Category Archives: Pest Alerts

Boxwood Blight – Pest Alert

What is boxwood blight?  Boxwood blight (also known as box blight and boxwood leaf drop) is a devastating disease of boxwood (Buxus spp.) that can cause leaf loss and eventual death of affected shrubs.  Boxwood shrubs are commonly grown as hedges and as individual plants in home landscapes and public gardens.  Boxwood blight can affect any type of boxwood (Buxus spp.) including European or common boxwood (Buxus sempervirens)Korean littleleaf boxwood (B. sinica var. insularis), and Japanese littleleaf boxwood (B. microphylla var. japonica).  In addition, the disease has been reported on Japanese and Allegheny pachysandra (Pachysandra terminalis and Pachysandra procumbens respectively), two common groundcovers.  Boxwood blight has been found in Europe and New Zealand, and was first confirmed in the U.S. in 2011.  The disease was first detected in Wisconsin (in Kenosha County) in 2018.  The disease has subsequently been found in Dane, Milwaukee and Ozaukee Counties.

Boxwood blight can cause severe leaf loss and eventual death of boxwood shrubs. (Photo courtesy of David Clement, University of Maryland Extension)
Boxwood blight can cause severe leaf loss and eventual death of boxwood shrubs. (Photo courtesy of David Clement, University of Maryland Extension)

What does boxwood blight look like?  Initially, brown spots appear on the leaves.  The spots eventually enlarge and merge together.  Infected leaves turn brown and fall off.  Boxwood blight can cause total leaf loss on a shrub within days of the first onset of symptoms.  Dark brown to black sunken areas can also form anywhere on the stems, leading to branch dieback  Boxwood blight often kills plants shortly after all of the leaves drop.  Damage from winter burn (see UW Plant Disease Facts D0127, Winter Burn), dog urine and other diseases such as Volutella blight may look superficially similar to symptoms of boxwood blight.

Where does boxwood blight come from?  Boxwood blight is caused by the fungus Calonectria pseudonaviculata (sometimes referred to as Cylindrocladium pseudonaviculatum or Cylindrocladium buxicola) which thrives in humid, warm conditions.  The fungus is typically introduced into any area on nursery plants that are infected, but not showing symptoms.  Holiday wreaths containing boxwood sprigs have also been documented as a source of the boxwood blight fungus.  Once the fungus has been introduced into the landscape, spores can be easily spread by splashing water (e.g., rain or sprinklers), wind or contaminated gardening tools (e.g., pruners, shovels, gloves).  The boxwood blight fungus can survive and produce spores in dead boxwood leaves and branches (including those that have fallen onto the ground) for several years.

How can I save a plant with boxwood blight?  Because boxwood blight is new to Wisconsin and relatively rare, eradicating the causal fungus may be possible.  Therefore, if you find boxwood blight, remove and destroy any affected shrubs.  Currently, free testing for boxwood blight is available through the UW-Madison Plant Disease Diagnostics Clinic (  Plants (roots and all) confirmed to have boxwood blight, as well as any leaves or branches that have fallen from these plants, should be removed and destroyed by burning (where allowed by local ordinance), deep burying (at least two feet deep) or double bagging (in plastic garbage bags), then landfilling.  DO NOT compost any parts of infected shrubs.  Thoroughly decontaminate any tools used in the removal process by treating them for at least 30 seconds in 70% alcohol (e.g., rubbing alcohol or certain spray disinfectants) or (as a last resort) in 10% bleach.  If you use bleach, be sure to thoroughly rinse and oil tools after pruning to prevent rusting.

How can I avoid problems with boxwood blight in the future?  

Consider using shrubs other than boxwood in your landscape.  If you decide to use boxwood, choose boxwood blight resistant varieties where possible.  In Wisconsin, hybrid boxwoods ‘Green Gem’ and ‘Karzgreen (Green Ice®), Japanese littleleaf boxwood varieties ‘Jim Stauffer’, ‘Little Missy’ and ‘Winter Gem’, and Korean littleleaf boxwood varieties ‘Eseles’ (Wedding Ring®), ‘Franklin’s Gem’, ‘Pincushion’, ‘Wee Willie’, ‘Winter Beauty’ and ‘Wintergreen’ are hardy to at least USDA hardiness zone 5 and have been documented to be resistant to boxwood blight.  Always buy boxwood shrubs from local, reputable suppliers who have thoroughly inspected boxwood plants for evidence of boxwood blight.

Leaf spots typical of boxwood blight on boxwood sprigs in a holiday wreath. (Photo courtesy Purdue PPDL)
Leaf spots typical of boxwood blight on boxwood sprigs in a holiday wreath. (Photo courtesy Purdue PPDL)

Isolate new boxwood shrubs from established boxwoods for several weeks before planting, as boxwood blight symptoms not become apparent until weeks after purchase.  DO NOT plant boxwoods in areas where boxwood blight has been a problem in the past, as the fungus can survive in boxwood debris (e.g., leaves and branches) for several years.  When planting, space boxwood plants far enough apart from each other, as well as other shrubs, so that branches on adjacent shrubs do not overlap.  This will increase air flow between plants and promote a drier environment that will be less favorable for boxwood blight development.  Avoid watering plants with sprinklers or overhead with hoses; instead use a soaker or drip hose.  This will limit splash of spores from plant to plant and also promote a drier environment that is less favorable for disease.

Be cautious when buying holiday wreaths or other garlands.  Avoid holiday decorations that contain boxwood, whenever possible.  If you are unsure whether a wreath that you have purchased contains boxwood, assume that it does, and dispose of it appropriately by burning, deep burying or double bagging and landfilling as described above.  Be careful to collect and dispose of any leaves or branches that may have fallen from wreaths as well.  Make sure that no potentially contaminated materials end up near boxwood shrubs in your yard.  Under NO circumstances should you attempt to compost any suspected boxwood materials.

Once boxwood blight has been reported near your location, you may want to consider using preventative fungicide treatments for management.  Fungicides containing chlorothalonil (alone or in combination with thiophanate-methyl or tebuconazole), fludioxonil, metconazole, and tebuconazole (as a stand-alone product) have been shown to provide good control of boxwood blight if applied prior to the development of any symptoms.  These fungicides will not cure existing disease.  If you decide to use fungicides, you will need to treat every seven to 14 days throughout the growing season.  DO NOT use fludioxonil, metconazole, or tebuconazole as the sole active ingredient for all treatments.  If you decide to use one of these active ingredients, alternate its use with at least one of the other active ingredients listed above (except DO NOT alternate metconazole and tebuconazole as these products are chemically related).  Alternating active ingredients will help minimize problems with fungicide-resistant strains of the boxwood blight fungus.  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.

Finally, routinely (e.g., weekly) check boxwood plants for boxwood blight.  Immediately remove any symptomatic plants and fallen leaves and branches, and dispose of them as described above.

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

Ralstonia Wilt – Pest Alert

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.

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 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:].

Sudden Oak Death – Pest Alert

What is sudden oak death?  Sudden oak death (SOD), also called Ramorum leaf blight or Ramorum dieback, is an oftentimes lethal disease that has caused widespread death of tanoak (Lithocarpus densiflorus), coast live oak (Quercus agrifolia), California black oak (Quercus kelloggii), and Shreve oak (Quercus parvula var. shrevei) in California.  The disease can affect or has been reported in association with a wide range of woody and herbaceous plants including, but not limited to bigleaf maple (Acer macrophyllum), Bodnant viburnum (Viburnum X bodnantense), ‘Brouwer’s Beauty’ pieris (Pieris floribunda X japonica), California bay laurel (Umbellularia californica), California buckeye (Aesculus californica), California coffeeberry (Rhamnus californica), California honeysuckle (Lonicera hispidula), canyon live oak (Quercus chrysolepis), coast redwood (Sequoia sempervirens), doublefile viburnum (Viburnum plicatum var. tomentosum), douglas-fir (Pseudotsuga menziesii var. menziesii), evergreen huckleberry (Vaccinium ovatum), Formosa firethorn (Pyracantha koidsumii), ‘Forest Flame’ pieris (Pieris formosa X japonica), Himalaya pieris (Pieris formosa), Japanese camellia (Camellia japonica), Japanese pieris (Pieris japonica), laurustinus (Viburnum tinus), madrone (Arbutus menziesii), manzanita (Arctostaphylos manzanita), rhododendron (Rhododendron spp.), Sasanqua camellia (Camellia sasanqua), toyon (Heteromeles arbutifolia), western starflower (Trientalis latifolia), and witch hazel (Hamamelis virginiana), Burkwood viburnum (Viburnum X burkwoodii), California hazelnut (Corylus cornuta), Camellia X williamsii, cascara (Rhamnus purshiana), Chinese pieris (Pieris formosa var. forrestii), common lilac (Syringa vulgaris), David viburnum (Viburnum davidii), drooping leucothoe (Leucothoe fontanesiana), European beech (Fagus sylvatica), European cranberrybush viburnum (Viburnum opulus), European turkey oak (Quercus cerris), European yew (Taxus baccata), fragrant viburnum (Viburnum farreri), grand fir (Abies grandis), Holm oak (Quercus ilex), horse-chestnut (Aesculus hippocastanum), lingonberry (Vaccinium vitis-ideae), mountain laurel (Kalmia latifolia), Northern red oak (Quercus rubra), Pieris formosa var. forrestii X Pieris japonica, poison oak (Toxicodendron diversiloba), Prague viburnum (Viburnum X pragense), reticulate camellia (Camellia reticulata), salmonberry (Rubus spectabilis), Southern red oak (Quercus falcata), strawberry tree (Arbutus unedo), sweet chestnut (Castanea sativa), Viburnum X carlcephalum X Viburnum utile, Victorian box (Pittosporum undulatum), wayfaringtree viburnum (Viburnum lantana), and wood rose (Rosa gymnocarpa).

Rapid wilting and die back of branch tips can be a symptom of ramorum dieback.
Rapid wilting and die back of branch tips can be a symptom of ramorum dieback.

SOD was first reported in the US in California and has subsequently been found in other US states, including in Wisconsin in 2019.  SOD has also been reported in Europe.

What does sudden oak death look like?  Symptoms of SOD vary depending upon the plant species infected.  On some hosts, infections occur primarily on leaves leading to light brown leaf spots and blotches.  These leaf symptoms may be indistinguishable from other, more common, leaf spots and blights, or may mimic sunburn or leaf scorch symptoms.  Twigs and branches that become infected often wilt, forming a “shepherd’s-crook”, and subsequently die back.  Infection of tree trunks leads to cankers (i.e., sore-like areas) that produce large amounts of an amber to black colored ooze.  This ooze can dry to form a stained area on the bark.  Removing the bark over the affected area will reveal discolored wood beneath that sometimes (but not always) has a black border.  Cankers can eventually expand to girdle trunks, thus resulting in the death of the tree or shrub.  Trunk infections appear not to extend into the root system of the plant.  Once SOD cankers develop, other pathogens may invade the infected areas, accelerating tree or shrub death and complicating the diagnosis of the disease.

Where does sudden oak death come from?  SOD is caused by the fungus-like water mold Phytophthora ramorum, which was first recognized as a pathogen in 1995.  Phytophthora ramorum can be spread over long distances through movement of infected plants or infested plant parts.  The organism can also be moved with contaminated soil (e.g., on vehicle tires, tools, or shoes), or in contaminated water.  Once established on plants in a given location, the organism produces reproductive structures (called sporangia) that can be moved from plant to plant by rain splash, or wind.  Phytophthora ramorum was introducing into Wisconsin in 2019 on nursery stock grown in the state of Washington.

Ramorum leaf blight symptoms can mimic those of other leaf spots and blights.
Ramorum leaf blight symptoms can mimic those of other leaf spots and blights.

How do I save a plant with sudden oak death?  If you believe you have seen a plant that has SOD, please IMMEDIATELY submit a sample to the UW-Madison Plant Disease Diagnostics Clinic (PDDC).  See below for address details.  Double bag suspect plant tissue in sealable plastic bags and place the bagged specimen in a box or envelope for shipping.  Include contact information (complete address, phone number, email address) in a separate sealable plastic bag with the sample.  Tape over all of the edges of boxes and envelopes used for shipping to keep everything sealed inside.  Write on the box or envelope that the box or envelope contains a suspect SOD sample.  If you have questions about collecting or submitting a sample, contact PDDC staff at (608) 262-2863 or at

Because Phytophthora ramorum is a regulated, quarantined pathogen, DO NOT remove the affected plant (or parts thereof) or take the plant from the site where it is located, other than to collect a specimen for submission for a diagnosis.  Be sure to decontaminate any tools or other items that come into contact with the plant (including those used to collect a diagnostic sample) by treating them for at least 30 seconds in 10% bleach.  Thoroughly rinse and oil tools after decontamination to prevent rusting.  If a plant tests positive for Phytophthora ramorum, it will be removed and destroyed to help prevent further spread of the pathogen.

How do I avoid problems with sudden oak death in the future?  Carefully inspect any new nursery stock upon delivery (or prior to purchase, if possible) for symptoms of SOD.  Keep new stock isolated from older stock as long as possible, to minimize possible movement of the pathogen should the disease develop after plants have arrived.  If you see any suspect symptoms, alert the PDDC so that arrangements can be made for proper testing for Phytophthora ramorum.

For more information or help in diagnosing sudden oak death:  Contact Brian Hudelson, UW-Madison Plant Disease Diagnostic Clinic, Department of Plant Pathology, University of Wisconsin-Madison, 1630 Linden Drive, Madison, WI  53706-1598, phone: (608) 262-2863, email:, see the USDA APHIS sudden oak death website, or contact your County Extension agent.

Weir’s Cushion Rust of Spruces – Pest Alert

What is Weir’s cushion rust?  Weir’s cushion rust is a needle disease that disfigures and reduces growth of spruce trees (Picea spp.) of all ages.  This disease has been known in both eastern and western regions of the United States, but was recognized in Wisconsin for the first time in 2002.

Yellow spots and bands in winter on spruce shoots affected by Weir's cushion rust.
Yellow spots and bands in winter on spruce shoots affected by Weir’s cushion rust.

What does Weir’s cushion rust look like?  Needles on current year’s shoots affected by Weir’s cushion rust may develop yellow spots or bands in the summer and fall.  These spots and bands may intensify to give needles a bright “green and gold” appearance the following spring, when tiny blister-like pustules (a type of fungal reproductive structure) develop in the yellow areas.  Microscopic examination of these pustules is required for diagnosis of the disease.  Affected one-year-old needles continue to yellow, turn brown, and fall off as the spring and summer progress.  Trees badly damaged by Weir’s cushion rust will have thin crowns due to repeated loss of the previous year’s needles.

Where does Weir’s cushion rust come from?  Weir’s cushion rust results from colonization of spruce needles by the fungus Ceropsora weirii (formerly Chrysomyxa weirii).  This fungus overwinters in needles infected during the previous growing season.  In late summer, or more typically the following spring, C. weirii produces spores in the pustules that develop on the needles.  These spores can be blown by wind or splashed by rain to newly emerging needles on the same tree or other trees.  Spore germination is followed by infection of young needles.

Can I save a tree affected by Weir’s cushion rust?  Apply fungicides containing chlorothalonil to trees affected by Weir’s cushion rust to prevent new needle infections.  Make the first application when 10% of the buds have broken and two additional applications at seven to 10 day intervals thereafter.  Fungicide applications do not kill the fungus in needles that are already infected, so be sure to begin applications promptly and complete the spray program, to ensure thorough coverage and protection of new foliage.  Please be sure to read and follow all fungicide label instructions to ensure that you use the product in the safest and most effective manner possible.  Needles infected by C. weirii eventually die.  The fungus does not continue to live or produce spores on these dead needles.  Therefore, destruction of dead needles is not necessary.

How do I avoid Weir’s cushion rust in the future?  DO NOT accept and plant landscape or nursery stock affected by Weir’s cushion rust.  Inspect established spruce trees (in both landscape and nursery settings) in late summer and fall for evidence of Weir’s cushion rust (e.g., yellow spots and bands on the current year’s needles).  Inspect suspect trees again in spring for these symptoms, as well as pustules of C. weirii on the previous year’s needles.  In nurseries, move affected trees to areas where the disease is not already present.  Use fungicide applications to prevent establishment of the fungus on new trees or in previously unaffected nurseries and landscapes.

For more information on Weir’s cushion rust:  Contact your county Extension agent. 

Thousand Cankers Disease – Pest Alert

What is thousand cankers disease?  Thousand cankers disease (TCD) is a serious disease of black walnut (Juglans nigra), a tree native to Wisconsin.  TCD has not yet been reported in Wisconsin, but has been found in the western United States where it was first described in 2008.  TCD more recently has been reported in the eastern U.S. in Indiana, Maryland, North Carolina, Ohio, Pennsylvania, Tennessee, and Virginia.  TCD has been fatal to black walnut in all known cases.  Other walnut species found in the western U.S. [e.g., California walnut (Juglans californica) and Arizona walnut (Juglans major)] appear to be much less susceptible.  Butternut (Juglans cinerea), another tree native to Wisconsin, is also known to be susceptible.

Discoloration and tunneling under the bark of a walnut branch associated with thousand cankers disease leads to disruption of water and nutrient movement and eventual tree death. (Photo courtesy of Karen Snover-Clift, Cornell University,
Discoloration and tunneling under the bark of a walnut branch associated with thousand cankers disease leads to disruption of water and nutrient movement and eventual tree death. (Photo courtesy of Karen Snover-Clift, Cornell University,

What does thousand cankers disease look like?  The first symptom of TCD is a yellowing of the leaves starting at the top of a walnut tree.  Eventually lower leaves yellow and branches die.  Death of the entire tree soon follows.  Branches on trees with TCD have tiny holes (about the size of a pencil tip) made by a small beetle, the walnut twig beetle (Pityophthorus juglandis), that is involved in the disease.  Beneath the bark of symptomatic branches, well-defined dark black or brown cankers (i.e., diseased areas) form.  Cankers eventually merge, disrupting movement of water and nutrients in the tree, leading to tree death.

Where does thousand cankers disease come from?  Thousand cankers disease is caused by a combined effects of a fungus (Geosmithia morbida) and the walnut twig beetle.  The insect carries the fungus on its body and introduces the fungus into a walnut tree as it tunnels into the bark to feed.  Walnut twig beetles spread the fungus locally as they move from tree to tree to feed.  The fungus does not appear to spread from tree to tree by root grafts.  Longer distance dispersal of the insect and fungus is possible when walnut seedlings, walnut firewood, and walnut wood products are moved by human activities.  Walnut fruits have not been reported as a source of the insect or fungus.

How can I save a tree with thousand cankers disease?  At this time, there are no formal recommendations for managing TCD.  Researchers are attempting to develop treatment methods, including use of insecticides, fungicides and nutrient management, to help prolong the life of infected trees.  Because TCD has not yet been reported in Wisconsin, the most important management strategy at this time is prevention.

How can I avoid problems with thousand cankers disease in the future?  The best way to prevent the spread of TCD (as well as other tree pests and diseases)s to not move firewood!  For information about the restrictions on moving firewood in Wisconsin visit the Wisconsin Department of Natural Resources website at  Also be cautious about moving walnut transplants or other walnut products (especially those with the bark still attached), particularly if they are coming from an area where TCD has been reported.

For more information on thousand cankers disease or if you suspect you have seen this disease:  Contact your county Extension agent or the UW-Madison Plant Disease Diagnostics Clinic (

Powdery Scab – Pest Alert

What is powdery scab?  Powdery scab is a potentially serious disease of potatoes that occurs worldwide in regions where potatoes are grown, including Wisconsin.  Although powdery scab primarily causes cosmetic, if unsightly, skin blemishes of potato tubers, the pathogen that causes the disease can transmit another, more serious potato pathogen, Potato mop-top virus (PMTV).  PMTV was first detected in Wisconsin in 2020.  This virus can cause severe losses and can limit a seed potato producer’s ability to sell to certain foreign markets.  In addition, blemishes caused by powdery scab can serve as entry points for other pathogens, such as those that cause late blight (see UW Plant Disease Facts D0068, Late Blight, pink rot, dry rot and black dot.

Powdery scab can lead to development of crater-like lesions on the surface of potato tubers. (Photo courtesy of Anette Phibbs)
Powdery scab can lead to development of crater-like lesions on the surface of potato tubers. (Photo courtesy of Anette Phibbs)

What does powdery scab look like?  Tubers are infected through lenticels, eyes, or wounds.  Initial symptoms of tuber infection are sunken purple-brown lesions that are followed by pimple-like swellings.  As lesions mature, they break through the potato skin and develop into shallow depressions that contain a mass of powdery spore balls (called cystosori) surrounded by thin, raised remnants of the outer tuber skin.  When infections develop in wet soils, the lesions deepen and become open cankers.  Infections of roots and stolons can also occur, and first appear as necrotic spots, that later become small, white to tan-colored galls.  As galls mature, they enlarge, turn brown and finally break open releasing cystosori into the soil.  Because powdery scab symptoms appear on below-ground parts of the potato, infections may not be noticed until harvest.  If symptoms have not fully developed by harvest, they may continue to develop in storage.  At various stages of development, powdery scab can be mistaken for common scab (see UW Plant Disease Facts D0083, Potato Scab), potato wart, black scurf, and root-knot nematode damage (see UW Plant Disease Facts D0097, Root-Knot Nematode).

Where does powdery scab come from?  Powdery scab is caused by the soil-borne slime mold, Spongospora subterranea f. sp. subterranea.  The pathogen can be introduced into a non-infested field on infected seed tubers; on equipment, shoes, and other clothing contaminated with infested soil; or in infested manure (cystosori can survive passage through animal guts).  S. subterranea f. sp. subterranea cystosori can survive for at least 6 years in the soil.  In addition, S. subterranea f. sp. subterranea can survive on a variety of solanaceous vegetables and weeds including volunteer potatoes, tomatoes, peppers, nightshade, ground cherry and jimsonweed.  Cool temperatures (52ºF to 64ºF) and wet conditions favor disease development.  When free water is available, cystosori release motile spores (called zoospores) that swim to and infect root hairs, stolons and tubers.  Alternating periods of wet and dry weather produce repeated cycles of zoospore release.  Environmental conditions appear to be more important in disease development than initial inoculum level.

Powdery scab symptoms on a red-skinned variety of potato. (Photo courtesy of Anette Phibbs)
Powdery scab symptoms on a red-skinned variety of potato. (Photo courtesy of Anette Phibbs)

How do I control powdery scab?  The best way to manage powdery scab is to prevent introduction of the pathogen into potato fields.  The introduction of the powdery scab pathogen into Wisconsin is thought to have occurred when growers planted infected seed tubers.  Therefore, carefully inspect seed tubers for powdery scab symptoms and be sure to plant disease-free seed potatoes into non-infested fields.  Once fields become infested, avoid these fields if possible, particularly those with poorly drained soils.  Contaminated fields should be rotated away from potatoes (and other susceptible hosts, such as tomatoes) for three to 10 years.  During this period, be sure to keep solanaceous weeds (e.g., nightshade, ground cherry) under control as these plants can serve as alternate hosts for the pathogen.  Once potato production resumes in infested fields, be sure not to over-irrigate, especially during tuber set.  Adopting a later planting date to take advantage of warmer temperatures may help reduce the level of powdery scab, but this may not be possible given other management constraints.  When attempting to dispose of infected tubers, do not compost these tubers.  If you decide to use infected tubers as feed, do not use manure from animals that have been fed the tubers, as cystosori can survive passage through animal guts.  Research by USDA-ARS scientists indicates that some mustard family crops (e.g., white mustard, rape, canola) that produce high levels of glucosinolates, when grown as green manures (fall-planted, spring-incorporated), may reduce levels of powdery scab.  Fungicides containing the active ingredient fluazinam have shown some efficacy against powdery scab, but results have been variable.  If you decide to use fungicides for control, be sure to select a product that is labeled for use on potatoes, and 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.

For more information on powdery scab:  See UW Bulletin A3833, Potato Disorders:  Common Scab and Powdery Scab (available at, or contact your county Extension agent.

Plum Pox – Pest Alert

What is plum pox?  Plum pox, also known as “sharka,” is one of the most devastating diseases of stone fruits (plums, peaches, nectarines, and apricots) worldwide.  This viral disease was first discovered on plums in Bulgaria in 1915 and subsequently has been observed in many parts of the world.  There are several variants of plum pox, but only one has been found in the United States.  This variant was first found in peach orchards in Pennsylvania in 1999 (the first report of plum pox in North America).  In 2006, the same variant was identified in Michigan and New York.  Primary hosts of the U.S. plum pox variant are peach, plum, and ornamental Prunus species.  Cherries and almonds are not considered natural hosts of this variant, but they can be artificially infected.  Other plum pox hosts include garden plants (e.g., tomatoes, peas, petunias, zinnias) and weeds (e.g., white clover, lamb’s quarters).  While plum pox does not kill stone fruit trees, it causes serious crop losses by making fruit deformed, discolored, tasteless, and unmarketable.  In 2019, after intense quarantine and destruction of infected trees and orchards, the United States Department of Agriculture declared that plum pox had been eradicated from the United States.

Plum pox symptoms on immature plum fruits (left), and a plum leaf (right).  (Photographs courtesy of R. Scorza and obtained from West Virginia University at
Plum pox symptoms on immature plum fruits (left), and a plum leaf (right). (Photographs courtesy of R. Scorza and obtained from West Virginia University at

What does plum pox look like?  Plum pox symptoms vary widely depending on host plant, plant age, plant nutrient status, environmental conditions, plum pox variant, and timing of infection.  Some infected plants do not exhibit any visible symptoms or may not develop symptoms until years after infection, making plum pox difficult to detect.  Additionally, symptoms may not be visible throughout an entire plant but limited to only a portion of the plant.  Once a plant starts to show symptoms however, it will continue to do so in subsequent years.  Of the stone fruits, plums are generally most severely affected by plum pox and show the most obvious symptoms.  Branches on infected trees may develop spots.  Leaves may develop yellow-green spots or blotches and mild, light-green discoloration near leaf veins (see photo above) that can be difficult to distinguish from other causes (e.g., nutrient deficiencies).  On peach trees, leaf crinkling, puckering, and curling may also occur.  Fruits may develop yellow rings or line patterns and become brown or necrotic (see photo above).  As fruits ripen, symptoms fade, but fruits drop from the tree prematurely.  Seeds may have white rings or line patterns.

Where does plum pox come from?  Plum Pox is caused by the Plum pox virus (PPV).  PPV-D (one of six PPV variants/strains) is the only strain that has been detected in the United States.   PPV can be moved long distances via infected nursery stock such as infected trees or budwood used for grafting.  Once introduced into an orchard, the virus is spread short distances by aphids.  Aphid transmission occurs more frequently in spring and autumn.  PPV can overwinter in various parts of a tree, including the roots.

How do I save a tree with plum pox?  Once a tree has been infected with PPV, it cannot be cured.  Timely and complete eradication of infected trees and even entire orchards is the only effective way to prevent further spread.  Diseased trees (including stumps) should be removed and destroyed (i.e., burned and/or buried).  Trees surrounding a problematic area should be monitored frequently for symptom development.  Other potential host plants (see above) should also be monitored for symptoms of disease.  If you see what you believe to be plum pox symptoms, contact your local plant disease diagnostic clinic immediately (see for the lab nearest you).  In Wisconsin, contact the UW-Madison Plant Disease Diagnostics Clinic (PDDC) at (608) 262-283 or  PPV is a federally regulated pathogen and if detected, infected plants must be destroyed to prevent further spread.  For more information on the federal regulation of PPV, see

How do I avoid problems with plum pox in the future?  After 20 years and elimination of over 1,500 acres of fruit trees, PPV has been eradicated in the United States.  Preventing reintroduction of the PPV in the United States is critical.  To prevent reintroduction of PPV, only use nursery stock that is certified virus-free.  Also consider planting resistant varieties, but keep in mind that existing resistant varieties can still carry the virus and be asymptomatic.  Additional control strategies for plum pox include managing aphids that can transmit PPV, following quarantine regulations, and routinely scouting and surveying orchards for plum pox and PPV.  Ongoing monitoring for plum pox in stone-fruit-producing states and regulating imported trees will help ensure that the United States remains free of PPV.

For more information or help in diagnosing plum pox:  Contact Leslie Holland [Department of Plant Pathology, University of Wisconsin-Madison, 1630 Linden Drive, Madison, WI 53706-1598, phone: (608) 265-2047, email:], the UW-Madison PDDC, or your county Extension agent.