Madison Plant Pathology

Last Revised: 02/29/2024

D-number: D0044

What is downy mildew?

The downy mildews are a group of diseases that cause destruction of the leaves, stems, flowers and fruits of many plant species worldwide. In Wisconsin, downy mildews have traditionally been problems on grapes, cucumbers (see UW Bulletin A3978, Cucurbit Downy Mildew: Identification and Management, available at https://learningstore.extension.wisc.edu/), roses and Viburnum. Downy mildews on basil (see UW Plant Disease Facts D15, Basil Downy Mildew) and impatiens (see UW Plant Disease Facts D66, Impatiens Downy Mildew) have more recently become problematic.

What does downy mildew look like?

Downy mildew symptoms begin as small, green or yellow, translucent spots that can eventually spread to an entire leaf, stem, flower or fruit. Infected plant parts may eventually brown or bronze. The causal organism appears on infected stems, flowers and fruits, and on undersurfaces of infected leaves, as a downy, white, gray or purple fuzz.

Where does downy mildew come from?

Downy mildew is caused by several closely related fungus-like water molds (e.g., Peronospora spp., Plasmopara spp. and Pseudoperonospora spp.) that survive in plant debris or on infected plants. Downy mildew organisms are fairly host specific. The downy mildew organism that infects one type of plant (e.g., rose) is not the same downy mildew organism that infects another (e.g., grape). However, if you see downy mildew on one plant, then environmental conditions (i.e., cool, wet weather) are favorable for development of downy mildews on a wide range of plants.

How do I save a plant with downy mildew?

Downy mildews can be controlled using fungicides, if treatments are applied early enough in disease development. Fungicides containing chlorothalonil and copper (particularly Bordeaux mix, a combination of copper sulfate and lime) are labeled for downy mildew control in home gardens. Timing and number of applications will vary depending on exactly the type of downy mildew you are attempting to control. Be sure to read and follow all label instructions of the fungicide that you select to ensure that you use the product in the safest and most effective manner possible.

How do I avoid problems with downy mildew in the future?

Consider buying downy mildew resistant varieties, when available. In new plantings, space plants far apart. In established plantings, prune or thin plants to increase airflow. Proper spacing and thinning will reduce humidity and promote rapid drying of foliage, which is less favorable for downy mildew development. Avoid overhead watering and apply water directly to the soil at the base of your plants. At the end of the growing season, remove and destroy infected plant debris as this can serve as a source of spores for the next growing season.

For more information on downy mildew:

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

This Fact Sheet is also available in PDF format:

Full Color PDF

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

Thanks to Lisa Johnson, Ann Joy and Kristin Kleeberger 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, Turf

Dodder

July 21, 2015 hudelson

UW Plant Disease Facts

Authors: Jennifer Clifford, Amilcar Sanchez, Trenton Stanger, and Brian Hudelson UW-Madison Plant Pathology

Last Revised: 02/29/2024

D-number: D0129

Spaghetti-like dodder plants parasitizing carrots.

What is dodder?

Dodder is the name of several species of parasitic plants that are widely distributed in North America and Europe. Plants parasitized by dodder include alfalfa, carrots, onions, potatoes, cranberries, a variety of herbaceous and woody ornamentals, and many weed species. Parasitized plants become weakened, have reduced yields (in the case of agronomic crops), and can potentially die.

What does dodder look like?

Dodders lack roots and leaves, and also lack chlorophyll, the green pigment found in most plants. Dodders have slender, yellow-orange stems that cover infected plants in a spreading, tangled, spaghetti-like mass. From May through July, dodders produce white, pink, or yellowish flowers.

Where does dodder come from?

Dodders produce large numbers of seeds that germinate in the spring to produce shoots that attach to suitable host plants. Dodders penetrate host tissue, and absorb nutrients via specialized structures called haustoria. Once established on a host, the bottom of a dodder plant dies (thereby severing its connection with the soil), and the dodder plant becomes dependent on the host plant for water and nutrients.

How do I save plants parasitized by dodder?

On woody ornamentals, simply prune out dodder-parasitized branches. When small patches of dodder occur among herbaceous plants, apply contact herbicides such as 2,4-D early in the season, preferably before dodder seedlings have parasitized host plants. Keep in mind that use of contact herbicides will likely also kill host plants. Alternatively, cut or burn dodder and parasitized plants to keep dodder from spreading, and to prevent seed production. For widespread dodder infestations, a combination of frequent tilling, burning and herbicide applications may be needed to achieve control. Be sure to read and follow all label instructions of the herbicide that you select to ensure that you use the product in the safest and most effective manner possible.

How do I avoid problems with dodder in the future?

Dodder’s wide host range and ability to survive as dormant seeds in soil make eradication difficult. Preventing introduction of dodder is the best method of control. Use dodder-free seed, and be sure to clean equipment thoroughly after working in a dodder-infested area. Try to restrict animal movement between infested and non-infested areas as well. Depending upon the specific crop or location, use of pre-emergent herbicides containing DCPA, dichlobenil, propyzamide, or trifluralin may be possible to prevent germination of dodder seeds. Destroy actively growing dodder and any parasitized plants before the dodder produces seeds. In agricultural settings where dodder has been a problem, rotate away from susceptible crops and grow non-host crops (e.g., corn, soybeans, or small grain cereals). In conjunction with rotation, adequate control of weed hosts is critical to achieve control.

For more information on dodder:

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

This Fact Sheet is also available in PDF format:

Full Color PDF

Thanks to Lis Friemoth, Matt Lippert and Dan O’Neil 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, Misc

Dead Man’s Fingers

July 19, 2015 hudelson

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)

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:

Full Color PDF

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

July 19, 2015 hudelson

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.

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:

Full Color PDF

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 - Broad Leafed Woody Ornamental, Disease - Herbaceous Ornamental, Disease - Needled Woody Ornamental

Chlorosis

July 14, 2015 hudelson

UW Plant Disease Facts

Authors: Brian Hudelson, UW-Madison Plant Pathology

Last Revised: 02/29/2024

D-number: D0030

What is chlorosis?

Chlorosis is a common nutritional disorder of many woody ornamentals in Wisconsin, particularly in the southern and eastern parts of the state. Pin oaks are most commonly affected by chlorosis, although many other trees and shrubs (e.g., white oaks, red maples, white pines and rhododendrons) are also very susceptible.

Yellowing of foliage characteristic of chlorosis.

What does chlorosis look like?

Symptoms of chlorosis are easy to distinguish from those of other diseases. Affected leaves turn yellow, except for the veins, which remain green. In severe cases, foliage may turn brown and die. Symptoms can occur on isolated branches, or over an entire tree.

What causes chlorosis?

Chlorosis occurs when a tree or shrub is lacking certain micronutrients, in many cases iron or manganese. Lack of micronutrients in a tree may reflect a lack of these nutrients in the soil due to poor fertility. Often however, there are sufficient micronutrients, but they cannot be absorbed by a plant’s roots. Poor absorption of micronutrients is common in Wisconsin because of the high pH (alkalinity) of many soils.

How do I save a tree or shrub with chlorosis?

Chlorosis is rarely fatal and can be treated. For treatments to be effective, you must determine the exact cause of the chlorosis. Have the soil around an affected plant tested for micronutrients and for pH prior to applying any treatment. If the soil test indicates a lack of specific micronutrients, fertilize with these micronutrients. For example, chelated iron compounds can be used to increase the amount of iron in soil. If the soil test indicates a high soil pH, lower the pH by applying sulfur or ammonium sulfate. See University of Wisconsin Garden Facts XHT1151, Reducing Soil pH, for details. Contact your county Extension agent for information on soil testing and for tips on a treatment once you have determined the specific cause of your chlorosis problem.

How do I avoid problems with chlorosis in the future?

Plant trees and shrubs that are less susceptible to chlorosis. Also, make sure your trees and shrubs receive sufficient water (approximately one inch per week), as this will help plants with micronutrient uptake. During dry periods, use a drip hose or soaker hose to apply supplemental water. Remove turf from around the bases of trees and shrubs out to at least the drip lines, and apply shredded hardwood, pine or cedar mulch in these areas to help maintain soil moisture. On heavy clay soils, use one to two inches of mulch. On other soils, use three to four inches of mulch. Be sure to keep mulch four inches from the trunks of trees. If you decide to plant susceptible trees or shrubs, watch them closely for the yellowing characteristic of chlorosis, and apply corrective treatments as soon as symptoms appear. Treatments should always be based on the results of soil micronutrient and pH tests.

For more information on chlorosis:

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

This Fact Sheet is also available in PDF format:

Full Color PDF

Thanks to Sharon Morrisey, Patti Nagai 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

Butternut Canker

July 6, 2015 hudelson

UW Plant Disease Facts

Authors: Rachel Leisso*, Montana State University Plant Sciences and Plant Pathology Brian Hudelson, UW-Madison Plant Pathology

Last Revised: 02/29/2024

D-number: D0027

What is butternut canker?

Butternut canker is a fungal disease of Juglans cinerea, the butternut tree (also known as white walnut or oilnut). The disease is thought to have originated outside of the U.S. The first documented case of butternut canker in Wisconsin occurred in 1967. Butternut canker affects butternuts throughout their natural range (much of the eastern US) and has killed up to 80% of the butternut trees in some states. Although butternut canker can occur on saplings of black walnut (Juglans nigra), the effects of the disease on this species are minimal.

Multiple butternut cankers on a butternut tree trunk can girdle and kill the tree.

What does butternut canker look like?

Infected areas (called cankers) tend to be most noticeable on large branches or tree trunks, where long fissures in the bark form. Several cankers may grow together leading to bark that is distorted and has a mangled appearance. Smaller cankers also form around leaf or branch scars. These cankers are sunken and black, with white margins and folds of bark around the edge. Cankers girdle branches and trunks, cutting off nutrients and water, leading to progressive dieback above the canker. Trunk cankers will eventually kill trees. A single trunk canker can kill a young sapling, while several cankers combined may be needed to kill an older tree.

Where does butternut canker come from?

Butternut canker is caused by the fungus Ophiognomonia clavigignenti-juglandacearum. Spores of the fungus are produced in cankers throughout the growing season and are spread from infected to healthy trees through rain-splash, insects, and wind. Wounds, as well as leaf and branch scars, often serve as points of entry for the fungus. Once a tree is infected, additional infections can occur as spores are washed by rain down to the main trunk of the tree.

How do I save a tree with butternut canker?

There is no cure for butternut canker. Trees with trunk cankers will most likely die. If branch cankers are found early, removal of the affected branches can prevent the spread of the disease to other parts of the tree. Prune out smaller branch cankers by cutting six to eight inches below the canker. Disinfect pruning tools after each cut by treating them for at least 30 seconds with a 10% bleach solution or preferably (because of its less corrosive properties) a 70% alcohol solution (e.g., rubbing alcohol or certain spray disinfectants). If you use bleach, be sure to thoroughly rinse and oil tools after pruning to prevent rusting.

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

There is little that you can do to prevent butternut canker. However, vigorously growing butternut trees are better able to slow the development of butternut canker, so make sure that trees are watered and fertilized properly. Established trees require approximately one inch of water per week. If natural rains do not provide this amount of water, consider applying supplemental water near the drip line of the tree using a drip or soaker hose. Fertilization of trees should be based on a soil nutrient test.

If you live in a region where butternut canker is not currently present, or where there are few butternut trees, your butternut tree may be free of cankers indefinitely. If you live in a region where butternut canker is prevalent, watch for trees that are not affected by the disease. These trees may have inherent resistance. Consider collecting seed from such trees and growing saplings from the seed. At least some of these saplings will likely also have resistance to the disease.

For more information on butternut canker:

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

This Fact Sheet is also available in PDF format:

Full Color PDF

*Completed as partial fulfillment of the requirements for Plant Pathology 699 – Plant Pathology Special Topics at the University of Wisconsin-Madison.

Thanks to Diana Alfuth, Lis Friemoth and Vijai Pandian 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

July 6, 2015 hudelson

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.

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:

Full Color PDF

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

Black Spot

July 6, 2015 hudelson

UW Plant Disease Facts

Authors: Adrian Crabb* and Brian Hudelson, UW-Madison Plant Pathology

Last Revised: 02/28/2024

D-number: D0020

What is black spot?

Black spot is a common and potentially serious leaf disease affecting many types of roses. Black spot is found wherever roses are grown.

Typical spotting and yellowing of rose leaves due to black spot.

What does black spot look like?

Black spot lesions (i.e., infected areas) are roughly circular and may be up to ½ inch in diameter. Lesions often have feathery margins and are dark brown to black in color. Black spot initially appears during periods of wet weather, particularly when rose leaves are first emerging. The disease starts on lower leaves but will spread to the entire plant. Severe black spot leads to yellowing of leaves and defoliation. Black spot can also develop on one-year old canes, leading to raised, purplish-red blotches that blacken and blister.

Where does black spot come from?

Black spot is caused by the fungus Diplocarpon rosae, which survives in rose leaf litter and in infected rose canes. Spores of the fungus are easily spread to emerging leaves by wind or splashing rain.

How do I save a rose with black spot?

If your rose has little or no defoliation, and the weather is dry, then no treatments are necessary. If your rose has a history of severe black spot, and the weather for the growing season is predicted to be wet, consider applying preventative fungicide treatments. Fungicides containing chlorothalonil, copper sulfate, and myclobutanil can be highly effective in controlling black spot. Neem oil is another organic product (in addition to copper sulfate) that has been shown to provide some black spot control. Treat every seven to 14 days from bud break until wet weather subsides. DO NOT use the same active ingredient for all treatments. Instead, alternate the use of at least two active ingredients (particularly if you decide to use myclobutanil) to help minimize problems with fungicide-resistant strains of the black spot pathogen. Be sure to read and follow all label instructions of the fungicide(s) that you select to ensure that you use the fungicide(s) in the safest and most effective manner possible.

How do I prevent problems with black spot in the future?

Select rose varieties that are hardy in your area and that have a high level of black spot resistance. The hybrid shrub rose Rosa ‘Meipotal’ (Carefree Delight^®), rugosa rose varieties such as ‘Blanc Double De Coubert’ and ‘Fru Dagmar Hastrup’, climbing roses such as Rosa × Kordesii ‘William Baffin’ and Rosa × Kordesii ‘Henry Kelsey’, and roses in the Knockout^® series all have excellent resistance to black spot. Plant roses in a high light environment, and space shrubs far apart. This will promote good airflow and quick drying of foliage. Promptly remove diseased leaf litter. Prune diseased branches six to eight inches below any obvious infections. Prune only in dry weather. Disinfest pruning tools between cuts by treating them for at least 30 seconds with 70% alcohol (preferable for metal tools) or a 10% bleach solution. Rubbing alcohol and spray disinfectants often contain at approximately 70% alcohol. If you use bleach, be sure to thoroughly rinse tools and oil them after pruning to prevent rusting. Water roses at the base using a soaker or drip hose to minimize wetting of leaves and reduce movement of spores of the black spot pathogen. DO NOT overhead water (e.g., with a sprinkler).

For more information on black 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:

Full Color PDF

*Completed as partial fulfillment of the requirements for Plant Pathology 300 – Introduction to Plant Pathology at the University of Wisconsin Madison.

Thanks to Mike Maddox, Judy Reith-Rozelle 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

Black Knot

July 6, 2015 hudelson

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)

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:

Full Color PDF

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/.