Category Archives: Disease – Broad Leafed Woody Ornamental

Lichens

There are many types of lichens. Crustose lichens (left) are crust-like and adhere tightly to the surface upon which they grow. Foliose lichens (right) are leaf-like and composed of flat sheets of tissue that are not tightly bound.
There are many types of lichens. Crustose lichens (left) are crust-like and adhere tightly to the surface upon which they grow. Foliose lichens (right) are leaf-like and composed of flat sheets of tissue that are not tightly bound.

What are lichens? Lichens are organisms that arise from mutually beneficial interactions between certain fungi and algae. The fungi provide the physical structures of the lichens, as well as protection for the algae. The algae, in turn, produce food for the fungi via photosynthesis.

What do lichens look like? Lichens come in four basic growth forms. Crustose lichens are crust-like and adhere tightly to the surface upon which they grow. Foliose lichens are leaf-like and composed of flat sheets of tissue that are not tightly bound together. Squamulose lichens are composed of scale-like parts. Fruticose lichens are composed of free-standing branching tubes.

Where do lichens come from? Lichens are everywhere. There are an estimated 13,500 to 17,000 species of lichens, and lichens can be found growing in tropical, temperate and polar regions throughout the world. Lichens will grow on almost any surface that is stable and reasonably well-lit. In temperate regions, lichens can often be found growing on the bark of trees or old fence posts. Others lichens grow in less hospitable places, such as bare rock surfaces or old headstones in graveyards, where they aid in the breakdown of rocks and the formation of soil.

There are many types of lichens. Crustose lichens (left) are crust-like and adhere tightly to the surface upon which they grow. Foliose lichens (right) are leaf-like and composed of flat sheets of tissue that are not tightly bound.
There are many types of lichens. Crustose lichens (left) are crust-like and adhere tightly to the surface upon which they grow. Foliose lichens (right) are leaf-like and composed of flat sheets of tissue that are not tightly bound.

How do I save a tree with lichens? DO NOT PANIC! Lichens do not harm trees; they are not pathogens or parasites, and do not cause disease. Lichens are self-reliant, with the algal component of the lichen producing food for the organism via photosynthesis. Lichens absorb water and minerals from rainwater and the atmosphere, and because of this, they are extremely sensitive to air pollution. As a result, the presence or absence of certain lichen species can be used as an indicator of levels of atmospheric pollutants. Information on the abundance and species of lichens growing in an area can give a good indication of the local air quality.

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

Deciduous Tree Leaf Disease Quick Reference

Anthracnose for Quick Guide Anthracnose
Hosts:  Most trees, commonly ash, maple and oak
Pathogens:  Gloeosporium spp. as well as other fungi
Signs/Symptoms:  Blotchy dead areas on leaves
For more information see:  UW Plant Disease Facts D0002
Purple-bordered leaf spot Purple-Bordered Leaf Spot
Host:  Amur, Japanese, red, silver and sugar maple
Pathogen:  Phyllosticta minima
Signs/Symptoms:  Discrete, circular leaf spots with purple borders
For more information see:  UW Plant Disease Facts D0089
 Tubakia leaf spot Tubakia (Actinopelte) Leaf Spot
Hosts:  Oak
Pathogen:  Tubakia spp. (Actinopelte spp.)
Signs/Symptoms:  Discrete circular, or irregular blotchy dead areas on leaves
For more information see:  UW Plant Disease Facts D0118
 Scab Apple Scab
Hosts:  Apple, crabapple, pear, mountain-ash
Pathogen:  Venturia inaequalis, V. pirina
Signs/Symptoms:  Circular, black leaf spots with feathery edges; eventual leaf loss
For more information see:  UW Plant Disease Facts D0004
 Cedar-Apple Rust Gymnosporangium Rusts
Hosts:  Apple, crabapple, hawthorn
Pathogens:  Gymnosporangium spp.
Signs/Symptoms:  Bright yellow-orange, circular leaf spots
For more information see:  UW Plant Disease Facts D0058
 Powdery Mildew Powdery Mildew
Hosts:  Most deciduous trees
Pathogens:  Several genera of powdery mildew fungi
Signs/Symptoms:  Uniform/blotchy powdery white areas on upper and lower leaf surfaces
For more information see:  UW Plant Disease Facts D0087
 Downy Leaf Spot Downy Leaf Spot
Hosts:  Hickory, walnut
Pathogen:  Microstroma juglandis
Signs/Symptoms:  Discrete powdery white areas on lower leaf surfaces
 Clorosis Chlorosis
Hosts:  Oak, red maple
Cause:   Iron or manganese deficiency, often induced by high soil pH
Signs/Symptoms:  Yellow leaves with dark green veins
For more information see:  UW Plant Disease Facts D0030
 Scorch Scorch
Hosts:  Most deciduous trees
Cause:   Water stress induced by drought, high soil salt content, or other water-limiting factors
Signs/Symptoms:  Dead tissue on leaf margins
 Tatters Tatters
Hosts:  Most deciduous trees, but commonly oak
Cause:  Possible early season cold injury
Signs/Symptoms:  Lacy, tattered-looking leaves
For more information see:  UW Plant Disease Facts D0111

For more information on deciduous tree leaf diseases:  See https://pddc.wisc.edu/ or contact your County Extension agent.

Elderberry Rust

What is elderberry rust?  Elderberry rust is a visually striking fungal disease that affects stems, leaves and flowers of plants in the genus Sambucus (i.e., elderberries).  The disease also affects sedges (Carex spp.).  On elderberries grown as ornamentals, as well as on sedges, the disease is primarily a cosmetic problem.  However, if elderberries are grown for fruit, the disease can disrupt flower and fruit formation, thus reducing fruit yield.

An elderberry rust gall on elderberry (left) and leaf spots caused by elderberry rust on sedge (right). Photos courtesy of Jenell Bindl (left) and Michele Warmund, University of Missouri (right)
An elderberry rust gall on elderberry (left) and leaf spots caused by elderberry rust on sedge (right). Photos courtesy of Jenell Bindl (left) and Michele Warmund, University of Missouri (right)

What does elderberry rust look like?  Elderberry rust is most noticeable on elderberries where it causes growth distortions and swellings (i.e., galls) on leaves and stems.  Galls are often very large, bright yellow and powdery from spores produced by the causal fungus.  In extreme cases, galls can resemble banana slugs that have attached themselves to branches.  Infected flowers become thick, swollen and green-tinged rather than white.  Affected plant parts are covered with a network of small (approximately 1/16 inch in diameter) ring-like spots.  These spots are reproductive structures of the rust fungus and produce the powdery spores that coat the galls.

On sedges, elderberry rust causes brownish leaf spots, often with yellow halos.  The spots eventually erupt releasing powdery, rusty-orange spores.

Where does elderberry rust come from?  Elderberry rust is caused by the fungus Puccinia sambuci, also known as Puccinia bolleyana.  The fungus overwinters in sedge debris, and spores produced in this debris blow to elderberry plants in the spring, leading to infection and gall formation.  Spores produced in elderberry galls blow back to sedges, where infection of newly produced leaves (and other plant parts) occurs.  These infections lead to spotting and to the formation of a third type of spore that reinfects sedges causing additional spotting.  Late in the season a fourth type of spore is produced that serves as the overwintering phase of the fungus.  Infection of both elderberries and sedges is favored by wet weather.

How do I save plants with elderberry rust?  Elderberry rust is not a lethal disease on either elderberry or sedge.  When galls form on elderberry, simply prune these out.  This will make elderberry plants more aesthetically pleasing and limit spread of the fungus to sedges.  When pruning, cut branches four to six inches below each gall.  Between cuts, decontaminate pruning tools by treating them for at least 30 seconds with 70% alcohol (e.g., rubbing alcohol straight out of the bottle), a spray disinfectant containing 60-70% active ingredient, or a 10% bleach solution (i.e., one part of a disinfecting bleach and nine parts water).  If you decide to use bleach, be sure to rinse your tools thoroughly after you are done pruning and then oil them to prevent rusting, which can be caused by bleach use.  Dispose of galls by burning (where allowed by local ordinance) or burying them.

How do I avoid problems with elderberry rust in the future?  In landscape settings, the best way to avoid problems with elderberry rust is to remove any sedges that are growing near elderberry plants.  The farther the distance between elderberries and sedges, the less likely that elderberry rust will be an issue.  Also, increase airflow around elderberry plants by thinning them and removing surrounding plants.  Increased airflow will dry plants more rapidly and make the environment less favorable for infection.  DO NOT use a sprinkler to water plants, as that wets leaves and provides a better environment for infections to occur.  Instead, use a soaker or drip hose to apply supplemental water to the soil at the drip lines of the plants (i.e., the edges of where the branches extend).  While fungicides are available for rust control in commercial elderberry production, these products are not recommended for use in home garden settings.

For more information on elderberry rust:  Contact your county Extension agent.

Tobacco Mosaic

Tobacco mosaic causing a blotchy light and dark coloring (mosaic) of tobacco leaves.
Tobacco mosaic causing a blotchy light and dark coloring (mosaic) of tobacco leaves.

What is tobacco mosaic?  Tobacco mosaic is a common viral disease of worldwide distribution that affects over 200 species of herbaceous and, to a lesser extent, woody plants.  Common hosts include tobacco, solanaceous vegetables (e.g., pepper, tomato) and vining vegetables (e.g., cucumber, melon, squash), as well as a wide range of ornamentals (e.g., begonia, coleus, geranium, impatiens, million bells, petunia).  The disease has its biggest impact on vegetables, where it can reduce yield and affect quality to the point that commercial crops cannot be marketed.

What does tobacco mosaic look like?  Symptoms of tobacco mosaic vary in type and severity depending on the plant infected, plant age, the variant of the virus involved, and environmental conditions.  On leaves, typical symptoms include blotchy light and dark areas (called mosaic); cupping, curling, elongation (strapping), roughening, wrinkling and other growth distortions; and smaller than normal size.  Fruits may have a blotchy color, ripen unevenly, be malformed or have an off flavor.  Entire infected plants are often stunted.  Other viral diseases like cucumber mosaic (see UW Plant Disease Facts D0036, Cucumber Mosaic) can cause symptoms similar to tobacco mosaic.  Often, multiple viral diseases can simultaneously affect a single plant.  Certain herbicide exposures (see UW Plant Disease Facts D0060, Herbicide Damage), nutrient deficiencies or toxicities, high temperature and even insect feeding can also cause similar symptoms.  Proper diagnosis of tobacco mosaic requires lab testing.

Where does tobacco mosaic come from?  Tobacco mosaic is caused by Tobacco mosaic virus (TMV), the first virus ever identified.  Numerous variants (strains) of the virus have subsequently been described.  TMV survives in infected plants (including viable seeds), as well as in debris from these plants.  Plant-based products (most notoriously tobacco products) can harbor the virus.  TMV is very stable and can survive for long periods of time; there are reports of TMV surviving and remaining infectious after 50 years in storage at 40°F.  Because of its stability, TMV can survive on and be picked up from hands, clothing, gardening tools, work surfaces and any other object (e.g., door knobs) that gardeners may handle.

TMV is highly transmissible and is commonly spread by handling infected plants, then healthy plants.  Spread via gardening tools is also very common.  No specific insects spread TMV (the way that aphids spread Cucumber mosaic virus).  However, bees and chewing insects (e.g., grasshoppers) can transmit TMV through casual contact or their feeding as they move from plant to plant.

Leaf growth distortions caused by tobacco mosaic.
Leaf growth distortions caused by tobacco mosaic.

How do I save a plant with tobacco mosaic?  There is no cure for tobacco mosaic.  Once infected, plants remain infected for life, and typically the virus spreads throughout the plant from the point of infection.  Infected plants and any associated debris should be burned (where allowed by local ordinance) or double-bagged and disposed of in a landfill.  DO NOT compost plants with this disease.  Thoroughly decontaminate any items that have come into contact with infected plants or their debris by treating them for a minimum of one minute with:

  • 2.75 tablespoons Alconox® (a lab detergent) plus 2.5 tablespoons sodium lauryl sulfate (SLS), also known as sodium dodecyl sulfate (SDS), in one gallon of water, or
  • 14 dry ounces of trisodium phosphate in one gallon of water.

These ingredients can be ordered on the internet.  If you decide to use SLS (SDS), be sure to wear gloves, safety goggles and a dust mask, and mix the solution in a well-ventilated area as SLS (SDS) is a known skin and eye irritant.  Once treated, rinse items with sufficient water to remove any residues.  Also, thoroughly wash your hands with soap and water, and launder any clothing that you wore while disposing of infected plants and debris.

How do I avoid problems with tobacco mosaic virus in the future?  Inspect plants prior to purchase for any symptoms of tobacco mosaic, and DO NOT buy symptomatic plants.  Purchase seed from a reputable supplier that routinely inspects their seed-producing plants for symptoms of viral (and other) diseases.  If you use tobacco products, DO NOT use them around plants.  Also, wash your hands thoroughly with soap and water prior to handling plants, and consider wearing freshly laundered clothing when gardening.  Finally, decontaminate (as described above) any items that might harbor TMV to help prevent spread.  Even if you do not use tobacco products, routine handwashing and decontamination of gardening tools and other items can help prevent tobacco mosaic from being a problem.

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

Sudden Oak Death – Pest Alert

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

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

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

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

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

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

How do I save a plant with sudden oak death?  If you believe you have seen a plant that has SOD, please IMMEDIATELY submit a sample to the UW-Madison Plant Disease Diagnostics Clinic (PDDC).  See below for address details.  Double bag suspect plant tissue in sealable plastic bags and place the bagged specimen in a box or envelope for shipping.  Include contact information (complete address, phone number, email address) in a separate sealable plastic bag with the sample.  Tape over all of the edges of boxes and envelopes used for shipping to keep everything sealed inside.  Write on the box or envelope that the box or envelope contains a suspect SOD sample.  If you have questions about collecting or submitting a sample, contact PDDC staff at (608) 262-2863 or at 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 treating them for at least 30 seconds in 10% bleach.  Thoroughly rinse and oil tools after decontamination to prevent rusting.  If a plant tests positive for Phytophthora ramorum, it will be removed and destroyed to help prevent further spread of the pathogen.

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

For more information or help in diagnosing sudden oak death:  Contact Brian Hudelson, UW-Madison Plant Disease Diagnostic Clinic, Department of Plant Pathology, University of Wisconsin-Madison, 1630 Linden Drive, Madison, WI  53706-1598, phone: (608) 262-2863, email: 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 treating them for at least 30 seconds in 70% alcohol (e.g., rubbing alcohol, certain spray disinfectants), a commercial disinfectant or 10% bleach.  If you use bleach, be sure to thoroughly rinse and oil your tools after pruning to prevent rusting.

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 by local ordinance) 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.

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

Boxwood Blight – Pest Alert

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

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

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

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

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

How can I avoid problems with boxwood blight in the future?  Consider using shrubs other than boxwood in your landscape.  If you decide to use boxwood, choose boxwood blight resistant varieties where possible.  In Wisconsin, hybrid 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 development.  Avoid watering plants with sprinklers or overhead with hoses; instead use a soaker or drip hose.  This will limit splash of spores from plant to plant and also promote a drier environment that is less favorable for disease.

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)

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 treating them for at least 30 seconds in 70% alcohol (e.g., rubbing alcohol or certain spray disinfectants) or 10% bleach.  Decontaminating tools will prevent movement of rust fungi from branch to branch or from plant to plant during pruning.  If you use bleach, be sure to thoroughly rinse and oil your tools after pruning to prevent rusting.

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 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 rust fungi.

For more information on Gymnosporangium rusts:  See UW Bulletin A2598, Apple, Crabapple, Hawthorn and Juniper Disorder:  Cedar-Rust Complex (available at https://learningstore.extension.wisc.edu) 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 United States 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 Athelia rolfsii (formerly 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, A. rolfsii most likely enters gardens on infected nursery stock or infested mulch.  Freezing temperatures will kill A. rolfsii mycelia, but sclerotia can survive temperatures as low as approximately 14ºF.

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)

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.  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 A. 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 product(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 A. 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 A. rolfsiiA. 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).
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 1/8 inch) yellowish spots that form on infected leaves.  These spots may remain relatively small, or may enlarge over the growing season to approximately 3/4 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 1/8 inch in diameter), and clustered together.  On silver maple, the black structures are often single, large (approximately 3/4 inch 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, Rhytisma americanum, Rhytisma acerinum, and (less commonly) Rhytisma punctatum 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 or shrub with tar spot?  DO NOT panic.  For most maples and other susceptible trees and shrubs, tar spot is not a serious disease.  It 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, alone or in combination with mancozeb, are labeled for tar spot control in Wisconsin.  However, fungicide treatments for this disease are rarely, if ever, warranted.  Consult with your county Extension horticulture/agriculture agent to determine if your tree warrants preventative treatments.  If warranted, three fungicide applications will be needed:  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 product in the safest and most effective manner possible.

For more information on tar spot of trees and shrubs:  Contact your county Extension agent.