Category Archives: Disease – Broad Leafed Woody Ornamental

Sudden Oak Death

What is sudden oak death?  Sudden oak death (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.

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.

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).

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

What does sudden oak death look like?  Symptoms of sudden oak death 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 sudden oak death 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?  Sudden oak death 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 fungus 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 sudden oak death, please IMMEDIATELY submit a sample to the Plant Disease Diagnostics Clinic (PDDC), care of the address in the box at the bottom of this page.  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 pddc@wisc.edu.

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 dipping them for at least 30 seconds in 10% bleach.  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 sudden oak death.  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: pddc@wisc.edu, see the USDA APHIS sudden oak death website https://www.aphis.usda.gov/aphis/ourfocus/planthealth/plant-pest-and-disease-programs/pests-and-diseases/phytophthora-ramorum/sod, or contact your County Extension agent.

Silver Leaf

What is silver leaf?  Silver leaf is a fungal disease that affects a wide range of deciduous trees.  The disease has its biggest impact in fruit trees such as apple, pear and cherry, but can also affect ornamental trees such as willow, poplar, maple, oak, and elm.  Silver leaf has traditionally been considered a disease of older trees that have been physically damaged or are in decline due to other diseases.  However, beginning in 2017, severe cases of silver leaf have been observed on young, healthy apple trees in commercial orchards in Wisconsin.

Young, vigorous high-density apple trees, with trees showing symptoms of silver leaf (on the right) adjacent to those that do not (on the left).
Young, vigorous high-density apple trees, with trees showing symptoms of silver leaf (on the right) adjacent to those that do not (on the left).

What does silver leaf look like?  The first symptom of silver leaf is a silver sheen that appears on leaves of affected trees. The number of leaves affected can vary dramatically from tree to tree.  The silver sheen develops when the epidermis of a leaf (i.e., the surface layer of cells) separates from the rest of the leaf, altering the way that the leaf reflects light.  The silvery leaves may also have brown, dead patches.  Leaf symptoms may appear one year, but may be less severe or even nonexistent in subsequent years.

Note that other tree stresses (particularly environmental stresses) can cause leaf symptoms similar to those of silver leaf.  An additional symptom that can help in identifying silver leaf is dark staining just under the bark of branches with symptomatic leaves. This staining can extend several inches down a branch.  Eventually, white edged, purple-brown, shelf-like conks (reproductive structures of the fungus that causes the disease) will appear on branches and/or trunks of the diseased trees.

Where does silver leaf come from?  Silver leaf is caused by the fungus Chondrostereum purpureum.  Spores of the fungus are released from conks during wet periods in the autumn and spring and infect trees at pruning scars or other open wounds (e.g., wounds from branches breaking during severe storms or due to heavy, wet spring snow). The fungus lives in the xylem (i.e., the water-conducting tissue) of infected branches, and its presence in the xylem leads to the dark staining as described above.  A toxin released by the fungus moves up into the leaves causing the epidermis separation that leads to the silver sheen of the leaves. Eventually, wood in infected branches begins to decay, at which point the fungus starts producing conks.

How do I save a tree with silver leaf?  On trees with limited damage, prune out branches showing leaf symptoms.  Also watch for any conks, and immediately remove branches where these are present.  Removing conks limits production of spores that can lead to infections in other trees.  When pruning, cut branches at least four inches below where you can see staining under the bark or where conks are visible.  Decontaminate pruning tools after each cut by dipping them for at least 30 seconds in 70% alcohol, a commercial disinfectant or 10% bleach.  You can also use spray disinfectants that contain approximately 70% alcohol.  Apply sprays to tools until they drip and then allow the tools to air dry.

The silver sheen of leaves typical of silver leaf (left) and conks (i.e., reproductive structures) of the silver leaf fungus, Chondrostereum purpureum (right).
The silver sheen of leaves typical of silver leaf (left) and conks (i.e., reproductive structures) of the silver leaf fungus, Chondrostereum purpureum (right).

In plantings where silver leaf symptoms are widespread, pruning out all symptomatic branches may not be practical, and the loss of that many branches might cause more harm than good. Also, trees sometimes show symptoms one year but then appear to recover in subsequent years. Therefore, instead of pruning symptomatic branches, consider marking diseased trees.  Carefully watch the marked trees each year to see if symptoms reoccur, or if the trees lose vigor. If trees lose vigor and/or conks become visible, then the trees should be removed.  Because the silver leaf fungus limits water movement in infected branches, make sure that affected trees receive adequate water.  In general trees should receive approximately one inch of water per week during the growing season from natural rain and/or irrigation.  Eventually infected trees will likely decline to the point where they should be removed.  In some instances, monitoring trees may not be feasible.  In such situations, removing trees the first year that they show silver leaf symptoms may be the best management option.

Any branches or trunk sections removed from trees with silver leaf should be disposed of by burning (where allowed) or burying.

How do I avoid problems with silver leaf in the future?  Whenever possible, prune trees during the winter during dry periods when temperatures are below 32°F.  If you must prune during the growing season, only prune during dry periods.  Pruning at these times will decrease the risk of infection by the silver leaf fungus through pruning wounds. DO NOT use pruning paints or sealants when pruning. At this time, there are no fungicides for silver leaf control.

Boxwood Blight

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

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)

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.

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 University of Wisconsin Garden Facts XHT1239, “Winter Burn”), dog urine and other diseases such as Volutella blight (see University of Wisconsin Garden Facts XHT1191, “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 Division of Extension Plant Disease Diagnostics Clinic (https://pddc.wisc.edu/).  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, 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 dipping them for at least 30 seconds in 70% alcohol (e.g., rubbing alcohol) or (as a last resort) in 10% bleach.  Spray disinfectants that contain at least 70% alcohol also can be used.  Spray tools until they drip and then allow them to air dry.

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 boxwood ‘Green Gem’, common boxwood variety ‘Katerberg’ North Star®, and Korean littleleaf boxwood varieties ‘Eseles’ Wedding Ring®, ‘Franklin’s Gem’, ‘Winter Gem’ and ‘Wintergreen’ are hardy (to USDA hardiness zone 5) and have been documented to be resistant to box blight.  Always buy boxwood shrubs from local reputable suppliers who have thoroughly inspected boxwood plants for evidence of boxwood blight.  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.  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.

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)

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.

Gymnosporangium Rusts

What are Gymnosporangium rusts?  Gymnosporangium rusts are a group of closely related diseases caused by fungi that infect both junipers (in particular red cedar) and woody plants in the rose family (such as, but not limited to, apple, crabapple, hawthorn and quince).  These fungi must infect both types of plants to complete their life cycles.  The most common Gymnosporangium rusts found in Wisconsin are cedar-apple rust, cedar-hawthorn rust and cedar-quince rust.  The names of these diseases are somewhat misleading, given that all three diseases can affect multiple rosaceous hosts in addition to those referenced in their names.

Yellow cedar-apple rust spots on an apple leaf (left) and slimy, orange, gelatinous cedar-apple rust galls on a juniper branch (right).
Yellow cedar-apple rust spots on an apple leaf (left) and slimy, orange, gelatinous cedar-apple rust galls on a juniper branch (right).

What do Gymnosporangium rusts look like?  On junipers, symptoms of Gymnosporagium rusts vary.  Cedar-hawthorn and cedar-apple rust fungi induce formation of irregularly-shaped brown galls, with cedar-hawthorn rust galls tending to be smaller in size (approximately 1/8 to 9/16 inch in diameter) than cedar-apple rust galls (approximately 1/4 to 2 inches in diameter).  Both types of galls produce distinctive slimy, orange, gelatinous appendages in the spring.  In contrast, the cedar-quince rust fungus causes juniper branch swellings.  Orange spores ooze from these swollen areas in the spring.

On rosaceous hosts, Gymnosporangium rust symptoms also vary.  Symptoms of cedar-hawthorn rust and cedar-apple rust appear in mid to late May, typically as circular, yellow-orange areas on leaves.  Eventually, tube-like structures (that have a fringe-like appearance) form on the undersides of leaves beneath the yellow spots.  Symptoms of cedar-quince rust typically become obvious later in the summer (most commonly on hawthorns) as swollen, spiny branches and/or fruits.

Where do Gymnosporangium rusts come from?  Several fungi in the genus Gymnosporangium cause Gymnosporangium rusts. These include Gymnosporangium juniperi-virginianae (cedar-apple rust), Gymnosporangium globosum (cedar-hawthorn rust), and Gymnosporangium clavipes (cedar-quince rust).  These fungi overwinter in infected branches and galls on junipers.  Spores oozed from the infected branches or produced in the gelatinous gall appendages drift to rosaceous hosts leading to leaf and fruit infections.  Similarly, spores produced in the tube-like structures/spines on rosaceous leaves and fruits drift to junipers leading to new branch infections and additional gall formation.

How do I save a tree or shrub with Gymnosporangium rust?  Gymnosporangium rusts are primarily cosmetic diseases that make susceptible plants unattractive, but rarely have long-term detrimental effects.  Gymnosporangium rusts on leaves can, for all practical purposes, be ignored.  Gymnosporangium rusts on juniper branches can be easily managed by pruning approximately four to six inches below swollen areas or galls.  Rosaceous hosts with infected branches can be pruned similarly.  Be sure to decontaminate pruning tools between cuts by dipping them for at least 30 seconds in 70% alcohol (e.g., rubbing alcohol) or 10% bleach.  Alternatively, you can spray tools with a disinfectant that contains approximately 70% alcohol, then allow them to air dry.  Decontaminating tools will prevent movement of rust fungi from branch to branch or from plant to plant during pruning.

Cedar-quince rust on hawthorn fruit.
Cedar-quince rust on hawthorn fruit.

How do I avoid problems with Gymnosporangium rusts in the future?  The best way to avoid Gymnosporangium rusts is to not grow junipers (particularly red cedar) and susceptible rosaceous hosts close to one another.  In urban settings where yards are small however, keeping both hosts adequately separated may be impossible.  Where Gymnosporangium rusts have consistently been a problem, consider using evergreens (e.g., pine, fir, spruce) and flowering trees and shrubs (e.g., cherry, plum, lilac) that are immune to these diseases.  If you decide that you want to mix junipers with apple, crabapple, hawthorn, and quince on your property, check at your local nursery for resistant varieties that will satisfy your landscaping needs.  In general, Chinese junipers (Juniperus chinensis) tend to be relatively resistant to Gymnosporangium rusts.

Fungicides treatments are also available to control Gymnosporangium rusts, although such treatments should be considered only as a last resort.  Among fungicides marketed for use by home gardeners, those containing chlorothalonil, copper, mancozeb, myclobutanil, propiconazole, and sulfur are labeled for use for Gymnosporangium rust control.  These products may be useful for controlling Gymnosporangium rusts on rosaceous hosts, but will likely not be particularly effective if used on junipers.  For optimal control on rosaceous hosts, apply treatments when flower buds first show color, when half of the flowers are open, at petal-fall, seven to 10 days after petal fall and finally 10 to 14 days later.  Be sure to read and follow all label instructions of the fungicide(s) that you select to ensure that you use the products(s) in the safest and most effective manner possible.  In particular, be sure that you select appropriate products when treating trees and shrubs with edible fruit.  If you decide to use propiconazole or myclobutanil, alternate use of these active ingredients with use of at least one of the other active ingredients listed above (but DO NOT alternate propiconazole with myclobutanil) to help minimize potential problems with fungicide-resistant strains of Gymnosporangium.

For more information on Gymnosporangium rusts: See UW-Extension Bulletin A2598, “Apple, Crabapple, Hawthorn and Juniper Disorder:  Cedar-Rust Complex” (available at https://learningstore.uwex.edu/Assets/pdfs/A2598.pdf) and “Juniper Diseases” (available at https://store.extension.iastate.edu/Product/5247), or contact your county Extension agent.

Southern Blight

The Southern blight fungus produces large numbers of spherical, light tan to dark red resting structures called sclerotia (red arrow).

What is Southern blight?  Southern blight is a lethal fungal disease that is most common in the tropics and subtropics.  This disease causes damage in the southern U.S. and can even cause problems in temperate locations like Wisconsin during periods of warm, moist weather.  Southern blight has a wide host range, affecting over 500 plant species.  Vegetable and fruit hosts include tomato, pepper, onion, beet, rhubarb, strawberry, lettuce, cucumber, melon, carrot, asparagus and parsley.  Ornamental hosts include aster, black-eyed Susan, dahlia, daylily, gladiolus, hosta, impatiens, peony, petunia, rose, salvia, sedum and viola.  Small woody trees and shrubs can be affected as well.


What does Southern blight look like?
  Southern blight initially leads to a water-soaked appearance on lower leaves or water-soaked lesions (spots) on lower stems.  Any plant part that is near or in contact with the soil may become infected.  Infected plants yellow and wilt, often within days of infection, particularly when the weather is moist and warm (80 to 95°F).  Fruit rots, crown rots and root rots are also typical symptoms of the disease.  Thick mats of white fungal threads (called mycelia) may grow from infected tissue, radiating from the plant onto the soil surface.  Sclerotia (small spherical structures that are about the size of mustard seeds) develop on infected tissue and on the soil surface.  Sclerotia range in color from light tan to dark reddish-brown to black.

Where does Southern blight come from?  Southern blight is caused by the fungus Sclerotium rolfsii, which lives in soil, on plants (including weeds), and in plant debris.  The fungus can be spread through movement of infested soil and plant debris, on infected plants, in contaminated irrigation water, and through use of contaminated tools.  In Wisconsin, S. rolfsii most likely enters gardens on infected nursery stock or infested mulch.  Freezing temperatures will kill S. rolfsii mycelia, but sclerotia can survive temperatures as low as approximately 14ºF.

How can I save a plant with Southern blight?  Identify the extent of an infestation based on visible dead/dying plants, fungal mycelia and sclerotia.  Remove all plants (including roots), as well as three inches of soil, from at least 12 inches beyond the infested area.  Start at the edge of the infested area and work toward the center.  Bag all plants and soil and dispose of these materials in a landfill.

Southern blight can be a serious disease of vegetables, including tomatoes, leading to wilting and plant death. (Photo courtesy of Gary E. Vallad, University of Florida)

Turn any remaining soil in the infested area eight to 12 inches to bury any sclerotia that you may have missed.  This will reduce the length of time that the sclerotia will survive.  Grow non-susceptible plants (e.g., larger woody ornamentals) in the affected area for two to three years to allow time for sclerotia to die naturally.  Fungicides containing azoles (e.g., propiconazole, tebuconazole), fludioxonil, flutolanil, mancozeb, PCNB, strobilurins (e.g., azoxystrobin, fluoxastrobin), thiophanate-methyl, and triadimefon are all labeled for Southern blight control, but may have varying levels of effectiveness.  All of these products will likely be more effective if applied as preventive treatments rather than in an attempt to “cure” existing disease.  If you decide to use fungicides, DO NOT use one active ingredient for all treatments.  Instead, alternate the use of two or more unrelated active ingredients to help minimize problems with fungicide-resistant strains of S. rolfsii.  DO NOT alternate active ingredients that are chemically related (e.g., propiconazole and tebuconazole, or azoxystrobin and fluoxastrobin).  Be sure to read and follow all label instructions of the fungicide(s) that you select to ensure that you use the fungicide(s) in the safest and most effective manner possible.

How can I prevent Southern blight in the future?  Inspect new plants for sclerotia and mycelia of S. rolfsii prior to transplanting.  Bag and dispose of diseased plants as described above.  Use high-quality mulches (e.g., shredded oak bark mulch, red cedar mulch) in your garden and avoid any mulches that you suspect might be contaminated with S. rolfsiiS. rolfsii thrives under moist conditions.  Therefore, thin existing gardens or space plants farther apart in new gardens to improve airflow and promote more rapid drying of foliage and soil.

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

Tar Spot of Trees and Shrubs

What is tar spot?   Tar spot is a common, visually distinctive and primarily cosmetic fungal leaf spot disease.  Tar spot can affect many species of maple, including (but not limited to) silver maple, sugar maple and Norway maple.  Boxelder (also known as ash-leaved maple), willow, holly and tulip-tree can also be affected by tar spot.

Symptoms of tar spot of silver maple caused by Rhytisma americanum (left) and tar spot of Norway maple caused by Rhytisma acerinum (right).

What does tar spot look like? Initial symptoms of tar spot are small (approximately 18 inch) yellowish spots that form on infected leaves. These spots may remain relatively small, or may enlarge over the growing season to roughly 34 inch in diameter. As tar spot develops, black structures (resembling blobs of tar) form.  On Norway maple, the black structures are typically numerous, small (approximately 18 inch in diameter), and clustered together.  On silver maple, the black structures are often single, large (approximately 3inch in diameter) and visibly raised.  If you carefully examine the larger tar-like areas on silver maple, you will see convoluted line patterns that resemble fingerprints.

Where does tar spot come from?  Several fungi in the genus Rhytisma cause tar spot.  On maples specifically, Rhytismaamericanum, Rhytismaacerinum, and (less commonly) Rhytismapunctatum cause tar spot.  Tar spot fungi commonly survive in leaf litter where they produce spores in the spring that lead to leaf infections.

How do I save a tree with tar spot?   DO NOT panic.  For most maples and other susceptible trees and shrubs, tar spot is not a serious disease, but primarily is a cosmetic disease that makes trees and shrubs look unsightly.  Tar spot does not kill trees or shrubs, nor does it typically even cause serious defoliation.

How do I avoid problems with tar spot in the future?  You can reduce or even eliminate tar spot by simply collecting up and properly disposing of leaves from affected trees and shrubs each fall.  These leaves can be burned (where allowed), buried or hot composted.  When composting, make sure that your compost pile reaches a high temperature (approximately 140°F).  Also, make sure that you routinely turn your compost pile so that leaves on the outside of the pile eventually end up in the center of the pile.  The combination of high temperature and decay of leaf tissue in a compost pile helps eliminate tar spot fungi.

Fungicides containing copper and mancozeb are labeled for tar spot control in Wisconsin.  However, fungicide treatments for this disease are rarely, if ever, warranted.  Consult with your county UW-Extension horticulture/agriculture agent to determine if your tree warrants preventative treatments.  If warranted, three fungicide applications will be necessary for control:  one at bud break, one when leaves are half expanded, and one when leaves are fully expanded.  Be sure to read and follow all label instructions of the fungicide that you select to ensure that you use the fungicide in the safest and most effective manner possible.

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

Apple Scab

What is scab? Scab is a potentially serious fungal disease of ornamental and fruit trees in the rose family. Trees that are most commonly and severely affected include crabapple, hawthorn, mountain ash, apple and pear. In ornamental trees, leaf loss caused by apple scab can make trees unsightly and aesthetically unappealing.  In fruit trees, leaf loss can reduce fruit yield.  In addition, the presence of apple scab on fruits can make the fruit difficult, if not impossible, to market.

Leaf spots typical of apple scab on apple. Typical apple scab leaf spots (left) and fruit lesions (right). The disease can cause total defoliation and make fruit unmarketable.

What does scab look like? Scab lesions (diseased areas) are often first noticed on leaves, where they most commonly occur on the upper leaf surface. Fruits are also very susceptible to infection. Lesions on both leaves and fruits are roughly circular with feathery edges, and have an olive green to black color. They can be as small as the size of a pinhead or as large a 12 inch in diameter. When disease is severe, lesions can merge and cover a large portion of the leaf or fruit surface. Defoliation of a tree (i.e., extensive leaf drop) often follows.

Where does scab come from? Scab is caused primarily by the fungus Venturia inaequalis.  Other species of Venturia can be involved in diseases similar to apple scab.  In particular, Venturia pirina causes a very similar disease (called pear scab) on pear.  Venturia inaequalis and its relatives survive the winter in leaf litter from infected trees.  Scab is most severe in cool, wet years.

How do I save a tree or shrub with scab?  Apple scab is not a lethal disease, even when trees totally defoliate.  Once symptoms are visible, it is too late to treat a tree.  Proper long-term management of apple scab is important however.  If left unchecked, defoliation due to apple scab year after year can stress a tree and make it more susceptible to other, more serious and lethal diseases and insect pests.

How do I avoid problems with scab in the future?  If your crabapple, apple or pear tree has a history of severe scab, consider replacing it with a resistant variety.  See UW-Extension Bulletins A2105 – ‘Apple Cultivars for Wisconsin”), A2488 – “Home Fruit Cultivars for Northern Wisconsin”, and A2582 – “Home Fruit Cultivars for Southern Wisconsin” (all available at https://learningstore.uwex.edu/) for recommendations.

If you have a susceptible tree that you want to maintain, carefully collect up and discard fallen, infected leaves each autumn.  These leaves are a major source of spores that can infect leaves the following growing season.  Also, be sure to routinely thin your trees to open up the canopy and allow better airflow.  Thinning will promote more rapid drying of leaves which is less favorable for apple scab development.

Even with proper fall leaf clean up and thinning, you may have to consider applying fungicide treatments to susceptible trees, particularly when the weather is cool and wet.  Captan, chlorothalonil, mancozeb, myclobutanil, propiconazole, or thiophanate methyl, are available for apple scab control, although not all of these active ingredients can be used on trees where fruit will eventually be eaten.  Be sure to read and follow all label instructions of the fungicide(s) that you select to ensure that you use the correct fungicide(s) in the safest and most effective manner possible.  Typically, you will need to treat every seven to 14 days from bud break until wet weather subsides.  DO NOT use myclobutanil, propiconazole, or thiophanate methyl 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 to help minimize problems with fungicide-resistant strains of the apple and pear scab fungi.

For more information on scab:  See UW-Extension Bulletins A2173 – “Crabapple Disorder:  Scab” and A2594 – “Mountain Ash Disorder:  Scab” (both available at https://learningstore.uwex.edu/), or contact your county Extension agent.

Eastern Filbert Blight

What is Eastern filbert blight?  Eastern filbert blight is a potentially serious fungal disease found throughout the United States, including Wisconsin.  It affects only Corylus species, commonly known as hazelnuts or filberts.  On hazelnuts native to Wisconsin such as American hazelnut (Corylus americana) and beaked hazelnut (Corylus cornuta), the disease causes little significant damage, but on the commonly grown European hazelnut (Corylus avellana), including Harry Lauder’s walking stick (Corylus avellanaContorta’), the disease is lethal.  Turkish filbert (Corylus colurna) also appears to be highly susceptible.

Eastern filbert blight can cause small black cankers that form in rows, or deep gouges in the bark of severely infected trees/shrubs.
Eastern filbert blight can cause small black cankers that form in rows, or deep gouges in the bark of severely infected trees/shrubs.

What does Eastern filbert blight look like?  Eastern filbert blight causes cankers (i.e., dead, collapsed areas of bark) on branches or main trunks.  Easily visible within the cankers are black, football-shaped stromata (the reproductive structures of the causal fungus).  The stromata often form in rows of two.  Cankers first appear on new twigs and expand over time.  American hazelnut trees/shrubs are able to live almost indefinitely with Eastern filbert blight, forming a small number of slowly-expanding cankers (if any cankers form at all) that lead to limited branch dieback.  On European hazelnut however, cankers will expand anywhere from one inch to three feet in a year, and can eventually form long, deep gouges or grooves on severely affected trees/shrubs.  European hazelnuts typically die due to girdling from Eastern filbert blight within five to 10 years.

Where does Eastern filbert blight come from?  Eastern filbert blight is caused by the fungus Anisogramma anomala.  Stromata formed by the fungus produce spores that are spread short distances by water splash and over longer distances by wind.  Humans also can spread Anisogramma anomala on their hands and clothing, on gardening tools, and by transporting wood from infected trees/shrubs.  Unlike other canker fungi that can infect through wounds, the Eastern filbert blight fungus primarily infects through immature tissue on actively growing shoots.  Cankers appear 12 to 18 months after infection.  Eastern filbert blight does not affect hazelnut leaves, fruits or nuts.

How do I save trees/shrubs with Eastern filbert blight?  There is no cure for Eastern filbert blight.  If only a few branches on a tree/shrub are affected, prune these branches two to three feet below each canker.  Disinfest tools after each cut by dipping them for at least 30 seconds in a 10% bleach solution or (even better) a 70% alcohol solution.  Alternatively, use a spray disinfectant containing roughly 70% ethanol, spraying tools until they drip and then allowing them to air dry.

If a tree/shrub is severely affected by Eastern filbert blight (e.g., when there are so many cankers on multiple branches that the tree/shrub would look ugly if pruned, when branch pruning would require removing part of the trunk, or when trunk cankers are present), removal of the tree/shrub is the preferred management strategy.

Pruned branches and removed trees/shrubs should be burned (where allowed by local ordinance), deep-buried, or chipped (as long as the chips are allowed to dry to kill the fungus).

How do I avoid problems with Eastern filbert blight in the future?  Consider planting native species of hazelnut (e.g., American and beaked hazelnut) that are naturally resistant to the disease.  If you decide to plant European hazelnut, select cultivars that have been bred for resistance.  ‘Jefferson’, ‘Santiam’, ‘Yamhill’, and ‘Theta’ are resistant, nut-bearing cultivars.  ‘Red Dragon’ is a resistant, ornamental cultivar.  Note that these cultivars are not hardy in all hardiness zones in Wisconsin.  Hybrid hazelnuts (crosses between American and European hazelnut) are becoming increasingly available, but should be used with caution because their susceptibility to Eastern filbert blight has not been adequately tested.

Once hazelnut trees/shrubs are established in your yard, routinely inspect the plants for infection and remove infected branches as they occur.  Watch for dying branches in the summer and cankers (particularly on or near the youngest growth) in the winter.  Inspecting trees during the winter is very important, because cankers are more visible at that time.

Fungicides can be used for management, but should only be used as a last resort.  Not all fungicides that are approved for Eastern filbert blight control are particularly effective, but chlorothalonil has been shown to be an effective preventative treatment, although it will not cure existing infections.  Note that not all formulations of chlorothalonil are approved for use on nut-bearing hazelnuts; many formulations can only be used on ornamental hazelnuts.  Therefore, if you decide to use chlorothalonil, be sure to select the appropriate formulation for your particular situation.  Apply the first treatment at bud break (i.e., when half the buds show a separation of leaves) and additional treatments (up to three) every two weeks thereafter.  If you plan to eat nuts from your hazelnut tree, make sure that your last fungicide treatment is applied at least 120 days before anticipated nut harvest.  For further details about recommended fungicides, spray rate recommendations, and diagrams of bud stages, see “Pest Management Guide for Hazelnuts in the Willamette Valley”, Oregon State University Extension Bulletin EM8328 available at https://catalog.extension.oregonstate.edu/em8328.

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

Volutella Blight

What is Volutella blight? Volutella blight is a common and potentially lethal disease of Japanese pachysandra (Pachysandra terminalis), an evergreen, semi-woody groundcover that is grown in shade gardens throughout hardiness zones 4 and 5 in Wisconsin. Volutella blight can severely limit the aesthetic appeal of pachysandra wherever it is grown.

Volutella blight often starts as lesions on individual leaves, but can eventually kill groups of pachysandra plants.
Volutella blight often starts as lesions on individual leaves, but can eventually kill groups of pachysandra plants.

What does Volutella blight look like? Volutella blight begins as discrete, water-soaked areas (lesions) on pachysandra leaves and stems that eventually turn black/brown and become necrotic (i.e., die). Lesions can be circular, oval or irregular in shape, and may have darker margins. Distinct concentric rings also may be visible in the lesions. Lesions on a single leaf can merge and cause the collapse of the entire leaf, while stem lesions can lead to the collapse of an entire pachysandra plant. When severe, Volutella blight can kill large patches of a pachysandra planting.

Where does Volutella blight come from? Volutella blight is caused by the fungus Volutella pachysandricola which survives in infected plants, as well as on and in pachysandra plant debris. During wet weather, this fungus produces masses of light-orange to pink fungal spores and hyphae (i.e., fungal threads) that may be visible to the naked eye.

How do I save a plant with Volutella blight? Remove diseased leaves, as well as any leaf debris, and prune back or dig up plants with stem infections. Be sure to prune only when plants are dry to minimize the spread of the causal fungus. When disease is severe, remove diseased plants, then follow with one or more applications of a broad-spectrum fungicide (e.g., one containing chlorothalonil). Select a product that is labeled for use on pachysandra, and follow all label instructions to ensure that you apply the product in the safest and most effective way possible.

How do I avoid problems with Volutella blight in the future? V. pachysandricola is a somewhat opportunistic pathogen, most often causing problems on pachysandra that is weakened or under stress. Therefore, reduce any stresses where possible. Reduce stress due to overexposure to sun by always planting pachysandra in a shady area. Do not overcrowd new plantings, and thin existing beds to not only reduce stress from competition, but also to promote good air flow and rapid drying of plants. Dry plants are less likely to become infected than plants that stay wet for extended periods. To prevent water stress during the summer and reduce winter injury over the winter, established pachysandra should receive approximately one inch of water per week during the growing season up until the point where there is snowfall or the ground freezes. Newly planted pachysandra should receive approximately two inches of water per week. Minimize salt usage on sidewalks or driveways near pachysandra beds to limit salt injury. Control insect pests and avoid over-pruning of pachysandra to limit wounds that might serve as entry points for V. pachysandricola.

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

Verticillium Wilt of Trees and Shrubs

What is Verticillium wilt? Verticillium wilt is a typically fatal disease that affects a wide range of deciduous woody ornamentals in Wisconsin. Trees most commonly and severely affected are maple and ash. Other trees and shrubs such as barberry, catalpa, elm, lilac, linden, smoke-tree and redbud susceptible. In addition, many herbaceous ornamentals, as well as vegetable crops (see University of Wisconsin Garden Facts XHT1146 “Vertcillium Wilt of Vegetables”), can be affected by this disease.

Sudden yellowing, wilting and death of leaves and branches, particularly starting in one section of a tree or shrub, is a typical symptom of Verticillium wilt.
Sudden yellowing, wilting and death of leaves and branches, particularly starting in one section of a tree or shrub, is a typical symptom of Verticillium wilt.

What does Verticillium wilt look like? The first signs of Verticillium wilt that you may notice are individual branches that suddenly wilt and die. Affected branches may occur on one side of the tree or may be scattered throughout the tree. If you carefully peel away the bark of these branches, you may see brown or green streaking in the sapwood just under the bark. Streaking is common in trees such as maple or redbud, but often is not visible in ash and lilac.

Where does Verticillium wilt come from? Verticillium wilt is caused primarily by two fungi, Verticillium dahliae and Verticillium albo-atrum. These fungi are commonly found in Wisconsin soils and in roots, branches and leaves of infected plants. These fungi enter trees and shrubs through their roots and grow in the xylem (i.e., the water-conducting tissue) of plants where they lead to blockage of water movement. This lack of water movement is what eventually leads to wilting.

How do I save a tree or shrub with Verticillium wilt? Trees and shrubs infected with Verticillium cannot be cured and will likely eventually die. However, you can extend the life of your plants by making sure that you water and fertilize properly. Make sure established trees and shrubs receive approximately one inch of water per week. If rainfall is insufficient, use a drip or soaker hose to apply supplemental water. Fertilize trees as needed, but be sure to base any fertilization on a soil nutrient test. To prevent competition for water and nutrients, remove lawn grass within the drip line of your trees and shrubs (i.e., the edge of where the branches extend) and replace it with shredded hardwood, pine or cedar mulch. On heavy clay soils, use three inches of mulch. On other soils, use three to four inches of mulch. Be sure to keep mulch two inches from the main trunks and crowns of trees and shrubs. In addition, prune out dead branches as they occur. Dispose of these branches by burning (where allowed by local ordinance) or landfilling them. DO NOT bury or compost these branches. Be sure to clean your pruning tools between cuts by dipping them for at least 30 seconds in a 10% bleach solution or (preferably due its less corrosive properties) 70% alcohol. Rubbing alcohol and many spray disinfectants contain approximately this alcohol concentration. Decontaminating your tools will help prevent spread of Verticillium from branch to branch, or more importantly from tree to tree, as you prune.

Internal streaking in the sapwood of a branch is typical of Verticillium wilt.
Internal streaking in the sapwood of a branch is typical of Verticillium wilt.

How do I avoid problems with Verticillium wilt in the future? The best way to avoid Verticillium wilt is to plant trees and shrubs that are immune or resistant. Resistant deciduous trees and shrubs (as adapted from “Diseases of Trees and Shrubs, Second Edition” by W. Sinclair and H. H. Lyon (Comstock Publishing Associates, 2005)) include apple, aspen, azalea, beech, birch, butternut, crabapple, dogwood, flowering quince, ginkgo, hackberry, hawthorn, hickory, holly, honeylocust, katsura tree, mountain-ash, oak, pear, poplar, sweetgum, sycamore, walnut, and willow. Conifers (e.g., pines, spruces and firs) appear to be immune to the disease. Also, DO NOT use mulches that may have been produced from infected trees or that are of unknown composition. Finally, immediately collect and discard leaves that have fallen from symptomatic trees. Both mulch and leaves are potential sources of Verticillium.

For more information on Verticillium wilt: See UW-Extension Bulletin A2537, “Maple and Other Tree Disorder: Verticillium Wilt” (available at http://learningstore.uwex.edu); University of Wisconsin Garden Facts XHT1146, “Verticillium Wilt of Vegetables” (available at http://hort.uwex.edu); or contact your county Extension agent.

Thanks to Jean Ferdinandsen, Patti Nagai and Scott Reuss for reviewing this document.