Category Archives: Disease – Vegetable

Ralstonia Wilt – Pest Alert

What is Ralstonia wilt?  Ralstonia wilt (also sometimes known as Southern wilt) is a usually lethal disease that affects over 250 plants in over 40 plant families.  Susceptible greenhouse-grown ornamentals include, but are not limited to, plants in the genera Capsicum, Cosmos, Cyclamen, Dahlia, Fuschsia, Gerbera, Hydrangea, Impatiens, Lantana, Nasturtium and Pelargonium.  Vegetables such as eggplant, pepper, potato and tomato, as well as tobacco, are also susceptible.  Ralstonia wilt was first reported on geraniums (Pelargonium spp.) in Wisconsin in 1999.  In 2020, the disease was reported on Fantasia © ‘Pink Flare’ geraniums in Michigan.  Potentially infected ‘Pink Flare’ geraniums were also distributed to 38 other states including Wisconsin.

Yellowing and wilting characteristic of Ralstonia wilt. Photo courtesy of WI DATCP
Yellowing and wilting characteristic of Ralstonia wilt. Photo courtesy of WI DATCP

What does Ralstonia wilt look like? Symptoms of Ralstonia wilt in geraniums are similar to those associated with bacterial blight (caused by Xanthomonas campestris pv. pelargonii).  Initially, lower leaves of infected plants yellow and wilt, then die.  Yellowing and death of upper leaves follow.  Symptoms may initially occur on only one side of the plant.  Internally, the water-conducting tissue of the plant browns, and then the entire stem rots from the inside out.  Eventually, infected plants die.

Where does Ralstonia wilt come from? Ralstonia wilt is caused by the bacterium Ralstonia solanacearum (formerly Pseudomonas solanacearum).  This bacterium is commonly found in tropical, sub-tropical and warm temperate climates, but is not believed to survive cold temperatures such as those typical of Wisconsin winters.  The bacterium can be moved in symptomless plants or cuttings, or in contaminated soil and plant debris (where the pathogen can remain dormant for many years).  Several subgroups (i.e., races and biovars) of R. solanacearum have been recognized, each with a different host range.  R. solanacearum race 3, biovar 2 is of particular concern because it causes a serious disease of potato called brown rot.  In addition, this race/biovar has been listed as a select agent by the U.S. government and is considered to have potential to be developed as a bioterrorist weapon against U.S. agriculture.

How do I save plants with Ralstonia wilt? There are no known treatments that will save plants affected by Ralstonia wilt.  If you believe your plants are suffering from this disease, immediately contact your local department of agriculture or county Extension agriculture or horticulture agent to arrange for confirmatory testing.  If you live in Wisconsin, you can contact the UW-Madison Plant Disease Diagnostics Clinic (see below for contact information) for assistance.  If your plants test positive for R. solanacearum race 3, biovar 2 the United States Department of Agriculture Animal and Plant Health Inspection Service (USDA APHIS) must be notified and this organization will provide guidance on proper disposal of contaminated plants, as well as decontamination of greenhouses or other sites where contaminated plants have been grown.

How do I avoid problems with Ralstonia wilt in the future? Start by purchasing and growing pathogen-free plant cuttings.  Keep plants from different suppliers physically separated by at least 4 ft. to minimize the risk of cross contamination should a shipment of plants prove to be contaminated.  Because R. solanacearum is easily moved with soil or water, minimize splashing or any other movement of water or soil from plant to plant when watering.  When taking cuttings or trimming plants, be sure to clean cutting tools between cuts using an approved disinfectant.  For a complete list of such products, contact the UW-Madison Plant Disease Diagnostics Clinic (see below for contact information).  Also wear disposable gloves (nitrile are best) when handling plants, and change gloves between working with different geranium varieties.  This will minimize the possibility of moving R. solanacearum by touch.  If gloves are not available, wash your hands frequently and thoroughly (especially between geranium varieties) with lots of soap and water or with an alcohol-based hand sanitizer.  Remove and destroy weeds or weed debris as these can harbor the pathogen.  Finally, do not grow plants in a greenhouse where the disease has occurred unless it has been properly decontaminated.

For more information on Ralstonia wilt or help in diagnosing this problem:  Contact Brian Hudelson, Plant Disease Diagnostics Clinic, University of Wisconsin-Madison, 1630 Linden Drive, Madison, WI  53706-1598 [phone: (608) 262-2863, fax: (608) 263-3322, email:  pddc@wisc.edu.

Southern Blight

The Southern blight fungus produces large numbers of spherical, light tan to dark red resting structures called sclerotia (red arrow).

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


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

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

How can I save a plant with Southern blight?  Identify the extent of an infestation based on visible dead/dying plants, fungal mycelia and sclerotia.  Remove all plants (including roots), as well as three inches of soil, from at least 12 inches beyond the infested area.  Start at the edge of the infested area and work toward the center.  Bag all plants and soil and dispose of these materials in a landfill.

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

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

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

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

Pink Eye of Potato

What is pink eye?  Pink eye is a disorder of potato tubers that can cause costly storage losses for potato growers and can reduce tuber quality to the point where tubers will be rejected by potato processors.  Pink eye not only directly affects tubers, but also makes tubers more susceptible to diseases such as Pythium leak, bacterial soft rot (see University of Wisconsin Garden Facts XHT1224), pink rot, and Fusarium dry rot.  These diseases cause additional storage losses and reduction in quality.

Pink eye is characterized by a pink discoloration of the skin of potato tubers.
Pink eye is characterized by a pink discoloration of the skin of potato tubers.

What does pink eye look like?  Pink eye is characterized by a short-lived external pink color that is often, but not always, found around the potato eyes of freshly harvested tubers.  Eyes at the bud ends of tubers (i.e., those farthest from where tubers are attached to stems) more commonly show pink eye symptoms.  Pink eye can eventually develop into corky patch/bull hide, which involves a thickening of areas of tuber skin extending approximately 1/10 of an inch into the tuber flesh.  Corky patch/bull hide can make tubers unmarketable for either fresh market or processing use.

External pink eye symptoms are often accompanied by brown patches in the tuber flesh immediately underneath the skin.  Browning due to pink eye can resemble browning due to other disorders such as internal brown spot or heat necrosis, but these latter disorders tend to occur deeper in the tuber (i.e., inside the vascular ring), rather than just underneath the skin.

Pink eye can also be confused with late blight.  If there is any question whether the problem might be late blight rather than pink eye, contact your county Extension agent for information on submitting a sample to a diagnostic lab for proper testing.

Where does pink eye come from?  Pink eye is a physiological disorder (i.e., an abnormality in plant growth), rather than a true disease that involves a disease-causing microorganism.  Pink eye arises during periods of excessive soil moisture and warm temperatures, especially during the later stages of tuber development.  Pink eye symptoms typically appear within 7 to 10 days after excessive rain.  Excessive soil moisture coupled with high soil temperature causes a lack of oxygen around potato tubers, leading to damage of cells in the tuber skin.  This cell damage

contributes to pink eye development.  Environmental conditions that lead to pink eye also promote tuber infections by the pathogens that cause Pythium leak, bacterial soft rot, pink rot, and Fusarium dry rot (all diseases associated with pink eye in storage).

How do I salvage potato tubers affected by pink eye?  Once pink eye symptoms develop, they are permanent.  If symptoms are minor, tubers may still be usable.  However, when pink eye symptoms are severe, symptomatic tubers will be rejected and discarded.

How do I avoid problems with pink eye in the future?  Growers have no control over the extreme precipitation and high temperatures that promote pink eye development.  However, growers can practice management strategies that minimize water-saturated soils and reduce warm soil temperatures, thus reducing the severity of pink eye.

To minimize water-saturated soils, deep till areas where pink eye has been a problem, areas where water tends to collect for extended periods, and areas where soils may be compacted (e.g., field entrances or head lands).  Deep tillage will break up subsoils in these areas that impede proper drainage during wet weather.  Proper drainage will limit periods when tubers will be oxygen deprived and thus more prone to pink eye development.  Also, avoid any activities that will cause soil compaction such as operation of heavy and large farm tractors and field equipment when soils are wet.  Minimizing water-saturated soils will not only reduce the likelihood of pink eye development but will also help limit development of other tuber diseases.

To promote cooler soil temperatures, be sure to manage diseases (e.g., potato early dying) that reduce canopy coverage.  Loss of canopy allows soils to warm faster on sunny days, thus leading to higher temperatures that are more favorable for pink eye development.

Finally, be sure to scout for pink eye symptoms prior to and during harvest.  Knowing the severity of pink eye in a field can help growers make informed decisions about the appropriate duration for tuber storage and the best end use for symptomatic tubers.

For more information on pink eye:  Contact your county Extension agent.

Hot-Water Seed Treatment for Disease Management

Growing vegetables from seed is a common practice for many home gardeners.  Unfortunately, vegetable seed (even though it appears perfectly healthy) can sometimes be contaminated with disease-causing organisms, particularly disease-causing bacteria.  Bacterial speck (see University of Wisconsin Garden Facts XHT1250, “Bacterial Speck of Tomato”), bacterial spot (see University of Wisconsin Garden Facts XHT1244, “Bacterial Spot of Tomato”), and stem canker of tomato, as well as bacterial spot of pepper and black rot of crucifers such as cabbage and broccoli (see University of Wisconsin Garden Facts XHT1225, “Black Rot of Crucifers”) are common bacterial diseases where pathogens can be introduced into a garden via contaminated seed.  Making sure your vegetable seed is pathogen free is an important first step in preventing these diseases from being a problem.

Hot-water treatments can eliminate disease-causing organisms from seed.
Hot-water treatments can eliminate disease-causing organisms from seed.

Hot-water seed treatment is one method that you can use to eradicate, or at least reduce the level of pathogens (particularly bacterial pathogens), in vegetable seed.  Some commercial vegetable seed companies routinely use this method (as well as other more stringent decontamination methods) to eradicate pathogens.  Hot-water seed treatments are effective because hot water soaks into the seed for a brief time and kills disease-causing organisms, without killing the seed itself.  Other common seed treatments (e.g., fungicide treatments) can also help reduce disease, but typically do not eliminate pathogens that have penetrated the seed coat.

Hot-water seed treatment works best for small seed.  It is not as effective for large or extremely fragile seed, pelleted seed, primed seed (i.e., seed treated to speed germination), fungicide-treated seed, and old seed.  When using hot water seed treatments, treat only the amount of seed that you plan on planting.  Treatment temperatures and durations will vary depending on the particular crop (see Table 1).

To most effectively hot-water treat seed, use a water bath (in home cooking often referred to as a “water oven”) with precise temperature and timing control.  Such equipment will provide the most consistent and uniform heating, but unfortunately can be very expensive.  Alternatively, you can use a large pan heated on a stove.  In order for this method to work, you will need to use a precise thermometer to accurately and frequently measure any changes in temperature.  In addition, you must mix the water thoroughly, adjust the stove settings appropriately and submerge the seed completely during the treatment process to ensure that the seed receive a constant and uniform temperature at all times.  Water that is too hot may injure the seed; water that is too cold will not eradicate pathogens.

To hot-water treat seed, use the following steps:

  •  Wrap seed in a permeable cloth (e.g., cheesecloth);
  • Thoroughly soak (removing any air) and pre-warm seed in 100°F tap water for ten minutes;
  • Transfer seed to tap water heated to the crop-specific prescribed temperature (see Table 1);
  • Place seed in cold tap water for five minutes to quickly end the heat treatment;
  • Apply fungicide seed treatments according to the manufacturer’s instructions (optional).  Spread seed out on a paper towel or screen to air dry;
  • Apply fungicide seed treatments according to the manufacturer’s instructions (optional).

Table 1.  Hot-water treatment temperatures and timings by crop*

Crop Temperature (°F) Time (minutes)
Brussel Sprouts 122 25
Broccoli 122 20
Cabbage 122 25
Carrot 122 20
Cauliflower 122 20
Celeriac 118 30
Celery 118 30
Chinese Cabbage 122 20
Collards 122 20
Coriander 127 30
Cress 122 15
Cucumber 122 20
Eggplant 122 25
Kale 122 20
Kohlrabi 122 20
Lettuce 118 30
Mint 112 10
Mustard 122 15
New Zealand Spinach 120 60-120
Parsley 122 30
Pepper 125 30
Radish 122 15
Rutabaga 122 20
Shallot 115 60
Spinach 122 25
Tomato 122 25
Turnip 122 20

*Table modified from http://vegetablemdonline.ppath.cornell.edu/NewsArticles/HotWaterSeedTreatment.html.

For more information on hot-water seed treatment:  Contact your county Extension agent.

 

Bacterial Spot of Tomato

What is bacterial spot?  Bacterial spot of tomato is a potentially devastating disease that, in severe cases, can lead to unmarketable fruit and even plant death.  Bacterial spot can occur wherever tomatoes are grown, but is found most frequently in warm, wet climates, as well as in greenhouses.  The disease is often an issue in Wisconsin.

Sunken, scabby bacterial spot lesions on ripening tomato fruit. (Photo courtesy of Mary Ann Hansen, Virginia Polytechnic Institute and State University)
Sunken, scabby bacterial spot lesions on ripening tomato fruit. (Photo courtesy of Mary Ann Hansen, Virginia Polytechnic Institute and State University)

What does bacterial spot look like?  Bacterial spot can affect all above ground parts of a tomato plant, including the leaves, stems, and fruit.  Bacterial spot appears on leaves as small (less than ⅛ inch), sometimes water-soaked (i.e., wet-looking) circular areas.  Spots may initially be yellow-green, but darken to brownish-red as they age.  When the disease is severe, extensive leaf yellowing and leaf loss can also occur.  On green fruit, spots are typically small, raised and blister-like, and may have a yellowish halo.  As fruit mature, the spots enlarge (reaching a maximum size of ¼ inch) and turn brown, scabby and rough.  Mature spots may be raised, or sunken with raised edges.  Bacterial spot symptoms can be easily confused with symptoms of another tomato disease called bacterial speck.  For more information on this disease, see University of Wisconsin Garden Facts XHT1250.

Where does bacterial spot come from?  Bacterial spot of tomato is caused by Xanthomonas vesicatoria, Xanthomonas euvesicatoria, Xanthomonas gardneri, and Xanthomonas perforans.  These bacterial pathogens can be introduced into a garden on contaminated seed and transplants, which may or may not show symptoms.  The pathogens enter plants through natural openings (e.g., stomates), as well as through wounds.  Disease development is favored by warm (75° to 86°F), wet weather.  Wind-driven rain can contribute to more severe disease as the pathogens are splashed and spread to healthy leaves and fruit.  Bacterial spot pathogens can survive well in tomato debris, but they survive very poorly in soil when not associated with debris.

How do I save plants with bacterial spot?  A plant with bacterial spot cannot be cured.  Remove symptomatic plants from the field or greenhouse to prevent the spread of bacteria to healthy plants.  Burn, bury or hot compost the affected plants and DO NOT eat symptomatic fruit.  Although bacterial spot pathogens are not human pathogens, the fruit blemishes that they cause can provide entry points for human pathogens that could cause illness.

On tomato leaves, bacterial spot leads to small, angular (i.e., straight-edged) spots with yellow haloes. (Photo courtesy of Michelle Grabowski, University of Minnesota Extension)
On tomato leaves, bacterial spot leads to small, angular (i.e., straight-edged) spots with yellow haloes. (Photo courtesy of Michelle Grabowski, University of Minnesota Extension)

How can I prevent bacterial spot in the future?

Plant pathogen-free seed or transplants to prevent the introduction of bacterial spot pathogens on contaminated seed or seedlings.  If a clean seed source is not available or you suspect that your seed is contaminated, soak seeds in water at 122°F for 25 min. to kill the pathogens.  To keep leaves dry and to prevent the spread of the pathogens, avoid overhead watering (e.g., with a wand or sprinkler) of established plants and instead use a drip-tape or soaker-hose.  Also to prevent spread, DO NOT handle plants when they are wet (e.g., from dew) and routinely sterilize tools with either 10% bleach solution or (better) 70% alcohol (e.g., rubbing alcohol).  Where bacterial spot has been a recurring problem, consider using preventative applications of copper-based products registered for use on tomato, especially during warm, wet periods.  Keep in mind however, that if used excessively or for prolonged periods, copper may no longer control the disease.  Be sure to read and follow all label instructions of the product that you select to ensure that you use it in the safest and most effective manner possible.  Burn, bury or hot compost tomato debris at the end of the season.  Wait at least one year before planting tomatoes in a given location again, and remove and burn, bury or hot compost any volunteer tomatoes that come up in your garden.

For more information on bacterial spot of tomato:  Contact your county Extension agent.

Bacterial Speck of Tomato

What is bacterial speck?  Bacterial speck is a common disease of tomato that occurs worldwide wherever tomatoes are grown.  The disease can substantially reduce yield when it severely affects leaves early in the growing season.  The disease can have an even greater impact on quality (and marketability for commercial tomato producers) when symptoms occur on tomato fruit.

Small, brown/black spots on a green tomato characteristic of bacterial speck. (Photo courtesy of S. T. Koike)
Small, brown/black spots on a green tomato characteristic of bacterial speck. (Photo courtesy of S. T. Koike)

What does bacterial speck look like?  Leaf symptoms of bacterial speck consist of small black spots (approximately ⅛ to ¼ inch in diameter) that often are more prominent on the undersides of leaves.  As the spots age, a yellow halo may develop around the edge.  Spots on fruit are very small (almost pinpoint-like) and do not penetrate very deeply into the tissue.  The spots can be raised, flat or sunken, and range in color from brown to black.  On unripe, green fruits, the spots often have darker green haloes, while on ripe fruits the spots can have subtle, yellow haloes.  Leaf symptoms of bacterial speck can be hard to distinguish from other tomato diseases.  Bacterial spot, (see University of Wisconsin Garden Facts XHT1244, “Bacterial Spot of Tomato”) and tomato spotted wilt (a viral disease) may cause similar leaf symptoms.  Laboratory testing may be needed to determine which disease is affecting your tomatoes.

Where does bacterial speck come fromBacterial speck of tomato is caused by the bacterium Pseudomonas syringae pv. tomato.  The bacterium is typically brought into a garden on contaminated tomato seeds or transplants, and thrives in cool (63oF to 75oF), wet weather.  It can be moved from plant to plant via splashing water (e.g., from rain or overhead watering with a sprinkler) or on hands and gardening tools when working with contaminated, and then healthy plants.  The bacterium can overwinter in dead tomato debris or on porous materials such as wooden plant stakes or trellises.

How do I save tomatoes with bacterial speck?  Once tomatoes are infected, there is no cure.  You may be tempted to cut off affected leaves as symptoms develop, but this will likely not do much to minimize or slow disease development, and may actually promote spread of the pathogen.  Often the best course of action is to allow the disease to run its course and simply salvage any unblemished fruit as they ripen over the summer.  DO NOT eat symptomatic fruit.  Although the bacterial speck pathogen is not a human pathogen, the fruit blemishes that it causes can provide entry points for human pathogens that could cause illness.

On tomato leaves, bacterial speck leads to small, angular (i.e., straight-edged) spots with yellow haloes. (Photo courtesy of Alan Collmer, Cornell University)
On tomato leaves, bacterial speck leads to small, angular (i.e., straight-edged) spots with yellow haloes. (Photo courtesy of Alan Collmer, Cornell University)

How can I prevent bacterial speck in the future?  Start by using high quality, pathogen-free seed or transplants from a reputable seed supplier or garden center.  If you have seed that you believe is contaminated with the bacterial speck bacterium and would still like to use it (e.g., it’s a favorite variety with difficult-to-find seed), consider treating the seed in hot water prior to planting to eliminate the pathogen.  Treat seed with 122°F water for 25 minutes.

To prevent spread of the bacterial speck pathogen from plant to plant in your garden, DO NOT use a sprinkler to water; instead use a soaker or drip hose to water at the bases of plants.  Also, only work with tomato plants when they are dry and consider routinely disinfecting garden tools with 10% bleach or (better) 70% alcohol (e.g., rubbing alcohol).  Spray disinfectants that contain approximately 70% alcohol can also be used for this purpose.

If you have a problem with bacterial speck, remove contaminated tomato debris from your garden at the end of the growing season.  This material can be deep buried, burned (where allowed by local ordinance) or hot composted.  DO NOT replant tomatoes in the same area the following growing season; instead grow a nonsusceptible vegetable crop.  This approach is referred to as non-host crop rotation.  For more information on this technique, see University of Wisconsin Garden Facts XHT1210, “Using Crop Rotation in the Home Vegetable Garden”.

As a last resort, consider chemical treatments for bacterial speck control.  If you decide to go this route, use a product that is labeled for use on tomatoes and that contains copper as the active ingredient.  To be most effective, the first treatment must be applied before symptoms have developed.  Apply additional treatments every 10 to 14 days as long as cool, moist conditions continue.  Keep in mind however, that if used excessively or for prolonged periods, copper may no longer control the disease.  Be sure to read and follow all label instructions on the product that you select to make sure you use it in the safest, most effective manner possible.

For more information on bacterial speck of tomato:  Contact your county Extension agent.

Cucumber Mosaic

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

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

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

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

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

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

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

For more information on cucumber mosaic:  Contact your county Extension agent.

Basil Downy Mildew

Basil Downy Mildew
Leaf curling and yellowing, as well as a gray-purple fuzz on the bottoms of leaves, is typical of basil downy mildew. (Photo courtesy of Debbie Roos)

What is basil downy mildew?  Basil downy mildew is a devastating disease that affects the leaves, branches, and stems of many types of basil (i.e., plants in the genus Ocimum) commonly used for cooking.  Green-leafed varieties of sweet basil are particularly susceptible to the disease, while purple-leafed varieties of basil, Thai basil, lemon basil, and spice basil are less susceptible.  Certain ornamental basils (e.g., hoary basil) appear to be highly resistant to the disease.  Basil downy mildew was first reported in the United States in 2007 and has since spread widely to wherever basil is grown, including Wisconsin.

What does basil downy mildew look like?  Symptoms of basil downy mildew typically develop first on lower leaves, but eventually an entire plant will show symptoms.  Initial symptoms include leaf yellowing (which gardeners often think is due to a nitrogen deficiency) followed by leaf browning.  Affected leaves also curl and wilt, and on the undersides of the leaves, a gray-purple fuzzy material will develop.

Where does basil downy mildew come from?  Basil downy mildew is caused by the fungus-like organism, Peronospora belbahrii.  This pathogen can be easily introduced into a garden each year via contaminated seed, on infected transplants, or via wind-borne spores (technically called sporangia).  Once introduced into a garden the pathogen can spread by wind, by rain splash, or via items (e.g., hands, clothing, garden tools) that come into contact with infected plant and then are used to work with healthy plants.  The pathogen thrives in humid, warm environments and can spread rapidly, decimating an entire basil crop.

How do I save plants with basil downy mildew?  There is no known cure for basil downy mildew.  If you see basil downy mildew, harvest any asymptomatic leaves on infected plants, as well as other healthy basil plants in your garden.  Use these materials immediately (e.g., to make pesto).  Remove and bag any symptomatic plant remains and dispose of this material in your garbage.

How do I avoid problems with basil downy mildew in the future?  Avoid planting sweet basil if possible.  Instead, plant other types of basil that are more resistant to basil downy mildew.  If you decide to grow sweet basil, try growing the variety ‘Eleonora’ which has been bred for at least some resistance to the disease.  If you grow basil from seed, check to see if the seed you are buying has been steam-treated to kill the downy mildew pathogen.  Be aware however, that this information may be difficult to find, because steam treatment of basil seed is relatively new and the use of this technique is not widely advertised (at least to home gardeners).

Whatever type of basil you choose, try to grow your plants in a manner that will keep them as dry as possible, thus creating an environment that is less favorable for the downy mildew pathogen to develop and infect.  Plant basil in a sunny location, space plants as far apart as possible and orient rows in the direction of prevailing winds to promote good airflow and rapid drying of plants when they get wet.  Avoid overhead watering (e.g., with a sprinkler) that will wet leaves and spread the pathogen; instead, use a drip or soaker hose to water.

Use of fungicide treatments to control basil downy mildew is NOT recommended.  Products that currently are available to homeowners, even when applied in the best manner possible, will likely not control the disease adequately, if at all.  Thus using these products would be a waste of time, effort and money.

For more information on basil downy mildew:  Contact your county Extension agent.

White Mold

What is white mold? White mold, also known as Sclerotinia stem rot, is a serious and often lethal fungal disease that affects over 400 species of plants in many plant families. White mold causes severe damage in commercial snap, kidney and lima bean production, as well as commercial sunflower production. White mold also can be a serious problem in home vegetable gardens on tomatoes, potatoes, cucurbits (e.g., cucumber, pumpkin and squash), carrots, lettuce, celery, brassicas (e.g., cabbage, broccoli, and cauliflower), basil, and rosemary. White mold is a common disease on many annual and perennial ornamentals as well.

Stem cankers, cottony mycelia and sclerotia (see arrows) of white mold on snap beans.
Stem cankers, cottony mycelia and sclerotia (see arrows) of white mold on snap beans.

What does white mold look like? Symptoms of white mold vary depending on the plant infected. White mold can lead to crown and stem cankers, root rots, wilts, damping-off of seedlings, and blossom and fruit rots. Plants affected by white mold can wilt rapidly due to stem-girdling cankers at or near the soil line. Brown spots can appear on flower petals and buds. Cottony masses of fungal threads (called mycelia) may appear on stems or on nearby soil. Hard, irregularly-shaped masses (called sclerotia) develop within or on the surface of infected plants. The sclerotia are white at first and then turn dark brown or black as they mature.

Where does white mold come from? White mold is caused by several species of the fungus Sclerotinia, most commonly Sclerotinia sclerotiorum. White mold fungi overwinter as sclerotia in dead plant material or in infested soil. Sclerotia can survive in soil for at least 3 years. During periods of cool, wet or humid weather, sclerotia germinate to form either fungal threads (called hyphae) or tiny, mushroom-like spore-producing structures (called apothecia). Apothecia can release millions of spores over a period of several days. Spores are dispersed primarily by wind, but also by rainsplash and insects. Both spores and hyphae can infect plant tissue, with infection often occurring through dead or declining plant parts (e.g., flowers and leaves), and then spreading to healthy tissue. Infections may also occur through healthy plant parts that are growing near or in contact with the soil. In some plants, seeds can be contaminated by sclerotia or mycelia of white mold fungi.

How do I save plants with white mold? White mold is difficult to manage once infections have occurred. Prune four to five inches below obviously disease tissue or remove entire plants if they are severely affected. DO NOT compost these materials because of the risk of spreading white mold fungi via long-lived sclerotia. Burn these materials instead.

White mold can be destructive on ornamentals such as Liatris.
White mold can be destructive on ornamentals such as Liatris.

How do I avoid problems with white mold in the future? Check seed for sclerotia and remove these structures if you find them. Make sure your soils are well-drained, and avoid overwatering as well as frequent, light watering. DO NOT overhead water (e.g., with a sprinkler). Instead water deeply early in the day with a drip or soaker hose. Promote good air circulation in your garden so that plants more rapid dry when they do get wet (e.g., from natural rain). Better air circulation can be achieved by choosing cultivars or varieties of plants that have a more open growth form, by spacing plants farther apart, by avoiding excessive use of nitrogen fertilizers (which can promote excessive production of lush, thick foliage), and by keeping weeds under control. Weed control also eliminates potential alternate hosts that white mold fungi can infect. As leaves and flowers or your plants naturally wither and die, remove them, as well as any other plant debris that may harbor white mold fungi. Finally, in beds where severe cases of white mold have occurred, remove and replace infested soil.

For more information on white mold: Contact your county Extension agent.

Verticillium Wilt of Vegetables

What is Verticillium wilt? Verticillium wilt is a typically fatal disease that affects a wide range of vegetable crops in Wisconsin. Vegetables commonly and severely affected by Verticillium wilt include tomato, potato, pepper, eggplant, cucumber, muskmelon, pumpkin and watermelon. Other vegetables such as asparagus, bean, beet, broccoli, Brussels sprouts, cabbage, cauliflower, celery, chive, collards, garlic, kale, kohlrabi, leek, lettuce, mustard, onion, parsley, peas, radish, rutabaga, spinach and turnip are susceptible to Verticillium wilt, but show less severe symptoms. Some vegetables, such as sweet corn are immune to the disease. Many woody and herbaceous ornamentals are also susceptible to Verticillium wilt (see also University of Wisconsin Garden Facts XHT1008 “Verticillium Wilt of Trees and Shrubs”).

A blotchy yellow leaf color can be an initial symptom of Verticillium wilt on eggplant.  (photo courtesy of Amanda Gevens)
A blotchy yellow leaf color can be an initial symptom of Verticillium wilt on eggplant. (photo courtesy of Amanda Gevens)

What does Verticillium wilt look like? Initial symptoms of Verticillium wilt include sudden yellowing of foliage, typically first appearing on one side of a plant. These leaves then wilt and die. Oftentimes symptoms mimic the natural aging process of leaves. Eventually however, the entire plant becomes affected and dies. By peeling away the outer layers of affected branches or vines, one may observe brown streaking in the vascular (water-conducting) tissue. Vascular browning is common in tomato and potato plants.

Where does Verticillium wilt come from? Verticillium wilt is caused primarily by two fungi, Verticillium dahliae and Verticillium albo-atrum. These fungi are commonly found in Wisconsin soils. V. dahliae and V. albo-atrum have wide host ranges including many common weeds, such as ragweed, cocklebur, and velvetleaf. Susceptible plants and their debris can serve as an overwintering site for these fungi. In potato, root lesion nematodes (Pratylenchus spp.) can contribute to the development of Verticillium wilt. However, the role of these nematodes in Verticillium wilt development in other vegetable crops is not well understood.

How do I save a plant with Verticillium wilt? Vegetables infected with Verticillium cannot be cured and will eventually die. However, one can extend the life of infected plants by fertilizing and watering them properly. Fertilization should be based on a soil nutrient test and should be tailored to the specific vegetables being grown. Most vegetables require approximately one inch of water per week. If natural rains are not sufficient, then supplemental water should be applied using a soaker or drip hose.

Verticillium wilt eventually leads to wilting and plant death. (photo courtesy of Amanda Gevens)
Verticillium wilt eventually leads to wilting and plant death. (photo courtesy of Amanda Gevens)

How do I avoid problems with Verticillium wilt in the future? Do not repeatedly grow crops that are highly susceptible to Verticillium wilt in the same area of your garden each year. Try to rotate highly susceptible crops with other less susceptible crops. Also, try to plant vegetable varieties that are resistant to Verticillium wilt. In particular, many modern tomato varieties are resistant to the disease. Watch for varieties with the letter “V” after the variety name. If Verticillium wilt resistant varieties of your favorite vegetables are not available, select varieties that mature early. Early maturing varieties are more likely to produce usable vegetables before they eventually succumb to the disease than late maturing varieties.

In addition to using resistant varieties, try to eliminate sources of Verticillium in your garden. Keep broadleaf weeds under control, as these plants are often susceptible to Verticillium wilt and can serve as a reservoir for the fungus. DO NOT use mulches that may have been produced from trees (e.g., maple and ash trees) that are susceptible to Verticillium wilt, and immediately collect and discard leaves that have fallen from symptomatic trees. Also, remove and discard debris from susceptible vegetable plants. Burn (where allowed by local ordinance) or landfill infected plants and infested plant debris. DO NOT bury or compost these materials. Finally, decontaminate any gardening items (e.g., shovels, hoes, plant stakes, pots, working surfaces, etc.) that may have come into contact with Verticillium to prevent spread of the fungus. Rinse these items with water to remove clinging soil, then treat them for at least 30 seconds with a 10% bleach solution or (preferable for metal tools due its less corrosive properties) 70% alcohol. Rubbing alcohol and many spray disinfectants contain approximately this alcohol concentration.

For more information on Verticillium wilt: See UW-Extension Bulletin A2537, “Maple and Other Tree Disorder: Verticillium Wilt” (available at http://learningstore.uwex.edu); University of Wisconsin Garden Facts XHT1146, “Verticillium Wilt of Vegetables” (available at http://hort.uwex.edu); or contact your county Extension agent.