Category Archives: Disease – Broad Leafed Woody Ornamental

Thousand Cankers Disease – Pest Alert

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

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

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

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

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

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

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

Tatters

Leaves with tatters appear shredded, or as if damaged by leaf-feeding insects.
Leaves with tatters appear shredded, or as if damaged by leaf-feeding insects.

What is tatters?  Tatters is a leaf disorder affecting primarily members of the white oak group of oaks (i.e., oaks with leaves with rounded lobes) including bur, white and swamp white oaks.  Members of the red oak group of oaks (i.e., oaks with leaves with pointed lobes), including red, black, pin and shingle oaks, as well as other types of trees, rarely display the disorder.  Tatters was first documented in Iowa, Indiana and Ohio in the 1980’s, and since then has been documented throughout much of the Midwest.

What does tatters look like?  Trees with tatters have leaves that are lacy and shredded.  Some leaves may appear as though the tissue between veins has been neatly ripped out, while other leaves have an irregular pattern of damage.  The amount of damage may vary from leaf to leaf and branch to branch.  Adjacent oak trees may show different amounts of damage due to genetic variability, variation in environmental conditions, or other external factors.  Tatters is commonly confused with herbicide damage (see UW Plant Disease Facts D0060, Herbicide Damage), or damage by leaf-feeding insects.

Where does tatters come from?  The cause of tatters has not been precisely determined.  Tatters is thought to be a physiological disorder caused by damage to leaf tissue (e.g., cold injury) in the bud-stage or during the opening of buds in the spring.

How do I save a tree with tatters?  DO NOT panic.  Trees affected with tatters often produce replacement leaves within two to three weeks after tattered leaves appear.  However, producing new leaves weakens trees and may make them more susceptible to other diseases and drought stress.  If your trees suffer from tatters, make sure they receive sufficient water (approximately one inch per week for established trees).  If rainfall is insufficient, use a drip hose or soaker hose to apply supplemental water around the drip line of the tree (i.e., the edge of where the branches extend).  To prevent competition for water and nutrients, remove grass within the drip line of your trees and replace it with shredded hardwood, pine or cedar mulch.  On heavy, clay soils, use one to two inches of mulch.  On lighter, sandy soils, use three to four inches of mulch.  Be sure to keep mulch four inches from the tree trunks.  Fertilize trees only based on a soil nutrient test.

How do I avoid problems with tatters in the future?  There is no known method for preventing tatters.  However, the occurrence of tatters one year does not guarantee that the same trees will suffer from tatters in subsequent years.

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

Powdery Mildew – Deciduous Woody Ornamentals

Many woody plants such as rose and lilac are susceptible to powdery mildew.
Many woody plants such as rose and lilac are susceptible to powdery mildew.

What is powdery mildew?  Powdery mildews are diseases that occur on the above-ground parts (especially the leaves) of many deciduous trees and shrubs, as well as herbaceous ornamental plants, indoor houseplants, and many agricultural crops.  Conifers are not affected by these diseases.

What does powdery mildew look like?  The name of these diseases is descriptive.  The upper and lower surfaces of leaves, as well as stems of infected plants, have a white, powdery appearance.  They look as though someone has sprinkled them with talcum powder or powdered sugar.

Where does powdery mildew come from?  Powdery mildews are caused by many closely related fungi that survive in plant debris or on infected plants.  These fungi are fairly host specific.  The powdery mildew fungus that infects one type of plant (e.g., lilac) is not the same powdery mildew fungus that infects another (e.g., phlox).  However, if you see powdery mildew on one plant, then weather conditions (high humidity) are favorable for development of the disease on a wide range of plants.

How do I save a plant with powdery mildew?  DO NOT panic!  For many trees and shrubs (e.g., lilac), powdery mildews are cosmetic, non-lethal disease.  For other plants (e.g., rose, ninebark) powdery mildews can cause severe leaf loss and even branch tip dieback.

Powdery mildew on ninebark can be so severe that it causes branch tip dieback.
Powdery mildew on ninebark can be so severe that it causes branch tip dieback.

When a highly valued plant has had severe leaf loss due to powdery mildew for several years, you may want to consider using a fungicide for control.  Fungicides containing chlorothalonil, copper, mancozeb, myclobutanil, triadimefon, sulfur or thiophanate-methyl are registered for powdery mildew control.  A combination of baking soda (1½ tablespoons) and a light weight (i.e., paraffin-based) horticultural oil (3 tablespoons) in water (1 gallon) has also been shown to be effective.  Most products should be applied every seven to 14 days from bud break until humid weather subsides.  DO NOT use myclobutanil, triadimefon, 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 listed active ingredients to help minimize problems with fungicide-resistant strains of powdery mildew fungi.  DO NOT alternate myclobutanil and triadimefon as these active ingredients are chemically related.  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.  Also consider pretesting any product that you decide to use on a small number of leaves before treating an entire tree or shrub to make sure there are no toxic effects, particularly when treating during warmer weather.

How do I avoid problems with powdery mildew in the future?  Consider buying plant varieties that are powdery mildew resistant.  This will not guarantee that your plants will be powdery mildew free every year, but should result in less severe disease when it occurs.  Reduce the humidity around your plants by spacing them further apart to increase air flow.  In established trees and shrubs, thin canopies to increase air flow.  Be sure not to over-water as this can lead to higher air humidity as well.  Finally, at the end of the growing season, remove and destroy any infected plant debris as this can serve as a source of spores for the next growing season.  You can burn (where allowed by local ordinance), bury or hot compost this material.

For more information on powdery mildew:  Contact your county Extension agent.

Rose Rust

What is rose rust?  Rose rust is a common fungal disease found in much of North America (including the continental United States) and Europe.  Rose rust affects many varieties of rose, though some varieties (e.g., hybrids) are more prone to the disease.  Rose rust has been a perennial problem along the Pacific Coast of the United States where mild temperatures and high moisture are favorable for rust development.  In the Midwest, extremes in winter and summer temperatures have historically tended to be less favorable for the disease.  However, recent climate changes in Wisconsin have led to rose rust becoming more commonplace in the state.

Yellow spots on upper leaf surfaces with corresponding powdery, orange to black spots on lower leaf surfaces are typical of rose rust.
Yellow spots on upper leaf surfaces with corresponding powdery, orange to black spots on lower leaf surfaces are typical of rose rust.

What does rose rust look like?  Rose rust often first appears on lower leaves, but eventually an entire plant can be affected.  Typical symptoms include general yellowing of leaves followed by eventual leaf death.  Affected rose stems (i.e., canes) can become curled and distorted.  As the disease progresses, powdery orange or black, circular spots (called pustules) containing spores of the fungus that causes the disease form on the undersides of leaves.  Corresponding yellow spots are visible on upper leaf surfaces above the pustules.  Pustules may also form on stems and green flower parts (sepals).  Rose rust usually develops in the spring and fall (when favorable mild temperatures and wet conditions are more common), but the disease can affect roses during the summer months as well.

Where does rose rust come from?  Rose rust is caused by several species of fungi in the genus Phragmidium.  These fungi specifically infect roses.  Rose rust is often introduced into a garden on infected shrubs purchased from a nursery or other rose supplier.  Once introduced into a garden, rose rust fungi can overwinter in rose leaf debris, as well as on infected rose canes.  In the spring, spores produced in debris and on canes can blow to newly emerging rose foliage, leading to new infections.

How do I save a plant with rose rust?  Control of rose rust is difficult once symptoms develop.  Prune out affected canes and remove leaves as symptoms develop to prevent the spread of rust fungi to other rose shrubs.  Destroy these materials by burning (where allowed by local ordinances) or burying them.  In the fall, remove and destroy any remaining dead leaves and other rose debris to eliminate places where rose rust fungi can overwinter.  If you notice a rust problem very early (before there are many symptoms), fungicide treatments may be useful for managing the disease; however, most fungicides work best when applied before any symptoms appear.  If you decide to use fungicides for rust control, select products that are labeled for use on roses and that contains the active ingredients chlorothalonil, mancozeb, myclobutanil, propiconazole, sulfur or triforine.  Treat every seven to 10 days, and DO NOT use the same active ingredient for all treatments, particularly if you decide to use myclobutanil, propiconazole or triforine.  Instead, alternate use of the two active ingredients listed above to help minimize potential problems with fungicide-resistant strains of rose rust fungi.  DO NOT alternate myclobutanil, propiconazole or triforine, as these active ingredients are chemically related.  Be sure to read and follow all label instructions of the fungicides that you select to ensure that you use these products in the safest and most effective manner possible.

How do I avoid problems with rose rust in the future?  Whenever possible, plant rose varieties that are less susceptible to rose rust (i.e., avoid hybrid varieties).  Always inspect new rose shrubs for rose rust (and other diseases) prior to purchase.  DO NOT bring diseased shrubs into your garden.  Plant rose shrubs far enough apart so that their foliage does not overlap, and thin your roses on a regular basis.  Proper planting and pruning promote good air circulation that will facilitate rapid drying of leaves and canes, thus making the environment less favorable for rust development.  Avoid working with your roses when they are wet as you are more likely to spread rust spores under these conditions.  Fertilize and water roses appropriately.  Well-cared-for plants tend to be less susceptible to disease.  When watering, apply water at the base of your shrubs (e.g., with a soaker or drip hose) rather than over the leaves (e.g., with a sprinkler).  Watering with a sprinkler tends to spread rust spores and wets leaves and canes, thus providing a more favorable environment for rust infections to occur.

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

Root and Crown Rots

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

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

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

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

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

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

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

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

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

Purple-Bordered Leaf Spot

What is purple-bordered leaf spot?  Purple-bordered leaf spot (also called eye spot or Phyllosticta leaf spot) is a common, but primarily cosmetic disease that affects maples (in particular Amur, Japanese, red, silver and sugar maple).  Phyllosticta leaf spot is similar in many ways to other foliar diseases of maple such as anthracnose (see UW Plant Disease Facts D0002, Anthracnose) and tar spot (see UW Plant Disease Facts D0110, Tar Spot of Trees and Shrubs).

Purple-bordered leaf spot (l).  Within the leaf spots (r), small, black, pimple-like fungal fruiting bodies form (red arrows).
Purple-bordered leaf spot (l). Within the leaf spots (r), small, black, pimple-like fungal fruiting bodies form (red arrows).

What does purple-bordered leaf spot look like?  Leaves with purple-bordered leaf spot have roughly circular dead areas (typically less than ¼ inch in diameter).  Spots have tan to brown centers and distinct purple, red, or brown margins.  Tiny, black, pimple-like reproductive structures (called pycnidia) often form within the spots, and are diagnostic.  As spots mature, the centers may fall out, leaving roughly circular holes.

Where does purple-bordered leaf spot come from?  Purple-bordered leaf spot is caused by the fungus Phyllosticta minima, which overwinters in leaf litter.  In the spring, rain and wind move spores of the fungus from the leaf litter to newly developing maple leaves, where infections occur.  Spores produced on infected leaves can lead to additional infections within the tree canopy throughout the growing season.

How do I save a tree with purple-bordered leaf spot?  DO NOT PANIC.  Although purple-bordered leaf spot may look unsightly, the disease is usually only a cosmetic problem, rarely causing significant damage to mature and vigorously-growing trees.  Occasionally, purple-bordered leaf spot may defoliate trees early in the growing season, but these trees are typically able to produce new leaves within a few weeks.  Defoliated trees should be watered and properly fertilized.  Established trees require approximately one inch of water per week; newly transplanted trees (i.e., trees planted within approximately the past three years) require approximately two inches of water per week.  If there is insufficient rain, apply water at the drip lines of trees (i.e., the edges of where the branches extend) using a drip or soaker hose.  Only fertilize trees based on a soil nutrient test.

How do I avoid problems with purple-bordered leaf spot in the future?  If available, select maple varieties that are resistant to purple-bordered leaf spot.  Compost, bury or burn (where allowed by local ordinance) leaf litter from infected trees in the fall or in the spring before trees releaf.  Newly planted maples and established maples that have been severely affected by purple-bordered leaf spot for several years may benefit from treatments with a fungicide containing chlorothalonil, copper, mancozeb, neem oil, sulfur or thiophanate-methyl.  Three treatments may be needed for adequate control:  one at bud break, one when leaves are half expanded, and one when leaves are fully expanded.  DO NOT use the same active ingredient for all treatments (particularly if you are using thiophanate-methyl).  Instead, alternate the use of at least two active ingredients.  This strategy will help minimize problems with fungicide-resistant strains of Phyllosticta minima.  Be sure to read and follow all label instructions of the fungicide(s) that you select to ensure that you use the product(s) in the safest and most effective manner possible.

For more information on purple-bordered leaf spot:  Contact your county UW-Extension agent.

Plum Pox – Pest Alert

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

Plum pox symptoms on immature plum fruits (left), and a plum leaf (right).  (Photographs courtesy of R. Scorza and obtained from West Virginia University at http://www.caf.wvu.edu/kearneysville/wvufarm1.html)
Plum pox symptoms on immature plum fruits (left), and a plum leaf (right). (Photographs courtesy of R. Scorza and obtained from West Virginia University at http://www.caf.wvu.edu/kearneysville/wvufarm1.html)

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

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

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

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

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

Oak Wilt

What is oak wilt?  Oak wilt is a lethal fungal disease that affects virtually all species of oaks.  Oaks in the red oak group (oaks with pointed leaf lobes) such as red, scarlet, black and Northern pin oak are most susceptible.  Oaks in the white oak group (those with rounded leaf lobes) such as white, bur, post, and swamp white oak are less susceptible.

What does oak wilt look like?  Initially, single branches on infected trees wilt and die.  Leaves on these branches often bronze, or turn tan or dull green, starting at the tips or outer margins.  Leaves may also droop, curl, or fall from the tree.  Infected trees eventually die.  Oak wilt can kill oaks in the red oak group in less than one month.  Oaks in the white oak group usually have less severe symptoms and rarely die in a single season.

Marginal leaf bronzing or tanning is often an early symptom of oak wilt.
Marginal leaf bronzing or tanning is often an early symptom of oak wilt.

Where does oak wilt come from?  Oak wilt is caused by the fungus Bretziella fagacearum which survives in infected living oaks and in oaks recently killed by the disease.  Sap beetles are attracted to mats of the oak wilt fungus in infected trees, pick up spores of the fungus on their bodies, then carry spores to healthy trees.  These beetles are attracted to trees that have been recently wounded by wind or storm damage, or by pruning.  Natural grafts between roots of oak trees growing near each other can also serve as a means by which the fungus moves from tree to tree.

How do I save a tree with oak wilt?  Removing infected oaks is often the best way to manage oak wilt.  Before removing trees, be sure to disrupt root grafts between infected and other nearby oaks.  This will help limit tree to tree movement of the fungus during the removal process.  Burn or bury wood from diseased oaks, if possible.  If you decide to keep the wood, remove the bark, pile it in one place and cover it with a heavy tarp, burying the tarp edges with soil until you use it.  This will limit sap beetle access to the pile and reduce the risk that these insects will acquire the oak wilt fungus.  Propiconazole injections can be used for oak wilt management, but these treatments work best when used before, rather than after, oak trees are infected.

How do I avoid problems with oak wilt in the future?  Prune oak trees only during the dormant season when sap beetles are not active.  If you must prune during the growing season (e.g., due to storm damage) IMMEDIATELY cover wounds with paint.  Sap beetles can visit wounded oaks within 10 minutes of wound formation.  Monitor oaks for oak wilt and remove infected trees promptly.

For more information on oak wilt:  See UW Bulletin G3590, Oak Wilt Management – What are the Options (available at https://learningstore.extension.wisc.edu/), or contact your county Extension agent.

Nectria Canker

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

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

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

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

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

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

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

Herbicide Damage

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

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

What does herbicide damage look like?  Symptoms of herbicide damage vary depending upon the plant affected and the herbicide used.  Common symptoms include stems that are flattened, or that twist or corkscrew.  Leaves may have abnormal shapes, sizes or textures.  In addition, leaves or leaf veins may yellow or redden.  In severe cases, plants may brown and die.  Some plants, such as tomatoes and grapes, are particularly susceptible to herbicide damage and can be used as indicators of unwanted herbicide exposure.

How does herbicide damage occur?  Herbicide damage results when an herbicide is misapplied.  Herbicides for control of broadleaf weeds are occasionally applied with fertilizers as part of a lawn care program.  If these products are applied too close to ornamentals or vegetables, or are applied when there is too much wind, then the herbicide can drift (move) from the area of application into a non-treated area.  Often, drifting herbicides are difficult to detect by eye because they are extremely fine mists.  They can better be detected by smell.  Some herbicides readily produce vapors that can begin to drift several hours after application.

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

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

For more information on herbicide damage:  See UW Bulletin A3286, Plant Injury Due to Turfgrass Broadleaf Weed Herbicides (available at https://learningstore.extension.wisc.edu/), or contact your county Extension agent.