Category Archives: Disease – Fruit Crop

Cucumber Mosaic

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

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

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

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

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

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

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

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

Common Leaf Spot of Strawberry

What is common leaf spot?  Common leaf spot of strawberry (also known as Mycosphaerella leaf spot, Ramularia leaf spot, strawberry leaf spot, bird’s-eye spot, gray spotness, and white spot) is a common fungal leaf disease that affects both wild and cultivated strawberries throughout the world.  Common leaf spot was once the most economically important strawberry disease, but the use of resistant strawberry varieties/cultivars and improvements in methods for growing strawberries have been effective in managing the disease and reducing its impact.  Today, the disease is often a cosmetic problem and typically has little impact on yield or fruit quality.

Typical common leaf spot symptoms: tan to gray spots with reddish purple margins. (Photo courtesy of Dr. Patricia McManus, Department of Plant Pathology, University of Wisconsin)
Typical common leaf spot symptoms: tan to gray spots with reddish purple margins. (Photo courtesy of Dr. Patricia McManus, Department of Plant Pathology, University of Wisconsin)

What does common leaf spot look like?  Symptoms of common leaf spot can occur on leaves, fruits, berry caps, petioles, and runners.  The most noticeable symptoms of the disease are small, round, necrotic (i.e., dead) spots on strawberry leaves.  Initially, these spots develop on the upper leaf surface and are deep purple to red in color.  The spots eventually develop tan, gray or almost white centers with distinct reddish-purple to brown borders.  During warm, humid weather, uniformly rusty-brown spots without purple margins or light colored centers may develop instead.  Spots can occur on the undersides of the leaves as well, but these spots tend to be less vibrant in color.  As the disease progresses, spots enlarge to ⅛ to ¼ inch in diameter and may merge together, in extreme cases leading to leaf death.  Spots on berry caps, petioles, and runners resemble those produced on upper leaf surfaces.  Shallow, black spots (¼ inch in diameter) may develop on infected fruits, and are often surrounded by brown or black, leathery tissue.

Where does common leaf spot come from?  Common leaf spot is caused by the fungus Mycospharella fragariae, which can enter a garden on infected strawberry plants or via windblown spores from nearby strawberries.  Once introduced into a garden, the fungus is spread predominantly by splashing water from rain or sprinklers used for watering.  M. fragariae is most active when temperatures range from 65°F to 75°F, with periods of high rainfall and humidity.  M. fragariae survives the winter on dead strawberry leaves and other plant parts, and it is moved to new foliage in the spring by early season rains.

How do I save strawberry plants with common leaf spot?  Once common leaf spot develops on strawberry plants, the plants cannot be cured.  If the disease is detected early, its development may be slowed using fungicides.  Keep in mind however, that common leaf spot is often merely a cosmetic issue and the use of fungicides may not be warranted.  If you decide that fungicide treatments are needed, select a product that is labeled for use on strawberries and that contains captan, myclobutanil or copper as the active ingredient.  Use copper-containing fungicides only prior to flowering.  If you decide to use a myclobutanil-containing product, alternate applications of this product with applications of a second fungicide containing another active ingredient.  This will help prevent selection of myclobutanil-resistant variants of the common leaf spot pathogen.  Be sure to read and follow all instructions on the label(s) 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 common leaf spot in the future?  When establishing your strawberry patch, consider planting resistant strawberry varieties and use certified, disease-free nursery stock.  Examples of resistant varieties include ‘Crimson King’, ‘Earliglow’, ‘Glooscap’, ‘Ogallala’, and ‘Ozark Beauty’.  Plant strawberries in full sunlight, in well-drained soils, and with proper spacing to optimize air circulation and create a drier environment that is less favorable for the common leaf spot pathogen.  See UW Bulletin A1597 (Growing Strawberries in Wisconsin), available at https://learningstore.extension.wisc.edu/, for additional details on proper planting.

Once plants are in the ground, avoid overhead watering (i.e., DO NOT use a sprinkler) as this will splash the common leaf spot pathogen from plant to plant, and provide a wet environment that is more favorable for the fungus to infect.  Instead, use a drip or soaker hose for watering.  For similar reasons, DO NOT work in your strawberry patch (e.g., weeding, thinning plants or harvesting fruit) when it is wet; wait until the patch is dry.

For June-bearing strawberries, bed renovation techniques (in particular mowing) can be useful in managing common leaf spot.  See UW Bulletin A1597 (mentioned above) for details on proper renovation techniques.  At the end of the growing season, remove strawberry plant debris to minimize sites where the fungus can survive the winter.  Deep bury, burn (where allowed by local ordinance) or hot compost this material.

For more information on common leaf spot of strawberry:  Contact your county Extension agent.

Cane Blight

What is cane blight?  Cane blight is a fungal disease that affects the health of canes (i.e., stems) of cultivated and wild Rubus species (e.g., raspberries and blackberries), wherever they are grown.  Black and purple raspberries appear to be more susceptible to cane blight than red raspberries, but all commonly cultivated raspberry cultivars can get the disease.  Although cane blight is not typically fatal, it may cause significant fruit yield losses if left unmanaged.

Cane death on a thornless blackberry caused by cane blight. Look for a dark brown infection line and dead/dying shoots above the point of infection. (Photo courtesy of Michael Ellis, The Ohio State University)
Cane death on a thornless blackberry caused by cane blight. Look for a dark brown infection line and dead/dying shoots above the point of infection. (Photo courtesy of Michael Ellis, The Ohio State University)

What does cane blight look like?  Symptoms of cane blight usually first appear in early summer after blossoming and leaf emergence, and in association with wounds caused by pruning or harvesting of fruit.  Look for sudden death of side branches and tips of fruit-bearing canes, as well as dark brown or purple spots (called cankers) on the canes below the dieback.  In wet weather, cankers may produce a grey ooze.  In dry weather, the cankers may appear fuzzy or powdery.  Dead canes may become brittle and snap off in windy conditions.

Where does cane blight come from?  Cane blight is caused by the fungus Leptosphaeria coniothyrium which survives the winter in infected canes.  During wet periods, the fungus produces windborne spores and infects through open wounds on canes caused by pruning, harvest damage, insect damage, or abrasions from canes rubbing together.  As the disease develops, the fungus produces additional spores that can spread to other wounds by wind and splashing water, leading to additional infections.

How do I save plants with cane blight?  DO NOT prune infected canes during the growing season as pruning wounds will provide the cane blight fungus easy entry into healthy tissue.  Label symptomatic canes as you see them and prune the canes to the ground during the dormant season (i.e., mid- to late winter).  Also prune any older cane stubs at this time to remove them as a source of abrasion and wounding for newer canes.  Pruning when plants are dormant allows ample time for wounds to close at a time of year when spores of the cane blight fungus are not being produced.  Use only sharp tools for pruning, and disinfest pruning tools after each cut by treating them for at least 30 seconds with 10% bleach or preferably (due to its less corrosive properties) 70% alcohol (e.g., rubbing alcohol or certain spray disinfectants).  If you use bleach, be sure to thoroughly rinse and oil your tools after pruning to prevent rusting.  Dispose of any canes that you prune by burning them (where allowed by local ordinance), burying them, or arranging to have them hauled away through municipal brush collection.  Be patient as you attempt to get cane blight under control; it may take two or more years of pruning and good sanitation to reduce cane blight to negligible levels.

How can I prevent cane blight in the future?  When establishing a new raspberry patch, choose a site that is well-drained and sunny, and make sure that the distance between rows is approximately 18 inches.  Also be sure to keep weeds under control.  Proper site selection, row spacing and weed control will promote good airflow and drainage, and will reduce excessive moisture that is favorable for spore production by the cane blight fungus.

In addition, maintain optimum soil fertility.  If you underfertilize plants, they will produce weak canes that are less able to fend off infections by the cane blight fungus.  If you overfertilize plants (particularly with nitrogen), they will produce succulent new growth that is more prone to breakage and more prone to wounding by certain insects that will use the new growth as food.  Remember that wounds of any kind can provide entry points for the cane blight fungus.  For details on properly fertilizing raspberries, see UW Bulletin A1610, Growing Raspberries in Wisconsin (available at https://learningstore.extension.wisc.edu/).

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

Wood Mulch and Tree Health

What are the benefits of wood mulch?  Wood mulch is typically available as chipped wood, or shredded or chunked bark, and can contribute to tree health in many ways.  When high quality, composted mulches are applied two to four inches deep in a ring three to six feet in diameter (or greater) from the trunk of a tree, mulch can help preserve moisture, control weeds, limit damage to the trunk from mowers and string trimmers and moderate the soil temperature.  Use four inches of mulch when soils are light and well-drained, and two inches of mulch on heavier, clay soils.

Use of properly composted mulches can be beneficial to trees and shrubs in the landscape.
Use of properly composted mulches can be beneficial to trees and shrubs in the landscape.

Can wood mulch harm trees?  Use of improperly composted mulches (some-times called “sour mulches”), can lead to tree nutrient deficiencies.  Sour mulches can also produce gases like methane and ammonia that can be toxic to plants.  Foliage on trees surrounded by sour mulches may initially turn yellow, then brown, die and fall off.  If your mulch smells like vinegar, ammonia or sulfur, it is likely a sour mulch and should be removed.  Replace the sour mulch with a high quality, composted mulch and consult with your local Extension agent about testing the soil for nutrient deficiencies.  Fertilize appropriately based on the results of these tests.

Improper application of mulch can also lead to problems.  Piling wood mulch up against the trunk of a tree can keep the bark underneath excessively wet.  This wetness can contribute to bark decay.  In addition, use of thick mulch layers (greater than four inches) can lead to overly wet soils that are favorable for development of root rots (see UW Plant Disease Facts D0094, Root and Crown Rot).  To avoid these problems, make sure mulch is applied at least four inches away from the trunk of a tree and that the mulch layer is the appropriate thickness for the soil type in your landscape (see above).

Does woody mulch harbor or attract insects?  Insects such as earwigs [see UW Bulletin A3640, Controlling Earwigs (available at https://learningstore.extension.wisc.edu/)], centipedes (see University of Wisconsin Garden Facts XHT1113, Centipedes), millipedes (see University of Wisconsin Garden Facts XHT1108, Millipedes) and sowbugs (see University of Wisconsin Garden Facts XHT1110, Sowbugs) can feed on decaying organic matter in mulches.  While these insects are often only nuisances, earwigs can feed on and cause damage to a variety of ornamentals, particularly to flowering plants.  If mulch is used near entrances to a home or around basement windows, these unwanted insects may get inside.  Termites ingest wood and can be attracted to wood mulch, but new termite colonies are not likely to become established due to use of wood mulches.  Typically, termites are not a problem in Wisconsin, and when colonies are found, they occur only in the southern half of the state.

Carpenter ants [see UW Bulletin A3641, Controlling Carpenter Ants (available at https://learningstore.extension.wisc.edu/)] and powderpost beetles (see University of Wisconsin Garden Facts XHT1053, Powderpost Beetles) are unlikely to utilize mulch as a food source because conditions required for their development would not be satisfied by wood mulch.  Carpenter ants do not ingest wood as a food source; instead, they chew non-living wood (in trees or landscape timbers, etc.) to excavate galleries in which they live and raise their young.  Since wood mulch is composed of small wooden pieces, it would not serve as a home.  To avoid potential insect problems, keep mulch as far away from the foundation of your home as possible and seal all holes and crevices that insects might use as entry points.  Also, periodically inspect landscape timbers and the house for termites.

Does woody mulch harbor tree pathogens?  Wood mulch may come from many sources, including trees and shrubs that have died from a wide range of diseases.  To be harmful to your trees, disease-causing organisms (pathogens) would have to survive in mulch and these organisms would have to move from the mulch either directly, or through the soil, to their new host – your tree.  There is currently very little research on this topic.

Elm trees killed by Dutch elm disease (see UW Plant Disease Facts D0045, Dutch Elm Disease), can serve as breeding areas for native and European elm bark beetles.  Bark beetles that breed in logs or firewood from these trees can pick up the fungi that cause Dutch elm disease (Ophiostoma ulmi and Ophiostoma novo-ulmi) and carry these fungi from tree to tree.  Chipping infected elm trees creates an unfavorable environment for bark beetles yet there is no scientific literature that describes the level of risk of transmitting the Dutch elm disease fungi from wood chips or bark chunks to healthy elms.

Oak trees killed by oak wilt (see UW Plant Disease Facts D0075, Oak Wilt) can be attractive to several sap-feeding beetles that can potentially pick up the oak wilt fungus (Bretziella fagacearum) and move it in the landscape.  This process is affected by moisture and temperature and would likely be disrupted by the chipping and composting process yet there is no scientific literature that describes the level of risk of transmitting the oak wilt disease fungus from wood chips or bark chunks to healthy oaks.

Research at the University of Wisconsin-Madison suggests that wood chip mulches produced from trees suffering from Verticillium wilt (see UW Plant Disease Facts D0121, Verticillium Wilt of Trees and Shrubs) can serve as a source of the fungus (Verticillium dahliae) that causes the disease.  These studies show that Verticillium can survive for at least one year in mulch and that use of this contaminated mulch can lead to Verticillium wilt in both woody and herbaceous plants.  Therefore use of mulches produced from trees with Verticillium wilt should be avoided.

For more information on wood mulch and tree health:  Contact your county Extension agent.

Root and Crown Rots

Discoloration of maple crown and roots typical of Phytophthora root/crown rot.
Discoloration of maple crown and roots typical of Phytophthora root/crown rot.

What is root/crown rot?  Root/crown rot is a general term that describes any disease of woody ornamentals where the pathogen (causal organism) attacks and leads to the deterioration of a plant’s root system and/or lower trunk or branches near the soil line.  Root rots can be chronic diseases or, more commonly, are acute and can lead to the death of the plant.

What does root/crown rot look like?  Gardeners often become aware root/crown rot when they see above ground symptoms.  Affected plants are often slow-growing or stunted and may show signs of wilting.  Often the canopy of an affected tree or shrub is thin, with foliage that is yellow or red, suggesting a nutrient deficiency.  Careful examination of the roots/crowns of these plants reveals tissue that is soft and brown.

Where does root/crown rot come from?  Several soil-borne water molds (i.e., fungi-like organisms) and true fungi can cause root/crown rots, including (most frequently) Phytophthora spp. and Pythium spp. (both water molds), and Rhizoctonia solani and Fusarium spp. (both true fungi).  These organisms have wide host ranges, and prefer wet soil conditions.  Water mold root rot organisms such as Pythium and Phytophtora produce thick-walled spores (called oospores) that can survive for long periods (years to decades) in soil.

How do I save a plant with root/crown rot?  REDUCE SOIL MOISTURE!  Provide enough water to fulfill a plant’s growth needs and prevent drought stress, but DO NOT over-water.  Remove excess mulch (greater than four inches) around trees and shrubs.  Excessive mulch can lead to overly wet soils.

A thinning canopy with red or yellow leaves can indicate a root/crown rot problem.
A thinning canopy with red or yellow leaves can indicate a root/crown rot problem.

Chemical fungicides (e.g., PCNB, mefenoxam, metalaxyl, etridiazole, thiophanate-methyl and propiconazole) and biological control agents (e.g., Gliocladium, Streptomyces, and Trichoderma) are labeled for root/crown rot control.  However, DO NOT use these products unless you know exactly which root/crown rot pathogen(s) is(are) affecting your trees and shrubs.  Contact your county Extension agent for details on obtaining an accurate root/crown rot diagnosis and for advice on which, if any, fungicides you should consider using.

How do I avoid problems with root/crown rots?  Buy plants from a reputable source, and make sure they are root/crown rot-free prior to purchase.  Establish healthy plants in a well-drained site, and when planting, place the root collar just at the soil surface.  To moderate soil moisture, add organic material (e.g., leaf litter or compost) to heavy soils to increase soil drainage, and DO NOT over-water.  Also, DO NOT apply more than three inches of mulch around trees and shrubs, and keep mulch from directly contacting the base of trunks and stems.  Prevent physical damage (e.g., lawnmower injury) that can provide entry points for root/crown rot pathogens.  Finally, minimize movement of root/crown rot fungi in your garden.  DO NOT move soil or plants from areas where plants are having root/crown rot problems.  DO NOT water plants with water contaminated with soil (and thus potentially with root/crown rot organisms).  After working with plants with root/crown rot, decontaminate tools and footwear by treating for at least 30 seconds with a 10% bleach solution or 70% alcohol (e.g., rubbing alcohol, certain spray disinfectants).  If you use bleach to decontaminate metal tools, be sure to thoroughly rinse and oil your tools after you are done gardening to prevent rusting.

For more information on root/crown rots:  See UW Bulletin A2532, Maple Decline:  Collar Rot and Basal Canker Complex (available at https://learningstore.extension.wisc.edu/), or contact your county Extension agent.

Raspberry Anthracnose

Anthracnose on raspberry canes. Note the sunken, gray centers and raised, purple edges. (Photo courtesy of Patricia McManus)
Anthracnose on raspberry canes. Note the sunken, gray centers and raised, purple edges. (Photo courtesy of Patricia McManus)

What is raspberry anthracnose?  Raspberry anthracnose is a common disease that causes significant reductions of yield and fruit quality on raspberries in the United States.  Black and purple raspberries (oftentimes misidentified as blackberries) are particularly susceptible to anthracnose, while red raspberries are less susceptible to the disease.  True blackberries are also occasionally affected by anthracnose.

What does raspberry anthracnose look like?  Anthracnose can affect any part of a raspberry plant; however, canes are most commonly affected.  Watch for scattered, purple, roundish spots (up to ⅜ inch in diameter) that are characteristic of the disease.  Over time, the spots develop ash-colored, sunken centers and raised purple margins.  When anthracnose is severe and develops early in the growing season, these sunken spots can merge and girdle raspberry canes, resulting in cane death.  When anthracnose develops later in the season spots may not develop sunken centers, but will overlap and merge.  If a large enough portion of a cane is affected, the bark may split.  This symptom is called “gray bark”.  Canes severely affected by anthracnose are more prone to winter injury.  On leaves, anthracnose symptoms typically appear in early to mid-summer as irregularly-shaped, yellow spots that are approximately 1/16 inch in diameter.  These spots enlarge and develop gray centers with reddish-purple borders.  Over time, the gray centers may fall out, giving the spots a “shot-hole” effect.

Where does raspberry anthracnose come from?  Anthracnose is caused by the fungus Elsinoe necator.  The fungus can be introduced into a raspberry patch on infected plants or by spores that are blown from other raspberry patches or from wild raspberry relatives (such as brambles).  Once established in a raspberry patch, E. necator can overwinter on diseased raspberry canes.  In the spring, spores of the fungus spread to new canes (which are more susceptible to infection due to their lack of a hard bark) and leaves by wind, rain splash, and insects.  Infections are more likely to occur during long periods of wet weather.  Additional spores produced as a result of these early season infections can lead to additional infections late in the summer (e.g., in late August and September).

How do I save a plant with raspberry anthracnose?  Anthracnose is difficult to manage once symptoms have developed.  Pruning symptomatic canes as they appear may provide some benefit, but only when disease levels are relatively low.  Prune four to six inches below where symptoms have appeared and decontaminate pruning tools between cuts by treating them for at least 30 seconds with 10% bleach or preferably (because of its less corrosive properties) 70% alcohol.  Rubbing alcohol and many spray disinfectants typically contain approximately 70% alcohol and are easy to use.  Canes can be burned (where allowed by local ordinance) or buried.  Fungicides are not effective for controlling anthracnose once infections have occurred and should be only be used preventatively (see below).

How do I avoid problems with raspberry anthracnose in the future?  When establishing your raspberry patch, choose a sunny, open area with a well-drained soil.  Remove plants related to raspberries (e.g., brambles) from the vicinity of your patch as these plants can potentially be a source of the anthracnose fungus.  Purchase disease-free raspberry plants from a reputable nursery or other raspberry supplier, and space these plants in rows that are 12 to 18 inches apart to promote good air flow, and rapid drying of plants.  Fertilize your raspberries appropriately [see UW Bulletin A1610, Growing Raspberries in Wisconsin (available at https://learningstore.extension.wisc.edu/) for details], but DO NOT overfertilize with nitrogen.  Heavy nitrogen fertilization will stimulate excessive growth of tender canes that are more susceptible to infection by the anthracnose fungus.  When watering, DO NOT use a sprinkler; instead use a soaker or drip hose that applies water to the soil rather than to onto leaves and canes.  Keep weeds under control so that they do not block air movement.

Once your raspberry patch is established, prune raspberries routinely (see UW Bulletin A1610 for details) to remove diseased and winter-injured canes as well as to promote better air flow.  If anthracnose has been a chronic problem in your raspberry patch, consider using a preventative fungicide treatment for control.  Use a single application of liquid lime sulfur (1⅓ cups per gallon of water) when leaf buds show ¼ to ½ inch of green tissue.  Be sure to read and follow all label instructions to ensure that you use the fungicide in the safest and most effective manner possible.

For more information on raspberry anthracnose:  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 http://www.caf.wvu.edu/kearneysville/wvufarm1.html)
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 http://www.caf.wvu.edu/kearneysville/wvufarm1.html)

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 http://npdn.org/ for the lab nearest you).  In Wisconsin, contact the UW-Madison Plant Disease Diagnostics Clinic (PDDC) at (608) 262-283 or pddc@wisc.edu.  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 https://www.aphis.usda.gov/aphis/ourfocus/planthealth/plant-pest-and-disease-programs/pests-and-diseases/

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: laholland@wisc.edu], the UW-Madison PDDC, or your county Extension agent.

Nectria Canker

What is Nectria canker?  Nectria canker is a common and potentially lethal disease that affects many species of trees and shrubs.  This disease can cause significant damage on newly planted, as well as established, trees and shrubs that are under stress.

Nectria canker on a honey locust tree. The site of infection was a pruning wound.
Nectria canker on a honey locust tree. The site of infection was a pruning wound.

What does Nectria canker look like?  Nectria canker is characterized by the formation of sunken areas (cankers) that form on twigs, branches, and trunks.  Cankers can form at leaf scars and wherever injuries occur.  Injuries can be caused by pruning (particularly improper pruning), frost, hail, cracking from heavy snow or ice, sunscald, insects, or animals.  Cankers appear first as slightly sunken areas on the bark, but can grow for years, becoming target-shaped or elongated.  Small branches girdled by cankers can wilt suddenly, fail to leaf out, and die.

Where does Nectria canker come from?  Nectria canker is caused by two fungi, Nectria cinnabarina and Nectria galligena.  These fungi survive in the margins of cankers where they produce numerous fruiting bodies (reproductive structures).  Fruiting bodies can be cream, coral, orange, or red, and eventually darken to brown or black with age.  Spores are dispersed by wind, water, and pruning tools.  Cankers expand slowly, usually when the host is dormant or under stress.  Infected plants may hold the fungus in check by producing wound-closing (callus) tissue around the infected area.

How do I save a tree with Nectria canker?  There is no cure for Nectria canker.  Remove smaller branch cankers by pruning six to eight inches below the canker.  Disinfect pruning tools after each cut by treating them for at least 30 seconds with 10% bleach solution or (preferably due to its less corrosive properties) 70% alcohol (e.g., rubbing alcohol, certain spray disinfectants).  If you use bleach, be sure to thoroughly rinse and oil your tools after pruning to prevent rusting.  Trees with trunk cankers may live many years with the disease.  Healthy trees are better able to slow the development of Nectria canker, so make sure that trees are watered and fertilized properly.

How do I avoid problems with Nectria canker in the future?  Choose plants that are well-adapted to your local climate.  Avoid any stresses to your trees and shrubs.  Prune trees and shrubs properly.  See UW-Garden Facts XHT1014, Pruning Deciduous Trees, and XHT1015, Pruning Deciduous Shrubs), and avoid injury to root and trunks from lawnmowers.  Remove grass from around the base of trees and shrubs, mulch properly, and water as needed to avoid drought stress.

For more information on Nectria canker:  See UW Bulletin A3281, Honey Locust Disorder:  Canker Diseases in Wisconsin (available at https://learningstore.extension.wisc.edu/), or contact your county Extension agent.

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., an 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 a non-treated area.  Often, drifting herbicides are difficult to detect by eye because they are extremely fine mists.  They 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?  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.  DO NOT 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 Bulletin A3286, Plant Injury Due to Turfgrass Broadleaf Weed Herbicides (available at https://learningstore.extension.wisc.edu/), or contact your county Extension agent.

Guignardia Leaf Spot

What is Guignardia leaf spot?  Guignardia leaf spot is a common fungal leaf disease that affects vining plants such as Boston ivy and Virginia-creeper.  The fungus that causes Guignardia leaf spot also causes a leaf spot and fruit rot of grape called black rot.

Roughly circular, red-bordered spots on Boston ivy typical of Guignardia leaf spot.
Roughly circular, red-bordered spots on Boston ivy typical of Guignardia leaf spot.

What does Guignardia leaf spot look like?  Symptoms of Guignardia leaf spot include roughly circular, or sometimes angular (i.e., straight-edged), ¼ to ½ inch diameter dead spots on affected leaves.  Spots often have a purple-red border, and the centers may eventually fall out.  Young leaves are more susceptible to infection than mature leaves.  If infections occur before leaves have fully expanded, leaves can become puckered and distorted.  Within the spots, a diffuse ring of black dots (reproductive structures of the fungus) is typically visible.

Where does Guignardia leaf spot come from?  Guignardia leaf spot is caused by the fungus Phyllosticta ampelicida (formerly Guignardia bidwellii), which survives in leaf litter.  Spores of the fungus are produced under cool, moist conditions and can be dispersed by wind or splashing water.

How do I save a plant with Guignardia leaf spot?  DO NOT panic!  Guignardia leaf spot is most often a cosmetic disease, making an affected plant look a little ragged, but not killing the plant.  Only occasionally will the disease be more severe, resulting in defoliation.

How do I avoid problems with Guignardia leaf spot in the future?  Remove and burn (where allowed by local ordinance), bury or hot compost fallen, infected leaves.  Thin plants to increase airflow and promote rapid drying of foliage.  This drier environment is less favorable for disease developments.  Water plants at the base using a soaker or drip hose to minimize wetting of leaves and reduce the movement of spores.  If a plant has been severely defoliated by Guignardia leaf spot for several years, preventative fungicide treatments may be necessary.  Make an initial fungicide application as leaves first begin to emerge.  If the weather is cool and wet, make additional applications at seven to 14 day intervals until hotter, drier weather develops or until leaves are fully expanded and mature.  Chlorothalonil, copper, mancozeb and thiophanate-methyl are labeled for Guignardia leaf spot control.  DO NOT use the same active ingredient for all treatments (particularly if you are using thiophanate-methyl).  Instead, alternate the use of at least two active ingredients.  This strategy will help minimize problems with fungicide-resistant strains of Phyllosticta ampelicida.  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.

For more information on Guignardia leaf spot:  Contact your county Extension agent.