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 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*

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

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