Category Archives: Disease – Vegetable

Herbicide Damage

What is herbicide damage?  Herbicide damage is any adverse, undesired effect on a plant that is caused by exposure of that plant to a pesticide designed for weed control (i.e., a herbicide).  Any plant can be subject to this problem.
 

Squash leaf distorted due to exposure to a common lawn herbicide.
Squash leaf distorted due to exposure to a common lawn herbicide.

What does herbicide damage look like?  Symptoms of herbicide damage vary depending upon the plant affected and the herbicide used.  Common symptoms include stems that are flattened, or that twist or corkscrew.  Leaves may have abnormal shapes, sizes or textures.  In addition, leaves or leaf veins may yellow or redden.  In severe cases, plants may brown and die.  Some plants, such as tomatoes and grapes, are particularly susceptible to herbicide damage and can be used as indicators of unwanted herbicide exposure.
 
How does herbicide damage occur?  Herbicide damage results when an herbicide is misapplied.  Herbicides for control of broadleaf weeds are occasionally applied with fertilizers as part of a lawn care program.  If these products are applied too close to ornamentals or vegetables, or are applied when there is too much wind, then the herbicide can drift (move) from the area of application into an untreated area.  Often, drifting herbicides are difficult to detect by eye because they are extremely fine mists and can better be detected by smell.  Some herbicides readily produce vapors that can begin to drift several hours after application. 

How do I save a plant that has been damaged by herbicides?  Don’t panic!  There is nothing you can do after plants have been exposed.  However, most plants accidentally exposed to broadleaf herbicides applied with lawn fertilizers do not receive a high enough dose to kill them.  Young growth exposed to the herbicide will be distorted and discolored, but subsequent growth will be normal.

How do I avoid problems with herbicide damage in the future?  When using a lawn herbicide, follow the application directions exactly.  Don’t apply the product too close to, or in a manner that will cause exposure to, non-target ornamentals or vegetables.  To avoid drift, apply the herbicide when there is as little wind as possible (< 5 mph).  Apply the herbicide at low pressure to minimize production of fine mists.  Finally use amine forms rather than ester forms of herbicides as amine forms are less likely to produce vapors.

For more information on herbicide damage:  See UW-Extension Bulletin A3286 or contact your county Extension agent.

Gray Mold (Botrytis Blight)

What is gray mold?  Gray mold (or Botrytis blight) is a common and often serious fungal disease that can affect plants of all kinds.  Gray mold is a particularly serious problem on flowering plants and those plants grown in greenhouses.

Severe gray mold can prevent rose blossoms from developing properly.
Severe gray mold can prevent rose blossoms from developing properly.

What does gray mold look like?  Gray mold causes brown spots on petals that enlarge, killing the petals, and eventually the rest of the flower.  Early infections may prevent flowers from opening.  On plants such as tulips, crocus, and daffodils, gray mold may spread from flowers into the bulbs leading to bulb decay.  On leaves, Botrytis causes irregularly-shaped necrotic (dead) areas that may have a bull’s-eye pattern.  Botrytis can also cause stem cankers (localized sunken areas) that may eventually enlarge to girdle the stem.

Where does gray mold come from?  Gray mold is caused by the fungus Botrytis cinerea, which survives on dead plant tissue as dark brown to black, multi-celled structures called sclerotia, and as thick, dark-walled, single-celled spores called chlamydospores.  Botrytis produces large numbers of dusty, gray reproductive spores that are spread by wind or splashing water.  Botrytis spores rapidly die when dried, and most readily infect delicate tissues such as petals.  In order to infect tough tissues such as healthy leaves, Botrytis spores require an external food source such as nutrients leaking from wounds or dying tissues such as old flower petals.

How do I save a plant with gray mold?  Promptly remove diseased leaves and flowers.  Prune diseased branches four to six inches below the infection leaving a clean cut.  Disinfest pruning tools between cuts by dipping them for at least 30 seconds in a 10% bleach solution or alcohol (spray disinfectants that contain at least 70% alcohol can also be used). 

How do I avoid problems with gray mold in the future?  Remove dead or dying tissue from plants and the soil surface.  Avoid wounding plants mechanically, or chemically by overfertilization or misuse of pesticide sprays.  Reduce humidity around plants and germinate seedlings under warm, relatively dry conditions.  Fungicides such as chlorothalonil, iprodione and mancozeb can be used to prevent infections.  However, Botrytis may develop resistance to these products (particularly iprodione).  Be sure to read and follow all label instructions of the fungicide that you select to insure that you use the fungicide in the safest and most effective manner possible. 

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

Foliar Nematodes

What are foliar nematodes?  Foliar nematodes are microscopic worm-like organisms in the genus Aphelenchoides.  They live in and on the leaves (and other above-ground plants parts) of over 450 plant species in more than 75 plant families. 

Angular dead areas on Brunnera leaves typical of infections by foliar nematodes.  (Photo courtesy of Monica Lewandowski, The Ohio State University Plant Pathology)
Angular dead areas on Brunnera leaves typical of infections by foliar nematodes. (Photo courtesy of Monica Lewandowski, The Ohio State University Plant Pathology)

They are commonly found on hostas, but can affect other herbaceous ornamentals (e.g., African violet, anemone, begonia, chrysanthemum, fern, orchid, veronica) and woody ornamentals (e.g., azalea, elm, privet), as well as fruit crops (e.g., sour cherry, strawberry) and vegetable crops (e.g., broccoli, celery, lettuce, onion, pinto bean, potato, squash, tomato).  Damage from foliar nematode is usually cosmetic and non-lethal making ornamental plants less attractive and less saleable.  In severe cases and on particularly susceptible hosts (e.g., strawberry), foliar nematodes can cause extensive leaf injury and defoliation, and can interfere with and limit flowering. 

What does foliar nematode damage look like?  Unlike most other nematodes which cause root damage, foliar nematodes cause damage to above-ground plant parts, especially leaves.  In young plants, foliar nematodes can cause new growth to curl, twist, and stunt.  In more mature plants, foliar nematodes cause small, discolored, angular (i.e., straight-edged) blotches on leaves.  The blotchy areas are typically bordered by veins.  Blotches eventually turn brown and dry, and may fall away, giving the leaf a “shot-holed” appearance.  Angular blotches often are not apparent until late in the growing season (e.g., August).

Where do foliar nematodes come from?  Foliar nematodes are typically first introduced into a garden on infected/infested plants brought from another location.  Foliar nematodes can then be spread from infected/infested plants to healthy plants by water splash from rain or overhead watering.  Infections most often occur during periods of high humidity or when films of water form on leaves allowing for nematode movement.  Foliar nematodes can also be spread when infected/infested plants are vegetatively propagated (i.e., when cuttings are taken from infested plants).  Foliar nematodes can survive in dry leaves, dormant buds, and in soil, but not in plant roots.  They can survive in unfavorable (e.g., dry or freezing) conditions and quickly become active when moisture becomes available. 

How do I save a plant with foliar nematodes?  Eliminating foliar nematodes is virtually impossible.  No chemical products are available for foliar nematode control in home gardens.  Hot water treatments have been developed to treat high-value plants, but are not recommended for home gardeners.  Exact temperatures/timings for these treatments vary depending on the type of plant being treated, and missteps in timing/temperature can either kill plants or can lead to less than 100% control of the nematodes. 

Striped dead areas on hosta leaves typical of infections by foliar nematodes.
Striped dead areas on hosta leaves typical of infections by foliar nematodes.

If you have plants infected with foliar nematodes, the best course of action is to dig them up, bag them and remove them from your garden as soon as you notice symptoms to reduce the risk of the nematodes spreading to healthy plants.  After working with infected plants, wash your hands with soap and water and decontaminate anything that has come into contact with the plants (e.g., tools, pots, bench surfaces, etc.) for 30 seconds with either 70% alcohol (e.g., rubbing alcohol) or 10% bleach.  Spray disinfectants that contain approximately 70% alcohol can also be used.  Because soil is virtually impossible to decontaminate, avoid planting susceptible hosts in an area where foliar nematodes have been a problem.

How do I avoid foliar nematode problems in the future?  The easiest way to avoid problems with foliar nematodes is to not bring them into your garden.  Carefully inspect plants for nematode symptoms before purchase, but keep in mind that plants may not show symptoms early in the growing season.  Avoid using overhead sprinklers as watering in this manner can splash foliar nematodes from plant to plant and promote spread.  Instead use a soaker or drip hose that applies water directly to the soil, rather than onto leaves.  Space plants far enough apart so that potential spread via water splash during natural rains is minimized, and avoid working with plants when they are wet.  DO NOT use foliar nematode-infected plants or even healthy-looking plants suspected to be infested with foliar nematodes when taking cuttings to propagate plants.

For more information on foliar nematodes:  Contact your county Extension agent.

 

 

Edema

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

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

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

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

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

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

For more information on edema:  See UW-Extension Bulletin A3287 (available at http://learningstore.uwex.edu) or contact your county Extension agent.

Early Blight

What is early blight?  Early blight is one of two common fungal diseases that can devastate tomatoes in both commercial settings and home gardens.  Early blight can also be a serious disease on other popular vegetables including eggplants, peppers, and potatoes.  The second common tomato blight, Septoria leaf spot, is detailed in University of Wisconsin Garden Facts XHT1073.

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

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

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

How do I save a plant with early blight?  Once symptoms of early blight appear, control is difficult.  Thinning of whole plants or removal of selected branches from individual plants may slow the disease by increasing airflow and thus reducing humidity and the length of time that leaves remain wet.  Fungicides labeled for use on vegetables and containing copper or chlorothalonil may also provide control of early blight if they are carefully applied very early in the course of the disease (before symptoms develop is best) and on a regular basis throughout the rest of the growing season.  If you decide to use fungicides for disease control, be sure to read and follow all label instructions of the fungicide that you select to ensure that you use the product in the safest and most effective manner possible.

How do I avoid problems with early blight in the future?  Early blight is best controlled using preventative measures.  Destroy infested plants by burning or burying them.  Rotate vegetables to different parts of your garden each year to avoid areas where infested debris (and thus spores of Alternaria solani) may be present.  Use early blight-resistant vegetable varieties whenever possible.  Increase spacing between plants to increase airflow and decrease humidity and foliage drying time.  Mulch your garden with approximately one inch of a high quality mulch, but DO NOT overmulch as this can lead to wet soils that can contribute to increased humidity.  Finally, where the disease has been a chronic problem, use of preventative applications of a copper or chlorothalonil-containing fungicide labeled for use on vegetables may be warranted.

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

 

Downy Mildew

What is downy mildew?  The downy mildews are a group of fungal diseases that cause destruction of the leaves, stems, flowers and fruits of many plant species worldwide.  In Wisconsin, downy mildews are common on grapes, cucumbers, roses and Viburnum.

Downy Mildew
Downy Mildew

What does downy mildew look like?  Downy mildew symptoms begin as small, green or yellow, translucent spots that can eventually spread to an entire leaf, stem, flower or fruit.  Infected plant parts may eventually brown or bronze.  The causal fungus appears on infected stems, flowers and fruits, and on undersurfaces of infected leaves, as a downy, gray-white fuzz.  Microscopic examination of this fuzz is necessary to confirm a downy mildew diagnosis.
 
Where does downy mildew come from?  Downy mildew is caused by several closely related fungi (e.g., Peronospora spp., Plasmopara spp. and Pseudoperonospora spp.) that survive in plant debris or on infected plants.  Downy mildew fungi are fairly host specific.  The downy mildew fungus that infects one type of plant (e.g., rose) is not the same downy mildew fungus that infects another (e.g., grape).  However, if you see downy mildew on one plant, then environmental conditions (i.e., cool, wet weather) are favorable for development of downy mildews on a wide range of plants. 

How do I save a plant with downy mildew?  Downy mildews can be controlled using fungicides.  Those containing chlorothalonil, copper sulfate and lime (e.g., Bordeaux mix), fosetyl-Al, mancozeb, metalaxyl, mefenoxam, or trifloxystrobin are labeled for downy mildew control.  Be sure to read and follow all label instructions of the fungicide that you select to insure that you use the fungicide in the safest and most effective manner possible.

How do I avoid problems with downy mildew in the future?  Consider buying downy mildew resistant varieties, when available.  In new plantings, space plants far apart.  In established plantings, prune or thin plants to increase airflow.  This will reduce humidity and promote rapid drying of foliage.  Avoid overhead watering and apply water directly to the soil at the base of your plants.  At the end of the growing season, remove and destroy infected plant debris as this can serve as a source of spores for the next growing season.

For more information on downy mildew:  See UW-Extension Bulletin A2129, or contact your county Extension agent.

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

Dodder

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

Spaghetti-like dodder plants parasitizing carrots.
Spaghetti-like dodder plants parasitizing carrots.

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

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

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

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

For more information on dodder:  Contact your county Extension agent.

Damping-Off

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

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

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

Where does damping-off come from? Damping-off is caused by several soil-borne fungi, most commonly including Pythium spp., Rhizoctonia solani and Fusarium spp. These fungi readily survive and are moved around in soil or on soil-contaminated items (e.g., pots, workbenches).

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

How do I avoid problems with damping-off in the future? When planting seeds, make sure that work areas, tools and pots are pathogen-free. Spray disinfectants that are at least 70% alcohol can be used to disinfest tools and workbenches. To disinfest previously used pots, wash them with soapy water, then soak them for at least 20 minutes in 10% bleach, then rinse them thoroughly to remove bleach residues. DO NOT reuse plastic pots if you have had problems with damping-off (or root rots) in the past, as they are difficult to disinfest. Be sure to use a well-drained, pasteurized potting mixture when planting. DO NOT use garden soils as they often contain damping-off pathogens. DO NOT plant seeds too deeply and germinate seeds at high temperatures, so that seedlings rapidly grow out of their susceptible phase. DO NOT overwater as damping-off fungi such as Pythium reproduce and infect more effectively in wet soils. Finally, if the techniques above do not work, then use of fungicide-treated seed, particularly those treated with captan, can dramatically reduce problems with damping-off.

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

Crown Gall

What is a crown gall? Crown gall is the most widely distributed bacterial disease of plants in the world, affecting over 100 species of fruit crops, and woody and herbaceous ornamentals, including rose, euonymus, lilac, poplar, viburnum, willow, apple, pear, brambles, stone fruits and grapes. Crown gall can cause severe damage on young plants, while mature woody ornamentals with the disease may show no ill effects.

Crown gall leads to tumor-like growths that form at or near the soil line.
Crown gall leads to tumor-like growths that form at or near the soil line.

What does crown gall look like? Crown gall gets its name from the round or irregularly shaped tumor-like growths (i.e., galls) that usually form on plant crowns just above or below the soil line. Galls can also form on roots, stems, trunks, or branches. Galls can be pea-sized, or as large as several inches in diameter. Galls interfere with water transport within the plant. Therefore, affected plants may suffer from water or nutrient deficiencies, and become stunted. Flower and fruit production may also be reduced.

Where does crown gall come from? Crown gall is caused primarily by the bacteria Agrobacterium tumefaciens, as well as by A. vitis (on grape) which survive in soil, and in or on susceptible plant roots. The bacterium is spread through movement of contaminated soil, water and infected plant material. The bacterium enters plants through wounds (e.g., mechanical injury, freeze injury, pruning cuts or nematode feeding sites) or natural plant openings (e.g., lenticels) and stimulates plant cells to undergo unregulated growth, leading to gall production. Once galls begin to form, they can continue to enlarge even if the bacterium is no longer present. Galls become visible anywhere from several weeks to one or more years after the time of infection.

How do I save a plant with crown gall? There is no cure for crown gall once galls begin to form. Galls can be pruned away, but new galls may reform elsewhere on the plant. To prevent spread of the crown gall bacterium, remove infected plants, surrounding soil, and as many of the infected plant’s roots as possible.

How do I avoid problems with crown gall in the future? Use disease-free, winter hardy stock from a reputable nursery and inspect the roots and crowns for galls before planting. Avoid wounding plants during transplant and cultivation. Decontaminate tools, equipment and shoes with 10% bleach or 70% alcohol for at least 30 seconds to prevent spread of the bacterium. Use of a biological control product can prevent new infections by A. tumefaciens at the time of planting. Current biological control products contain A. radiobacter, a close relative of the crown gall organism. These products are not effective on all hosts, or against all variants of the pathogen. In areas with infested soil, rotation to a non-susceptible plant (such as grass) for 3 years, may provide a good means of disease control. In commercial settings, soil fumigation may provide limited control of the crown gall bacterium in soil. However, fumigation does not kill the bacterium in roots that remain in the soil after removal of infected plants. Fumigation is not recommended for homeowners.

For more information on crown gall: Contact your county Extension agent.

Corky Ringspot

What is corky ringspot? Corky ringspot (also known as spraing) is a potentially serious viral disease of potato that has recently been detected in Wisconsin. The disease can cause severe losses due to the fact that it reduces potato tuber quality, making tubers unsuitable for use in potato chip production and undesirable to consumers as table stock. Variants of this disease [usually referred to as tobacco rattle (see University of Wisconsin Garden Facts XHT1180)] affect a variety of other plants including vegetable crops (e.g., beans, beets, peppers, and spinach), many herbaceous ornamentals (e.g., astilbe, bleeding heart, coral bells, daffodil, epimedium, gladiolus, hyacinth, marigold, tulip, and vinca) and many weed species (e.g., chickweed, cocklebur, henbit, nightshade, pigweed, purslane, prickly lettuce, shepherd’s-purse and sowthistle).

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

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

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

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

How do I avoid problems with corky ringspot in the

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

future? The best way to prevent problems with corky ringspot is to avoid introducing TRV onto your property. Be sure to grow potatoes from seed that is certified as being free of TRV. Currently, seed potatoes produced in Wisconsin are considered TRV-free. Also avoid introducing the virus on infected ornamental plants. Carefully inspect ornamentals (see above for a partial list of susceptible species) prior to purchase for symptoms caused by TRV and DO NOT buy symptomatic plants. Alternatively (and preferably), avoid growing susceptible species altogether, and grow plants that are not susceptible to TRV. Non-susceptible plants include, but are not limited to, annual phlox, carnation, carrot, devil’s trumpet (downy thorn-apple), Scotch spearmint, sorrel, sweet William, zinnia and zombie cucumber. To limit potential spread of TRV, routinely decontaminate tools (e.g., knifes or other cutting tools) that come into contact with potentially infected plant material (e.g., whole tubers that are cut into seed pieces, or ornamentals that are being divided). Also decontaminate tires, tools (e.g., spades or hoes) and any other object (e.g., shoes or boots) that might transport stubby-root nematode-infested (and thus TRV-infested) soil from field to field. First rinse any excess plant tissue or soil from these items, then treat them for at least 30 seconds in a solution that is a combination of 1% sodium lauryl sulfate (use 10% shampoo as a source of this chemical) and 1% Alconox (an industrial detergent). Trisodium phosphate (available at most local hardware stores) can also be used. Also consider routinely testing soils for the presence of stubby root nematodes. Knowing the level of these nematodes in a field can provide information on the likelihood that TRV will spread should the virus be introduced. Finally, DO NOT ever produce seed potatoes in fields with a history of corky ringspot or other TRV diseases. Also avoid using infested fields for potato or other vegetable production. If you decide to use a TRV-infested field for non-seed potato production, be sure to grow a TRV-resistant potato variety. The potato varieties ‘Castile’, ‘Millennium Russet’, ‘Red Pearl’, ‘Symfonia’, and ‘St. Johns’ have all been reported to have at least moderate levels of resistance to TRV.

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