Boxwood Blight Host: Boxwood Pathogens: Calonectria pseudonaviculata Signs/Symptoms: Circular, brown leaf spots followed by leaf drop and shrub death For more information see: UW Plant Disease Facts D0023
Lipstick Rust Host: Chinese juniper, apple, crabapple Pathogen: Gymnosporangium yamadae Signs/Symptoms: Brown blobs with orange gelatinous masses (juniper), red leaf spots (apple, crabapple)
Late Blight Host: Tomato, potato Pathogen: Phytophthora infestans Signs/Symptoms: Water-soaked spots on leaves, leathery areas on tomato fruits, rapid plant death For more information see: UW Plant Disease Facts D0068
Septoria Leaf Spot and Early Blight Host: Tomato Pathogens: Septoria lycopersici and Alternaria solani Signs/Symptoms: Spotting and eventual total collapse of leaves, working from the bottom of the plant up For more information see: UW Plant Disease Facts D0100/46
Septoria Leaf Spot of Lilac Host: Lilac Pathogen: Septoria sp. Signs/Symptoms: Dead spots on leaves, potentially leading to complete leaf browning
Wood Rots Hosts: Woody trees and shrubs Pathogens: Miscellaneous wood rot fungi Signs/Symptoms: Shelf-like growths on trunks and branches
Canker Diseases Hosts: Woody trees and shrubs Pathogens: Miscellaneous canker fungi Signs/Symptoms: Sunken areas on trunks/branches For more information see: UW Plant Disease Facts D0027, D0037, D0042, D0055, D0074, D0114
Virus Diseases Hosts: All plants, particularly herbaceous ornamentals Pathogen: Miscellaneous plant viruses Signs/Symptoms: Blotchy leaf color, growth distortions For more information see: UW Plant Disease Facts D0036, D0063, D0067, D0115, D0116, D0130
Rhizosphaera Needle Cast Hosts: Colorado blue spruce, other spruces Pathogen: Rhizosphaera kalkhoffii Signs/Symptoms: Browning/purpling of interior needles of lower branches, followed by needle drop For more information see: UW Plant Disease Facts D0093
Chlorosis Hosts: Pin oak, red maple, birch, azalea, white pine, blueberry Pathogen: None Signs/Symptoms: Yellow leaves with dark green veins For more information see: UW Plant Disease Facts D0084
Improper Planting Hosts: Woody trees and shrubs Pathogen: None Signs/Symptoms: No root flare at the soil line, girdling roots, frost cracks, canopy thinning, early fall color, branch dieback, tree/shrub decline and death
Septoria Leaf Spot and Early Blight Host: Tomato Pathogens: Septoria lycopersicia and Alternaria solani Signs/Symptoms: Spotting and eventual total collapse of leaves working from the bottom of the plant up For more information see: UW Garden Facts D0100/D0046
Late Blight Hosts: Tomato, potato Pathogen: Phytophthora infestans Signs/Symptoms: Water-soaked spots on leaves, leathery areas on tomato fruits, rapid plant death For more information see: UW Garden Facts D0068
Blossom End Rot Host: Tomato, pepper, eggplant, cucumber, squash Cause: Calcium deficiency Signs/Symptoms: Decayed areas on the bottom sides of vegetable fruits For more information see: UW Garden Facts D0022
Powdery Mildew Hosts: Any vegetable, particularly vine crops, peas Pathogens: Miscellaneous powdery mildew fungi Signs/Symptoms: Powdery white growth on leaves For more information see: UW Garden Facts D0086
Common Corn Smut Hosts: Corn Pathogen: Ustilago maydis Signs/Symptoms: Pasty white masses on corn ears eventually decomposing into a brown powder For more information see: UW Garden Facts D0031
Black Rot Hosts: Crucifers (e.g., cabbage, broccoli, cauliflower) Pathogen: Xanthomonas campestris pv. campetris Signs/Symptoms: V-shaped yellow/dead areas on leaves progressing into plant deterioration and death For more information see: UW Garden Facts D0019
Potato Scab Hosts: Potato, carrot, beet, other root crops Pathogen: Streptomyces scabies Signs/Symptoms: Brown, rough, scab-like areas on tubers and roots For more information see: UW Garden Facts D0083
Verticillium Wilt Host: Tomato, pepper, eggplant, potato, vine crops Pathogen: Verticillium spp. Signs/Symptoms: Leaf yellowing and wilting of plants followed by eventual plant death For more information see: UW Garden Facts D0122
Aster Yellows Hosts: Carrot Pathogens: Aster yellows phytoplasma Signs/Symptoms: Yellow/orange/purple leaves, stunted roots with tufts of white hairy roots For more information see: UW Garden Facts D0007
Bacterial Wilt Hosts: Vine crops Pathogen: Erwinia tracheiphila Signs/Symptoms: Sectional wilting and eventual death of plants after cucumber beetle feeding For more information see: UW Garden Facts D0014
Basil Downy Mildew Host: Basil Pathogen: Peronospora belbahrii Signs/Symptoms: Downward-cupped, yellow leaves with purple-gray fuzz on leaf undersurfaces For more information see: UW Garden Facts D0015
What is tobacco mosaic? Tobacco mosaic is a common viral disease of worldwide distribution that affects over 200 species of herbaceous and, to a lesser extent, woody plants. Common hosts include tobacco, solanaceous vegetables (e.g., pepper, tomato) and vining vegetables (e.g., cucumber, melon, squash), as well as a wide range of ornamentals (e.g., begonia, coleus, geranium, impatiens, million bells, petunia). The disease has its biggest impact on vegetables, where it can reduce yield and affect quality to the point that commercial crops cannot be marketed.
What does tobacco mosaic look like? Symptoms of tobacco mosaic vary in type and severity depending on the plant infected, plant age, the variant of the virus involved, and environmental conditions. On leaves, typical symptoms include blotchy light and dark areas (called mosaic); cupping, curling, elongation (strapping), roughening, wrinkling and other growth distortions; and smaller than normal size. Fruits may have a blotchy color, ripen unevenly, be malformed or have an off flavor. Entire infected plants are often stunted. Other viral diseases like cucumber mosaic (see UW Plant Disease Facts D0036, Cucumber Mosaic) can cause symptoms similar to tobacco mosaic. Often, multiple viral diseases can simultaneously affect a single plant. Certain herbicide exposures (see UW Plant Disease Facts D0060, Herbicide Damage), nutrient deficiencies or toxicities, high temperature and even insect feeding can also cause similar symptoms. Proper diagnosis of tobacco mosaic requires lab testing.
Where does tobacco mosaic come from? Tobacco mosaic is caused by Tobacco mosaic virus (TMV), the first virus ever identified. Numerous variants (strains) of the virus have subsequently been described. TMV survives in infected plants (including viable seeds), as well as in debris from these plants. Plant-based products (most notoriously tobacco products) can harbor the virus. TMV is very stable and can survive for long periods of time; there are reports of TMV surviving and remaining infectious after 50 years in storage at 40°F. Because of its stability, TMV can survive on and be picked up from hands, clothing, gardening tools, work surfaces and any other object (e.g., door knobs) that gardeners may handle.
TMV is highly transmissible and is commonly spread by handling infected plants, then healthy plants. Spread via gardening tools is also very common. No specific insects spread TMV (the way that aphids spread Cucumber mosaic virus). However, bees and chewing insects (e.g., grasshoppers) can transmit TMV through casual contact or their feeding as they move from plant to plant.
How do I save a plant with tobacco mosaic? There is no cure for tobacco mosaic. Once infected, plants remain infected for life, and typically the virus spreads throughout the plant from the point of infection. Infected plants and any associated debris should be burned (where allowed by local ordinance) or double-bagged and disposed of in a landfill. DO NOT compost plants with this disease. Thoroughly decontaminate any items that have come into contact with infected plants or their debris by treating them for a minimum of one minute with:
2.75 tablespoons Alconox® (a lab detergent) plus 2.5 tablespoons sodium lauryl sulfate (SLS), also known as sodium dodecyl sulfate (SDS), in one gallon of water, or
14 dry ounces of trisodium phosphate in one gallon of water.
These ingredients can be ordered on the internet. If you decide to use SLS (SDS), be sure to wear gloves, safety goggles and a dust mask, and mix the solution in a well-ventilated area as SLS (SDS) is a known skin and eye irritant. Once treated, rinse items with sufficient water to remove any residues. Also, thoroughly wash your hands with soap and water, and launder any clothing that you wore while disposing of infected plants and debris.
How do I avoid problems with tobacco mosaic virus in the future? Inspect plants prior to purchase for any symptoms of tobacco mosaic, and DO NOT buy symptomatic plants. Purchase seed from a reputable supplier that routinely inspects their seed-producing plants for symptoms of viral (and other) diseases. If you use tobacco products, DO NOT use them around plants. Also, wash your hands thoroughly with soap and water prior to handling plants, and consider wearing freshly laundered clothing when gardening. Finally, decontaminate (as described above) any items that might harbor TMV to help prevent spread. Even if you do not use tobacco products, routine handwashing and decontamination of gardening tools and other items can help prevent tobacco mosaic from being a problem.
For more information on tobacco mosaic: Contact your county Extension agent.
What is tomato spotted wilt? Tomato spotted wilt is a common viral disease of worldwide distribution that can affect over 1000 plant species. Economically important hosts include a wide range of vegetables, fruits, field crops, and ornamentals. Many weeds are also potential hosts. Tomato spotted wilt is especially important in greenhouse production where it can cause significant economic losses in horticultural and floral crops. In potato production, tomato spotted wilt is uncommon but of enough concern that routine monitoring for the disease in greenhouse and field settings is warranted.
What does tomato spotted wilt look like? Symptoms of tomato spotted wilt vary widely depending on host species, host variety and when during development a plant is infected. Typical symptoms on leaves include yellow, brown or black ringspots (i.e., ring-like areas of discolored tissue). On some hosts, the ringspots can form a target-like pattern. Distorted leaf growth can also be a typical symptom.
On potato leaves, tomato spotted wilt can lead to necrotic (i.e., dead) areas with or without yellow haloes. These symptoms can resemble symptoms of early blight (see UW Plant Disease Facts D0046, Early Blight). Black streaks on petioles or stems and branch tip dieback are also common symptoms on potato (as well as other hosts). Potato tuber symptoms include surface rings or dark patches, and internal discolored patches, rings, spots or flecks.
Where does tomato spotted wilt come from? Tomato spotted wilt is caused by Tomato spotted wilt virus (TSWV), which is closely related to Impatiens necrotic spot virus (INSV), the cause of impatiens necrotic spot (see UW Plant Disease Facts D0067, Impatiens Necrotic Spot). TSWV is found in the sap of infected plants and is most commonly spread by thrips (see University of Wisconsin Garden Facts XHT1022, Managing Thrips in Greenhouses). At least eight thrips species can be involved in TSWV transmission. Thrips larvae acquire the virus as they feed on infected plants, then transmit the virus as they move to and feed on healthy plants. Once thrips acquire the virus, they can transmit it for their entire lifespans. The severity of tomato spotted wilt depends on the size and activity of thrips populations at a location, as well as on the number of infected plants (often weeds, but potentially infected potatoes as well) serving as reservoirs for the virus. TSWV can also be transmitted mechanically (e.g., on tools used to trim branches or cut potato tubers), but this method of transmission is much less common than transmission by thrips.
How can I save plants with tomato spotted wilt? There is no known cure for tomato spotted wilt. Infected plants should be removed and destroyed to eliminate a reservoir for the virus that can subsequently contribute to spread to other plants. Infected plants can be burned (where allowed by local ordinance), deep buried or hot composted.
How can I avoid problems with tomato spotted wilt in the future? Prevent introducing TSWV and thrips into your greenhouse by carefully inspecting any new plants for potential problems. Test suspect plants for TSWV using dipstick tests [available from Agdia, Inc. (www.agdia.com)] or by submitting a sample to the UW-Madison Plant Disease Diagnostics Clinic (pddc.wisc.edu). Remove and dispose of any infected plants.
To prevent new infections via thrips, remove weeds in and outside of greenhouses to eliminate TSWV and thrips reservoirs. Place 400-mesh screens on vents to limit thrips movement. Also limit thrips movement on clothing by avoiding colors (e.g., pink, blue, yellow, white or green) that can attract thrips. Monitor for thrips using blue and yellow sticky cards placed above plants throughout the greenhouse and near doors and vents. Use chemical and/or biological control products to control thrips. See University of Wisconsin Garden Facts XHT1022, Managing Thrips in Greenhouses, for details on what products to use and when to use them. Multiple applications will likely be necessary because thrips eggs are not killed by insecticides. If you find plants that you suspect are infected with TSWV, remove and dispose of these plants immediately.
To prevent mechanical transmission of TSWV, be sure to routinely decontaminate any items (e.g., pruners, knives, pots, work surfaces) that come into contact with plants by treating them for a minimum of one minute with a solution of one of the following:
2.75 tablespoons Alconox® (a type of lab detergent) plus 2.5 tablespoons sodium dodecyl sulfate (SDS) [also known as sodium lauryl sulfate (SLS)] in one gallon of water, or
14 dry ounces of trisodium phosphate in one gallon of water.
In field settings, tomato spotted wilt does not appear to be a significant problem on potato (although it can be a significant problem on other crops). Use of resistant or tolerant potato varieties may reduce yield loses, but unfortunately there is little information on which potato varieties are resistant/tolerant to the disease. Removing symptomatic plants can reduce potential reservoirs of TSWV, but may not be feasible. More important in field settings may be to keep weeds under control. Current research suggests that TSWV does not persist long-term in the field unless there are perennial weeds to serve as TSWV reservoirs.
For more information on tomato spotted wilt: Contact your county Extension agent.
What is Ralstonia wilt? Ralstonia wilt (also sometimes known as Southern wilt) is a typically 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.
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 it 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 four feet 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: email@example.com].
What is Southern blight? Southern blight is a lethal fungal disease that is most common in the tropics and subtropics. This disease causes damage in the southern United States and can even cause problems in temperate locations like Wisconsin during periods of warm, moist weather. Southern blight has a wide host range, affecting over 500 plant species. Vegetable and fruit hosts include tomato, pepper, onion, beet, rhubarb, strawberry, lettuce, cucumber, melon, carrot, asparagus and parsley. Ornamental hosts include aster, black-eyed Susan, dahlia, daylily, gladiolus, hosta, impatiens, peony, petunia, rose, salvia, sedum and viola. Small woody trees and shrubs can be affected as well.
What does Southern blight look like?Southern blight initially leads to a water-soaked appearance on lower leaves or water-soaked lesions (spots) on lower stems. Any plant part that is near or in contact with the soil may become infected. Infected plants yellow and wilt, often within days of infection, particularly when the weather is moist and warm (80 to 95°F). Fruit rots, crown rots and root rots are also typical symptoms of the disease. Thick mats of white fungal threads (called mycelia) may grow from infected tissue, radiating from the plant onto the soil surface. Sclerotia (small spherical structures that are about the size of mustard seeds) develop on infected tissue and on the soil surface. Sclerotia range in color from light tan to dark reddish-brown to black.
Where does Southern blight come from?Southern blight is caused by the fungus Athelia rolfsii (formerly Sclerotium rolfsii), which lives in soil, on plants (including weeds), and in plant debris. The fungus can be spread through movement of infested soil and plant debris, on infected plants, in contaminated irrigation water, and through use of contaminated tools. In Wisconsin, A. rolfsii most likely enters gardens on infected nursery stock or infested mulch. Freezing temperatures will kill A. rolfsii mycelia, but sclerotia can survive temperatures as low as approximately 14ºF.
How can I save a plant with Southern blight? Identify the extent of an infestation based on visible dead/dying plants, fungal mycelia and sclerotia. Remove all plants (including roots), as well as three inches of soil, from at least 12 inches beyond the infested area. Start at the edge of the infested area and work toward the center. Bag all plants and soil and dispose of these materials in a landfill. Turn any remaining soil in the infested area eight to 12 inches to bury any sclerotia that you may have missed. This will reduce the length of time that the sclerotia will survive. Grow non-susceptible plants (e.g., larger woody ornamentals) in the affected area for two to three years to allow time for sclerotia to die naturally. Fungicides containing azoles (e.g., propiconazole, tebuconazole), fludioxonil, flutolanil, mancozeb, PCNB, strobilurins (e.g., azoxystrobin, fluoxastrobin), thiophanate-methyl, and triadimefon are all labeled for Southern blight control, but may have varying levels of effectiveness. All of these products will likely be more effective if applied as preventive treatments rather than in an attempt to “cure” existing disease. If you decide to use fungicides, DO NOT use one active ingredient for all treatments. Instead, alternate the use of two or more unrelated active ingredients to help minimize problems with fungicide-resistant strains of A. rolfsii. DO NOT alternate active ingredients that are chemically related (e.g., propiconazole and tebuconazole, or azoxystrobin and fluoxastrobin). Be sure to read and follow all label instructions of the fungicide(s) that you select to ensure that you use the product(s) in the safest and most effective manner possible.
How can I prevent Southern blight in the future?Inspect new plants for sclerotia and mycelia of A. rolfsii prior to transplanting. Bag and dispose of diseased plants as described above. Use high-quality mulches (e.g., shredded oak bark mulch, red cedar mulch) in your garden and avoid any mulches that you suspect might be contaminated with A. rolfsii. A. rolfsii thrives under moist conditions. Therefore, thin existing gardens or space plants farther apart in new gardens to improve airflow and promote more rapid drying of foliage and soil.
For more information on Southern blight: Contact your county Extension agent.
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 UW Plant Disease Facts D0010, Bacterial Soft Rot), pink rot, and Fusarium dry rot. These diseases cause additional storage losses and reduction in quality.
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.
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 (see UW Plant Disease Facts D0068, 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 seven 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 (see UW Plant Disease Facts D0010, 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 other 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.
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 UW Plant Disease Facts D0011, Bacterial Speck of Tomato), bacterial spot (see UW Plant Disease Facts D0012, 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 UW Plant Disease Facts D0019, 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 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 somewhat expensive. Alternatively (but much more of a challenge), you can try to 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;
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*
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.
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 UW Plant Disease Facts D0011, Bacterial Speck of Tomato.
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 your garden or greenhouse to prevent the spread of bacteria to healthy plants. Burn (where allowed by local ordinance), 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.
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, consider treating the seed in hot water prior to planting to eliminate the pathogens. For details on this process, including the proper temperature and length of treatment, see UW Plant Disease Facts D0064, Hot-Water Seed Treatment for Disease Management. 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. 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 due to its less corrosive properties) 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 (where allowed by local ordinance), 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.
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.
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 UW Plant Disease Facts D0012, 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 from? Bacterial 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 (63°F to 75°F), 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.
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. For details on this process, including the proper temperature and length of treatment, see UW Plant Disease Facts D0064, Hot-Water Seed Treatment for Disease Management.
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 due to its less corrosive properties) 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 ensure that you use it in the safest, most effective manner possible.
For more information on bacterial speck of tomato: Contact your county Extension agent.