Category Archives: Disease – Vegetable


What is edema?  Edema (or oedema) is a physiological disorder that frequently occurs in houseplants, greenhouse plants, and other plants sheltered under plastic.  This disorder also affects field-grown vegetable crops under certain environmental conditions.  Edema is often a cosmetic problem, but in extreme cases, edema can ruin a greenhouse crop and cause severe economic losses.

Edema pustules on the underside of a geranium leaf.
Edema pustules on the underside of a geranium leaf.

What does edema look like?  Small translucent, fluid-filled blisters form on the undersides of mostly older leaves, often beginning at the leaf margins.  Blisters can also occur on stems and occasionally on flowers.  When observed against the light, edema lesions are lighter in color than the surrounding leaf tissue.  The blisters may increase in size or merge, burst, and then scar, turning tan in color and corky in texture.  Some or all leaves may eventually shrivel or roll, and fall off.  Extensive blistering and scarring may limit the plant’s ability to photosynthesize.

Where does edema come from?  When the soil is warm and moist, water absorbed by a plant’s roots may exceed the water lost through a plant’s leaves.  Conditions preventing effective water loss include high relative humidity, low light intensity, cool air temperatures and poor ventilation.  Periods of cloudy weather, or an increase in relative humidity resulting from cooling air temperatures, can make plants susceptible to edema.  Edema has also been associated with the use of oil sprays that interfere with normal leaf water loss.

How do I save a plant with edema?  Edema is typically not fatal but will make plants less attractive.  To limit problems with edema, water less frequently in cloudy periods or under low light intensity.  Water in the morning so that the soil in which plants are potted will drain by nightfall when cooling temperatures can lead to increased relative humidity.

How do I avoid problems with edema in the future?  Use a growth medium that drains well.  Reduce relative humidity near leaf surfaces by increasing plant spacing and air circulation.  Increase light and air temperatures to help increase normal water loss.  Water less frequently during cool, humid weather.  Empty standing water in saucers under pots 30 minutes after watering.

For more information on edema:  See UW Bulletin A3287, Plant Disorder:  Oedema (available at, or contact your county Extension agent.

Early Blight

What is early blight?  Early blight is one of two common fungal diseases that can devastate tomatoes in both commercial settings and home gardens.  Early blight can also be a serious disease on other popular vegetables including eggplants, peppers, and potatoes.  The second common tomato blight, Septoria leaf spot, is detailed in UW Plant Disease Facts D100, Septoria Leaf Spot.

Early blight of tomato. Note spots with concentric rings (arrow) and yellow haloes.
Early blight of tomato. Note spots with concentric rings (arrow) and yellow haloes.

What does early blight look like?  Symptoms of early blight first appear at the base of affected plants, where roughly circular, brown spots appear on leaves and stems.  As these spots enlarge, concentric rings appear giving the areas a target-like appearance.  Often spots have a yellow halo.  Eventually multiple spots on a single leaf will merge, leading to extensive destruction of leaf tissue.  Early blight can lead to total defoliation of lower leaves and even the death of an infected plant.

Where does early blight come from?  Early blight is caused by the fungus Alternaria solani, which survives on infected plants or in plant debris.  Early blight symptoms typically begin as plant canopies start to close.  Denser foliage leads to high humidity and longer periods of leaf wetness that favor the disease.

How do I save a plant with early blight?  Once symptoms of early blight appear, control is difficult.  Thinning of whole plants or removal of selected branches from individual plants may slow the disease by increasing airflow and thus reducing humidity and the length of time that leaves remain wet.  Fungicides labeled for use on vegetables and containing copper or chlorothalonil may also provide control of early blight if they are carefully applied very early in the course of the disease (before symptoms develop is best) and on a regular basis throughout the rest of the growing season.  If you decide to use fungicides for disease 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 early blight in the future?  Destroy infected plants and plant debris by burning (where allowed by local ordinance) or burying them.  Move vegetables to different parts of your garden each year to avoid areas where the early blight fungus may be present.  See University of Wisconsin Garden Facts XHT1210, Using Crop Rotation in the Home Vegetable Garden, for details on this technique.  Use early blight-resistant vegetable varieties whenever possible.  Increase spacing between plants to increase airflow, and decrease humidity and foliage drying time.  Mulch your garden with approximately one inch of a high quality mulch, but DO NOT overmulch as this can lead to wet soils that can contribute to increased humidity.  Finally, where the disease has been a chronic problem, use of preventative applications of a copper or chlorothalonil-containing fungicide labeled for use on vegetables may be warranted.

For more information on early blight:  Contact your county Extension agent.


Downy Mildew

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, 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.

Downy Mildew
Downy Mildew

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 your county Extension agent.


Spaghetti-like dodder plants parasitizing carrots.
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 your county Extension agent.


What is damping-off?  Damping-off is a common and fatal disease that affects all types of plant seedlings.  The disease is most prevalent when seeds are germinated in cool, wet soils.  Fortunately, seedlings are susceptible to damping-off for only a short period following emergence.  As plants age, their susceptibility to damping-off declines.

Lower stem collapse of Zinnia seedlings due to damping-off.
Lower stem collapse of Zinnia seedlings due to damping-off.

What does damping-off look like?  Seedlings killed by damping-off initially are healthy but shortly after emergence become infected at or just below the soil line.  The lower stems of the seedlings collapse, and the seedlings fall over onto the soil surface.  The seedlings subsequently die.

Where does damping-off come from?  Damping-off is caused by several soil-borne water molds and fungi, including (but not limited to) Pythium spp., Rhizoctonia solani and Fusarium spp.  These organisms readily survive and are moved in soil or on soil-contaminated items such as pots, tools and workbenches.

How do I save seedlings with damping-off?  Seedlings with damping-off will die and cannot be saved.  Proper prevention is the only way to avoid problems with this disease.

How do I avoid problems with damping-off in the future?  When planting seeds, make sure that work areas, tools and pots are pathogen-free.  Decontaminate tools and workbenches by treating them for at least 30 seconds with 10% bleach or (preferably due to its less corrosive properties) 70% alcohol (e.g., rubbing alcohol or certain spray disinfectants).  Decontaminate pots by washing them with soapy water to remove bits of old soil, soaking them for at least 20 minutes in 10% bleach, and then rinsing them thoroughly to remove bleach residues.  DO NOT reuse plastic pots if you have had problems with damping-off or root rots (see UW Plant Disease Facts D0095, Root Rots in the Garden, for details) in the past, as they are difficult to decontaminate.

When planting, use a well-drained, pasteurized potting mixture.  DO NOT use garden soils as they often contain damping-off pathogens.  DO NOT plant seeds too deeply, and germinate seeds at high temperatures, so that seedlings rapidly grow out of the stage where they are susceptible to damping-off.  DO NOT overwater as damping-off organisms are more active in wet soils.  If the techniques described above do not work, then consider using fungicide-treated seed.  In particular, plants grown from captan-treated seeds tend to have fewer problems with damping-off.

For more information on damping-off:  Contact your county Extension agent.

Crown Gall

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 your county Extension agent.

Corky Ringspot

What is corky ringspot?  Corky ringspot (also known as spraing) is a potentially serious viral disease of potato that has recently been detected in Wisconsin.  The disease can cause severe losses due to the fact that it reduces potato tuber quality, making tubers unsuitable for use in potato chip production and undesirable to consumers as table stock.  Variants of this disease [usually referred to as tobacco rattle (see UW Plant Disease Facts D0116, Tobacco Rattle)] affect a variety of other plants including vegetable crops (e.g., beans, beets, peppers, and spinach), many herbaceous ornamentals (e.g., astilbe, bleeding heart, coral bells, daffodil, epimedium, gladiolus, hyacinth, marigold, tulip, and vinca) and many weed species (e.g., chickweed, cocklebur, henbit, nightshade, pigweed, purslane, prickly lettuce, shepherd’s-purse and sowthistle).

Internal necrosis of tubers, often in fleck or arc patterns is typical of corky ringspot.
Internal necrosis of tubers, often in fleck or arc patterns is typical of corky ringspot.

What does corky ringspot look like?  Symptoms of corky ringspot vary depending on the variety/cultivar of potato affected, and depending on environmental conditions.  Foliar symptoms are rare, but on occasion can include reduced leaf size, puckering and mottling (i.e., blotchy light and dark coloring).  More commonly, corky ringspot manifests itself underground as corky arcs, rings or flecks that form on or within tubers.  Thinner-skinned and lighter-colored potato varieties are more likely to exhibit obvious ring symptoms on the surfaces of tubers.  Symptoms similar to those caused by corky ringspot can be caused by other potato viruses such as alfalfa mosaic virus, potato mop-top virus, and certain strains of potato virus Y.

Where does corky ringspot come from?  Corky ringspot is caused by the Tobacco rattle virus (TRV) which is spread primarily by stubby-root nematodes, a group of microscopic, worm-like organisms in the genera Trichodorus and Paratrichdorus.  These nematodes feed on the roots of infected plants (vegetables, ornamentals or weeds), acquiring TRV, then move to non-infected plants where their subsequent feeding spreads the virus.  TRV also can be spread mechanically when knives or other tools that are used to cut tubers for seed pieces, or that are used to divide ornamental plants, become contaminated.  In addition, on ornamentals, TRV can be spread by pruning and grafting, and via movement of seed from infected plants.

How do I save potatoes with corky ringspot?  Once potatoes have become infected with TRV, they remain infected indefinitely.  Infected plants cannot be treated in any way to eliminate the virus and should be removed and disposed of by burning (where allowed by local ordinance), burying or composting.  Before destroying symptomatic plants, you may want to have them tested to verify the presence of TRV.  Note that ELISA (a technique commonly used to test for other potato viruses) is not a reliable test for TRV; polymerase chain reaction (PCR) should be used to test for this virus.  Once TRV is introduced into a field, it is likely to remain there indefinitely.  Stubby-root nematodes can carry the virus for extended periods and weed species can serve as reservoirs of the virus indefinitely.

Tubers with corky ringspot may, but do not always have target-like ring patterns on their surfaces.
Tubers with corky ringspot may, but do not always have target-like ring patterns on their surfaces.

How do I avoid problems with corky ringspot in the future?  The best way to prevent problems with corky ringspot is to avoid introducing TRV onto your property.  Be sure to grow potatoes from seed that is certified as being free of TRV.  Currently, seed potatoes produced in Wisconsin are considered TRV-free.  Also avoid introducing the virus on infected ornamental plants.  Carefully inspect ornamentals (see above for a partial list of susceptible species) prior to purchase for symptoms caused by TRV and DO NOT buy symptomatic plants.  Alternatively (and preferably), avoid growing susceptible species altogether, and grow plants that are not susceptible to TRV.  Non-susceptible plants include, but are not limited to, annual phlox, carnation, carrot, devil’s trumpet (downy thorn-apple), Scotch spearmint, sorrel, sweet William, zinnia and zombie cucumber.

To limit potential spread of TRV, routinely decontaminate tools (e.g., knifes or other cutting tools) that come into contact with potentially infected plant material (e.g., whole tubers that are cut into seed pieces, or ornamentals that are being divided).  Also decontaminate tires, tools (e.g., spades or hoes) and any other object (e.g., shoes or boots) that might transport stubby-root nematode-infested (and thus TRV-infested) soil from field to field.  First rinse any excess plant tissue or soil from these items, then treat them for at least 30 seconds in a solution that is a combination of 1% sodium lauryl sulfate and 1% Alconox (an industrial detergent).  Trisodium phosphate (available at most local hardware stores) can also be used.

Also consider routinely testing soils for the presence of stubby root nematodes.  Knowing the level of these nematodes in a field can provide information on the likelihood that TRV will spread should the virus be introduced.

Finally, DO NOT ever produce seed potatoes in fields with a history of corky ringspot or other TRV diseases.  Also avoid using infested fields for potato or other vegetable production.  If you decide to use a TRV-infested field for non-seed potato production, be sure to grow a TRV-resistant potato variety.  The potato varieties ‘Castile’, ‘Millennium Russet’, ‘Red Pearl’, ‘Symfonia’, and ‘St. Johns’ have all been reported to have at least moderate levels of resistance to TRV.

For more information on corky ringspot:  Contact your county Extension agent.

Common Corn Smut

Common corn smut leads to tumor-like galls on corn filled with a sooty, black powder.
Common corn smut leads to tumor-like galls on corn filled with a sooty, black powder.

What is common corn smut?  Common corn smut is a fungal disease that affects field, pop, and sweet corn, as well as the corn relative teosinte (Zea mexicana).  Common corn smut is generally not economically significant except in sweet corn where relatively low levels of disease make the crop aesthetically unappealing for fresh market sale and difficult to process for freezing or canning.  Interestingly, the early stages of common corn smut are eaten as a delicacy in Mexico where the disease is referred to as huitlacoche (see UW Plant Disease Facts D0065, Huitlacoche).

What does common corn smut look like? Common corn smut leads to tumor-like swellings (i.e., galls) on corn ears, kernels, tassels, husks, leaves, stalks, buds and, less frequently, on aerial roots.  Some galls (particularly those on leaves) are small and hard.  More typically, however, galls are fleshy and smooth, silvery-white to green, and can be four to five inches in diameter.  As fleshy galls mature, their outer surfaces become papery and brittle, and their inner tissues become powdery and black.  Galls eventually rupture, releasing the powder (i.e., the spores of the causal fungus).

Where does common corn smut come from?  Common corn smut is caused by the fungus Ustilago maydis, which can survive for several years as spores in soil and corn residue.  Spores are spread by wind or through water splashing up onto young plants.  Spores can also be spread through the manure of animals that have eaten infected corn.  U. maydis most typically infects corn ears via the silks.  The fungus can also enter plants through wounds caused by insect feeding, hail, or injuries from machinery.  Infection is favored by warm weather (79-93°F) and moderate rainfall.  Corn grown in low fertility soils or soils with excessive nitrogen also has a greater likelihood of infection.

How do I save plants with common corn smut?  Once galls have formed, treatment is not possible.  Remove and burn (where allowed by local ordinance) or bury smut galls before they burst to prevent spores from spreading and overwintering.  In order for eradication to be effective, you will need to coordinate efforts with your neighbors.  Fungicides are currently not an effective control against common smut.

How do I avoid problems with common corn smut in the future?  Plant resistant corn varieties whenever possible.  Check with your favorite sweet corn seed supplier for available varieties.  Also, avoid injuring plants (e.g., when weeding) to reduce possible entry points for the smut fungus.  Maintain well-balanced soil fertility (specifically nitrogen) based on a soil nutrient test.  Use crop rotation (see University of Wisconsin Garden Facts XHT1210, Using Crop Rotation in the Home Vegetable Garden for details) to allow time for corn smut spores to naturally die off in the soil.

For more information on common corn smut:  Contact your county Extension agent.

Blossom End Rot

Blossom end rot of tomato.
Blossom end rot of tomato.

What is blossom end rot?  Blossom end rot is a physiological disorder of tomato in which the tissue of the blossom end of the fruit (the portion of the fruit opposite the stem) breaks down and rots, thus reducing yield.  Pepper, eggplant and vine crop (e.g., cucumber, pumpkin, squash, watermelon) fruits can also be affected.

What does blossom end rot look like?  Blossom end rot often occurs on the first fruits formed on plants.  Initially, water-soaked spots (resembling small bruises) appear, most often on the bottoms of fruits.  On peppers these spots can resemble sunscald and can form on the sides of the fruits near the blossom end.  Spots enlarge, becoming dark brown to black, sunken and leathery.  Half the fruit may eventually be affected.  Sometimes, when a fruit is cut, the exterior will be sound, but the interior will be discolored and shrunken.  Often, bacteria and fungi invade the discolored areas, leading to further tissue decay.

What causes blossom end rot?  Blossom end rot is caused by a lack of calcium in the fruit.  This lack of calcium may be due to low calcium levels in the soil.  More often however, there is plenty of calcium in the soil, but its availability for uptake and transport to fruits is impaired.  Drought stress, alternating soil moisture extremes, and damage to a plant’s roots all can inhibit calcium uptake, as can waterlogged or cold soils, and high concentrations of ammonium (NH4+), potassium (K+), and magnesium (Mg++) in soil.  Movement of calcium within plants depends on active transpiration (i.e., loss of water through above-ground plant parts).  Because leaves transpire more than fruits, calcium moves more easily into leaves where it remains.  Calcium is not later redistributed from leaves to fruits.  This preferential distribution of calcium to leaves can be made worse by over-fertilizing with nitrogen which promotes excessive production of leaves.  In addition, high relative humidity OR low relative humidity in combination with hot, windy weather can limit transpiration, thus preventing calcium from reaching fruits.

How can I control blossom end rot?  Avoid conditions where there is too much or too little water.  Water evenly and mulch the soil to retain moisture during dry periods.  Avoid practices that would damage roots (e.g., cultivating too near plants thereby cutting roots).  Use nitrate (NO3) rather than ammonium (NH4+) forms of nitrogen fertilizer.  DO NOT over-fertilize.  Have your soil tested periodically to determine if there is sufficient calcium in the soil.  If not, add calcium (e.g., bonemeal or lime).  Check the soil pH on a regular basis, particularly if you use lime as a calcium source.  Use of lime tends to increase soil pH.  A pH of about 6.5 is ideal for growing most vegetables.  Finally, grow vegetable varieties/cultivars that are tolerant of calcium deficiencies and less likely to show blossom end rot symptoms.

For more information on blossom end rot:  Contact your county Extension agent.

Black Walnut Toxicity

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:  See UW Bulletin A3182, Walnut and Butternut Toxicity (available at, or contact your County Extension agent.