Category Archives: Disease – Broad Leafed Woody Ornamental

Chlorosis

What is chlorosis?  Chlorosis is a common nutritional disorder of many woody ornamentals in Wisconsin, particularly in the southern and eastern parts of the state.  Pin oaks are most commonly affected by chlorosis, although many other trees and shrubs (e.g., white oaks, red maples, white pines and rhododendrons) are also very susceptible.

Yellowing of foliage characteristic of chlorosis.
Yellowing of foliage characteristic of chlorosis.

What does chlorosis look like?  Symptoms of chlorosis are easy to distinguish from those of other diseases.  Affected leaves turn yellow, except for the veins, which remain green.  In severe cases, foliage may turn brown and die.  Symptoms can occur on isolated branches, or over an entire tree.

What causes chlorosis?  Chlorosis occurs when a tree or shrub is lacking certain micronutrients, in many cases iron or manganese.  Lack of micronutrients in a tree may reflect a lack of these nutrients in the soil due to poor fertility.  Often however, there are sufficient micronutrients, but they cannot be absorbed by a plant’s roots.  Poor absorption of micronutrients is common in Wisconsin because of the high pH (alkalinity) of many soils.

How do I save a tree or shrub with chlorosis?  Chlorosis is rarely fatal and can be treated.  For treatments to be effective, you must determine the exact cause of the chlorosis.  Have the soil around an affected plant tested for micronutrients and for pH prior to applying any treatment.  If the soil test indicates a lack of specific micronutrients, fertilize with these micronutrients.  For example, chelated iron compounds can be used to increase the amount of iron in soil.  If the soil test indicates a high soil pH, lower the pH by applying sulfur or ammonium sulfate.  See University of Wisconsin Garden Facts XHT1151, Reducing Soil pH, for details.  Contact your county Extension agent for information on soil testing and for tips on a treatment once you have determined the specific cause of your chlorosis problem.

How do I avoid problems with chlorosis in the future?  Plant trees and shrubs that are less susceptible to chlorosis.  Also, make sure your trees and shrubs receive sufficient water (approximately one inch per week), as this will help plants with micronutrient uptake.  During dry periods, use a drip hose or soaker hose to apply supplemental water.  Remove turf from around the bases of trees and shrubs out to at least the drip lines, and apply shredded hardwood, pine or cedar mulch in these areas to help maintain soil moisture.  On heavy clay soils, use one to two inches of mulch.  On other soils, use three to four inches of mulch.  Be sure to keep mulch four inches from the trunks of trees.  If you decide to plant susceptible trees or shrubs, watch them closely for the yellowing characteristic of chlorosis, and apply corrective treatments as soon as symptoms appear.  Treatments should always be based on the results of soil micronutrient and pH tests.

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

Butternut Canker

What is butternut canker?  Butternut canker is a fungal disease of Juglans cinerea, the butternut tree (also known as white walnut or oilnut).  The disease is thought to have originated outside of the U.S.  The first documented case of butternut canker in Wisconsin occurred in 1967.  Butternut canker affects butternuts throughout their natural range (much of the eastern US) and has killed up to 80% of the butternut trees in some states.  Although butternut canker can occur on saplings of black walnut (Juglans nigra), the effects of the disease on this species are minimal.

Multiple butternut cankers on a butternut tree trunk can girdle and kill the tree.
Multiple butternut cankers on a butternut tree trunk can girdle and kill the tree.

What does butternut canker look like?  Infected areas (called cankers) tend to be most noticeable on large branches or tree trunks, where long fissures in the bark form.  Several cankers may grow together leading to bark that is distorted and has a mangled appearance.  Smaller cankers also form around leaf or branch scars.  These cankers are sunken and black, with white margins and folds of bark around the edge.  Cankers girdle branches and trunks, cutting off nutrients and water, leading to progressive dieback above the canker.  Trunk cankers will eventually kill trees.  A single trunk canker can kill a young sapling, while several cankers combined may be needed to kill an older tree.

Where does butternut canker come from?  Butternut canker is caused by the fungus Ophiognomonia clavigignenti-juglandacearum.  Spores of the fungus are produced in cankers throughout the growing season and are spread from infected to healthy trees through rain-splash, insects, and wind.  Wounds, as well as leaf and branch scars, often serve as points of entry for the fungus.  Once a tree is infected, additional infections can occur as spores are washed by rain down to the main trunk of the tree.

How do I save a tree with butternut canker?  There is no cure for butternut canker.  Trees with trunk cankers will most likely die.  If branch cankers are found early, removal of the affected branches can prevent the spread of the disease to other parts of the tree.  Prune out smaller branch cankers by cutting six to eight inches below the canker.  Disinfect pruning tools after each cut by treating them for at least 30 seconds with a 10% bleach solution or preferably (because of its less corrosive properties) a 70% alcohol solution (e.g., rubbing alcohol or certain spray disinfectants).  If you use bleach, be sure to thoroughly rinse and oil tools after pruning to prevent rusting.

How do I avoid problems with butternut canker in the future?  There is little that you can do to prevent butternut canker.  However, vigorously growing butternut trees are better able to slow the development of butternut canker, so make sure that trees are watered and fertilized properly.  Established trees require approximately one inch of water per week.  If natural rains do not provide this amount of water, consider applying supplemental water near the drip line of the tree using a drip or soaker hose.  Fertilization of trees should be based on a soil nutrient test.

If you live in a region where butternut canker is not currently present, or where there are few butternut trees, your butternut tree may be free of cankers indefinitely.  If you live in a region where butternut canker is prevalent, watch for trees that are not affected by the disease.  These trees may have inherent resistance.  Consider collecting seed from such trees and growing saplings from the seed.  At least some of these saplings will likely also have resistance to the disease.

For more information on butternut canker:  Contact your county Extension agent.

Black Walnut Toxicity

What is black walnut toxicity?  Black walnut trees (Juglans nigra) produce a toxic substance (called juglone) that prevents many plants from growing under or near them.  Related trees like butternut (J. cinerea) and shagbark hickory (Carya ovata) also produce juglone, but in lower concentrations than black walnut.  Juglone occurs in all parts of black walnut trees, but especially in buds, nut hulls and roots.  The toxic effects of a mature black walnut tree can extend 50 to 80 feet from the trunk of the tree, with the greatest toxicity occurring within the tree’s dripline.  In this area, plants susceptible to juglone may wilt or die; plants tolerant to juglone will grow normally.  Vegetables such as tomato, potato, eggplant and pepper, and ornamentals such as lilac, peony, rhododendron and azalea are particularly sensitive to juglone.

Wilting of eggplant due to black walnut toxicity.
Wilting of eggplant due to black walnut toxicity.

What do the effects of black walnut toxicity look like?  Plants sensitive to juglone may be stunted, have yellow or brown, twisted leaves, exhibit wilting of some or all plant parts, and die over time.  Often, the vascular (i.e., water-conducting) tissue of affected plants will be discolored.  Symptoms may occur rapidly, even within a few days after sensitive species are transplanted into a walnut tree’s root zone.  Alternatively, some plants may survive for years near a young walnut tree but then wilt and die as the tree increases in size.  Black walnut toxicity can be confused with wilts caused by bacterial and fungal pathogens (e.g., see UW Plant Disease Facts D0121, Verticillium Wilt of Trees and Shrubs, and D0122, Verticillium Wilt of Vegetables), herbicide injury (see UW Plant Disease Facts D0060, Herbicide Damage), or drought.

How do I avoid problems with black walnut toxicity?  There is no cure for a plant affected by walnut toxicity.  Removing a walnut tree may not be practical, as the tree could be the focal point in a landscape.  In addition, even if a walnut tree is removed, juglones will not immediately be eliminated, because it is next to impossible to remove all root pieces from the soil and remaining pieces may continue to exude toxins for several years as they decay.

When establishing a garden around a walnut tree, try to plant species that are tolerant to juglone (see table on the reverse side).  If you are growing sensitive species near a walnut tree, transplant them elsewhere in your garden.  If you must grow sensitive plants near a black walnut, keep beds free of walnut leaves and hulls, and remove walnut seedlings as they appear.  Grow shallow rooted woody and herbaceous plants, and improve drainage to help diminish the effects of juglone.  Alternatively, consider building raised beds with wood, stone, or concrete barriers that limit root growth through and under the beds.

When disposing of bark and wood from a walnut tree, do not use these materials for mulch.

The information in the following table is intended to provide guidance in selecting plants to grow near walnut trees.  Inclusion of plants in this table is based on observation, not on formal testing.  In addition, the plant lists in this table are by no means exhaustive.  Oftentimes the juglone sensitivity or tolerance of specific plants has never been observed or documented.  Finally, sources often disagree on whether particular plants (e.g., columbine, lily, narcissus, tulip) are juglone sensitive or tolerant.  Some varieties may be susceptible while others may be tolerant.  Most plant species with conflicting information regarding their sensitivity or tolerance to juglone have not been included in the table.

  Sensitive to Juglone Tolerant of Juglone
Vegetables asparagus, cabbage, eggplant, pepper, potato, rhubarb, tomato beans, beet, carrot, corn, melon, onion, parsnip, squash
Flowers autumn crocus, chrysanthemum, forget-me-not, petunia, peonies aster, astilbe, bee balm, begonia, black-eyed Susan, bluebell, calendula, crocus, daylily, ferns, grape hyacinth, some hosta varieties, hollyhock, impatiens, Jack-in-the-pulpit, Jacob’s ladder, marigold, morning glory, pansy, phlox, Siberian iris, squill, sweet woodruff, trillium, zinnia
Trees alder, apple and crabapple, basswood, pine, spruce, silver maple, white birch black locust, catalpa, Eastern redbud, hackberry, Canadian hemlock, hickory, most maples, oaks, pagoda dogwood, poplar, red cedar
Shrubs and Vines azalea, blackberry (and most berries other than black raspberry), cotoneaster, hydrangea, lilac, mountain laurel, potentilla, privet, rhododendron, yew arborvitae, bittersweet, black raspberry, clematis, currant, forsythia, euonymus, greenbrier, most honeysuckle, pachysandra, rose-of-Sharon, sumac, most viburnum, Virginia creeper, wild grape, wild rose, willow, witch hazel
Field Crops and Grasses alfalfa, tobacco fescue, Kentucky bluegrass, orchard grass, soybean, timothy, wheat, white clover

For more information on black walnut toxicity:  See UW Bulletin A3182, Walnut and Butternut Toxicity (available at https://learningstore.extension.wisc.edu/), or contact your County Extension agent.

Black Spot

What is black spot?  Black spot is a common and potentially serious leaf disease affecting many types of roses.  Black spot is found wherever roses are grown.

Typical spotting and yellowing of rose leaves due to black spot.
Typical spotting and yellowing of rose leaves due to black spot.

What does black spot look like?  Black spot lesions (i.e., infected areas) are roughly circular and may be up to ½ inch in diameter.  Lesions often have feathery margins and are dark brown to black in color.  Black spot initially appears during periods of wet weather, particularly when rose leaves are first emerging.  The disease starts on lower leaves but will spread to the entire plant.  Severe black spot leads to yellowing of leaves and defoliation.  Black spot can also develop on one-year old canes, leading to raised, purplish-red blotches that blacken and blister.

Where does black spot come from?  Black spot is caused by the fungus Diplocarpon rosae, which survives in rose leaf litter and in infected rose canes.  Spores of the fungus are easily spread to emerging leaves by wind or splashing rain.

How do I save a rose with black spot?  If your rose has little or no defoliation, and the weather is dry, then no treatments are necessary.  If your rose has a history of severe black spot, and the weather for the growing season is predicted to be wet, consider applying preventative fungicide treatments.  Fungicides containing chlorothalonil, copper sulfate, and myclobutanil can be highly effective in controlling black spot.  Neem oil is another organic product (in addition to copper sulfate) that has been shown to provide some black spot control.  Treat every seven to 14 days from bud break until wet weather subsides.  DO NOT use the same active ingredient for all treatments.  Instead, alternate the use of at least two active ingredients (particularly if you decide to use myclobutanil) to help minimize problems with fungicide-resistant strains of the black spot pathogen.  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 do I prevent problems with black spot in the future?  Select rose varieties that are hardy in your area and that have a high level of black spot resistance.  The hybrid shrub rose Rosa ‘Meipotal’ (Carefree Delight®), rugosa rose varieties such as ‘Blanc Double De Coubert’ and ‘Fru Dagmar Hastrup’, climbing roses such as Rosa × Kordesii ‘William Baffin’ and Rosa × Kordesii ‘Henry Kelsey’, and roses in the Knockout® series all have excellent resistance to black spot.  Plant roses in a high light environment, and space shrubs far apart.  This will promote good airflow and quick drying of foliage.  Promptly remove diseased leaf litter.  Prune diseased branches six to eight inches below any obvious infections.  Prune only in dry weather.  Disinfest pruning tools between cuts by treating them for at least 30 seconds with 70% alcohol (preferable for metal tools) or a 10% bleach solution.  Rubbing alcohol and spray disinfectants often contain at approximately 70% alcohol.  If you use bleach, be sure to thoroughly rinse tools and oil them after pruning to prevent rusting.  Water roses at the base using a soaker or drip hose to minimize wetting of leaves and reduce movement of spores of the black spot pathogen.  DO NOT overhead water (e.g., with a sprinkler).

For more information on black spot of rose:  Contact your county Extension agent.

Black Knot

A typical older black knot gall with colonization by whitish secondary fungi. (Photo courtesy of Patricia McManus)
A typical older black knot gall with colonization by whitish secondary fungi. (Photo courtesy of Patricia McManus)

What is black knot?  Black knot is a disfiguring and potentially lethal disease of trees and shrubs in the genus Prunus.  This genus includes stone fruits such as wild, fruit-bearing and ornamental plums and cherries.

What does black knot look like?  During the first year of infection, black knot-infected trees develop greenish-brown to brown swellings on affected branches and trunks.  During the second year, these swellings enlarge into the ugly, black, erupting tumors (galls) characteristic of the disease.  These galls resemble animal feces attached to branches (affectionately referred to as “poop-on-a-stick”).  Older (greater than two years old) gall tissue often dies and then is colonized by fungi that give the gall a whitish or pinkish color.  Severe black knot infections may cause general tree decline or death if galls girdle large limbs, or tree trunks.

Where does black knot come from?  Black knot is caused by the fungus Apiosporina morbosa, which survives in black knot galls on infected Prunus trees.  Spores of the fungus are released from these galls and infect new branches in late spring or early summer during periods of wet weather and mild temperatures (55°F to 75°F).

How do I save a tree with black knot?  To manage existing black knot galls, simply remove the galls each winter from infected trees, then burn (where allowed by local ordinance) or bury them.  Prune branches six to eight inches below each gall.  On trunks, you can use a chisel to remove tissue from at least one inch beyond the infected area.  Keep in mind however, that doing this creates a wound that can provide an entry point for other serious Prunus pathogens such as the bacteria that cause bacterial canker (see UW Plant Disease Facts D0009, Bacterial Canker).  After removing galls, be sure to clean pruning tools between cuts by treating them for at least 30 seconds with 70% alcohol (e.g, rubbing alcohol or spray disinfectants) or a 10% bleach solution.  This will prevent accidental movement of the black knot fungus from branch to branch, or from tree to tree as galls are removed.  If you use bleach, be sure to thoroughly rinse your tools after you are done pruning and oil them to prevent rusting.

How do I avoid problems with black knot in the future?  In established plantings, remove any volunteer or wild cherry or plum trees from within 500 feet of susceptible fruit-bearing or ornamental cherries or plums.  When purchasing new cherries and plums, carefully inspect trees prior to purchase to be sure they are free of black knot.  When available, buy black knot-resistant ornamental cherry or plum species or varieties such as Prunus ‘Accolade’, Prunus sargentii, and Prunus maackii.  DO NOT attempt to control this disease using fungicide treatments, as these treatments are expensive and not likely to be effective.

For more information on black knot:  See UW Bulletin A3639 (Growing Apricots, Cherries, Peaches, and Plums in Wisconsin), available at https://learningstore.extension.wisc.edu/, or contact your county Extension agent.

Bacterial Wetwood

What is bacterial wetwood?  Bacterial wetwood, also known as “slime flux”, is a visually frightening-looking, but typically non-lethal, disorder of many types of deciduous trees.  This disorder can reduce the aesthetic appeal of landscape trees, and more seriously, can substantially reduce the value of forest trees used for lumber.  Bacterial wetwood most commonly affects elm and poplar, but can also be a serious problem on aspen, maple, and mulberry.

Bacterial wetwood leads to discolored, rancid-smelling areas on tree trunks.
Bacterial wetwood leads to discolored, rancid-smelling areas on tree trunks.

What does bacterial wetwood look like?  Trees suffering from bacterial wetwood have areas where liquid oozes from their trunks.  This ooze may flow freely at certain times of the growing season, but then may stop flowing at others.  The ooze leads to streaked, discolored, water-soaked areas on tree trunks.  The ooze is often colonized by bacteria, as well as yeasts and other fungi.  These organisms can give the ooze a slimy, sometimes brightly-colored (i.e., pink or orange) appearance as well as a highly disagreeable, rancid smell.  Internally, bacterial wetwood can be associated with localized areas of wood decay.

Where does bacterial wetwood come from?  Bacterial wetwood arises when localized wet areas develop in the heartwood or sapwood of tree trunks.  These areas are colonized by a diverse assortment of bacteria (e.g., Enterobacterium, Klebsiella, Pseudomonas and many others) that can enter trees through root, branch or trunk wounds.  As these bacteria feed and grow, often under anaerobic conditions (i.e., conditions without oxygen), they can produce gases such as methane, carbon dioxide, or nitrogen gas.  These gases build up pressure, causing movement of interior liquids to the exterior of the trunk where they escape through wounds and cracks.

How do I save a tree with bacterial wetwood?  Bacterial wetwood is a chronic disorder and affected trees cannot be cured.  To limit the unsightly staining of bark caused by bacterial wetwood, try to identify where the ooze is exiting from the trunk and insert a long, plastic tube at this location to direct the ooze away from the trunk and to the ground at the base of the tree.  There has been speculation that the build-up of gases due to bacterial wetwood might cause a tree to explode.  However, there have been no reliable reports of this ever happening.

How do I avoid problems with bacterial wetwood in the future?  There is little you can do to prevent problems with bacterial wetwood.  Many affected trees were likely invaded by wetwood-associated bacteria in the seedling stage.  Developing a healthy tolerance for bacterial wetwood, when it occurs, is perhaps the best method for coping with this disorder.

For more information on bacterial wetwood:  Contact your county Extension agent.

Bacterial Canker

What is bacterial canker?  Bacterial canker is a common and sometimes lethal disease of trees in the genus Prunus including cherry, plum and peach.  Bacterial canker is sometimes also referred to as “gummosis”, “blossom blast”, “dieback”, “spur blight” and “twig blight”.

Ooze (see arrow) on Prunus branches or trunks can indicate a bacterial canker problem.
Ooze (see arrow) on Prunus branches or trunks can indicate a bacterial canker problem.

What does bacterial canker look like?  Often branch dieback is the first symptom of bacterial canker that homeowners notice.  However, other more subtle symptoms of flowers, leaves, fruits and branches typically precede this dieback.  Initially, infected trees are symptomless.  Infected flowers often open but then collapse.  Infected leaves become spotted and yellowed.  The centers of leaf spots often shothole (i.e., fall out).  If spotting is severe, leaves may fall off.  Infected fruits develop dead spots surrounded by water-soaked tissue.  Spots can eventually develop into a fruit rot.  Branch or trunk infections often occur at pruning sites and lead to cankers (i.e., sunken, dead areas).  Cankers often produce a gummy, resinous ooze.  Wood in the cankered area is typically discolored.  Flower, fruit and branch infections can become systemic, leading to twig dieback, death of larger branches, or even death of an entire tree.

Where does bacterial canker come from?  Bacterial canker is caused by the bacteria Pseudomonas syringae pv. syringae (Pss) and P. syringae pv. mors-prunorum (Psm).  These bacteria overwinter in cankers, in asymptomatic, systemically infected branches, and in buds of susceptible trees.  Pss is also known to occur naturally on the leaves of many plants including many weed species.  Both Pss and Psm can subsequently be spread by rain splash, wind, or insects.  The bacteria can also be spread to healthy branches when contaminated pruning tools are used.  Infections most often occur during cool, wet conditions.

How do I save a plant with bacterial canker?  Prune infected branches at least 12 inches below cankers or other dead tissue, and dispose of branches by burning (where allowed by local ordinance) or burying them.  Prune branches only during the winter (e.g., Jan. and Feb.) or during dry periods in late summer (e.g., Aug.).  DO NOT prune during the cool, wet periods (e.g., spring and fall).  Disinfest pruning tools after each cut by treating them for at least 30 seconds with a 10% bleach solution or preferably 70% alcohol (e.g., rubbing alcohol straight from the bottle or a spray disinfectant).  If you decide to use bleach, be sure to thoroughly rinse and oil your tools after pruning is complete to prevent rusting.

How do I avoid problems with bacterial canker in the future?  Healthy trees are better able to slow the development of bacterial canker.  Therefore make sure that trees are watered and fertilized properly.  Keep weeds and other plants that may harbor bacterial canker pathogens from around the base of susceptible trees.  Copper-containing sprays have been advocated for bacterial canker management.  However, in many areas, copper-resistant strains of Pss and Psm are present and therefore copper sprays are ineffective.

For more information on bacterial canker:  See UW Bulletin A3639, Growing Apricots, Cherries, Peaches, and Plums in Wisconsin (available at https://learningstore.extension.wisc.edu/), or contact your county Extension agent.

Bacterial Blight

Death of lilac branch tips and leaves due to bacterial blight.
Death of lilac branch tips and leaves due to bacterial blight.

What is bacterial blight?  Bacterial blight, also known as blossom blight or shoot blight, is a common and often serious disease of Chinese, Japanese, Persian and common lilac, as well as walnut, apple, pear, plum and cherry.  White flowering varieties of common lilac are most susceptible to the disease.

What does bacterial blight look like?  Initial symptoms of bacterial blight may include dark brown necrotic (dead) leaf spots with yellow halos.  If leaf spots develop before leaves are fully expanded, leaf curling and twisting may result.  More advanced symptoms include necrotic blotches starting at the leaf margins and advancing inward, as well as black streaking on twigs.  In its most severe form, bacterial blight can result in the death of branch tips, leaves and blossoms.

Where does bacterial blight come from?  Bacterial blight is caused by the bacterium Pseudomonas syringae pv. syringae (Pss), which survives in diseased stem tissue (cankers), plant debris, and soil.  Pss can be spread by insects and on pruning tools but is more commonly spread by wind and rain.  Often Pss is found on the surface of healthy plants and does not cause disease.  Infections can occur when the bacterium enters tissue through natural openings or through wounds caused by insects, pruning, wind damage or hail.

How do I save a plant with bacterial blight?  Prune diseased twigs 10 to 12 inches below the point of visible symptoms, and dispose of the branches by burning (where allowed by local ordinance) or burying them.  Always prune in dry weather, and disinfest pruning tools after each cut by treating them for at least 30 seconds with a 10% bleach solution or preferably 70% alcohol (e.g., rubbing alcohol straight from the bottle or a spray disinfectant).  If you decide to use bleach, be sure to thoroughly rinse and oil your tools after pruning is complete to prevent rusting.

How do I avoid problems with bacterial blight in the future?  When planting lilacs, provide adequate spacing between shrubs.  Thin individual shrubs each winter to promote good air circulation (see University of Wisconsin Garden Facts XHT1015, Pruning Deciduous Shrubs, for pruning tips).  Properly water, fertilize and mulch shrubs to avoid stress that may predispose them to disease.  Avoid overhead watering that may keep leaves wet.  If you have had chronic problems with bacterial blight, you may want to use a combination of copper and mancozeb-containing fungicides for control.  Apply fungicides two to three times at seven to 10 day intervals as leaves emerge, but before symptoms develop.  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 bacterial blight:  Contact your county Extension agent.

Ash Yellows

What is ash yellows?  Ash yellows is a chronic, systemic disease that affects ash trees of all ages.  White ash is particularly susceptible to ash yellows.  Ash yellows likely occurs wherever ash is grown and has been reported widely in the United States and southern Canada.  The organism that causes ash yellows also causes a disease called lilac witches’-broom.

Brooming symptoms in an ash tree caused by ash yellows.
Brooming symptoms in an ash tree caused by ash yellows.

What does ash yellows look like?  Symptoms of ash yellows usually occur within three years of infection.  Infected trees typically grow at a much slower rate than non-infected trees, although this may be difficult to detect in an isolated, single tree.  The rate of growth of an infected tree may be as little as one half that of a healthy tree.  Leaves on infected trees are frequently smaller, thinner and lighter green than normal.  Often, but not always, affected trees will produce branches in tufts, a symptom that is called “brooming”.  Eventually, branches in the crown will die and this dieback can continue until the entire crown is dead.

Where does ash yellows come from?  Ash yellows is caused by the phytoplasma, Candidatus Phytoplasma fraxini.  Phytoplasmas are bacteria-like organisms that live and survive in the phloem (the food-conducting tissue) of infected plants.  Leafhoppers are thought to be the primary means by which this pathogen is moved from tree to tree.

How do I save a tree with ash yellows?  There is no known cure for ash yellows, but some infected trees may live and grow slowly with the disease for many years.  Ash trees suspected of having ash yellows should be tested for the disease, and those trees that test positive should be removed immediately to prevent spread of the ash yellows phytoplasma to other trees in the area.  Wood harvested from infected trees does not serve as a source of the phytoplasma and can be used for woodworking or firewood, or chipped for mulch.

How do I avoid problems with ash yellows in the future?  Avoid growing ash trees in areas where ash yellows is prevalent.  When choosing a lilac, select a variety of common lilac as these varieties appear to have tolerance to the ash yellows phytoplasma.  Avoid using S. josikaea, S. reticulata and S. sweginzowii (or hybrids of these species with either S. komarowii or S. villosa), as these lilacs appear to be highly susceptible.  It is unclear if the use of insecticides (or other means) to control leafhoppers can help control the spread of this pathogen.

For more information on ash yellows and ash yellows testing:  Contact your county Extension agent.

Armillaria Root Disease

What is Armillaria root disease?  Armillaria root disease, also known as shoestring root rot, is an often lethal disease of tree and shrubs.  It can affect almost any conifer or hardwood species, from seedling to maturity.  Herbaceous plants can also be affected.  Trees and shrubs stressed due to drought or defoliation can be particularly susceptible to Armillaria root disease.

White mats of fungal tissue called mycelial fans (arrow) may be present within and beneath the bark of stems and roots affected by Armillaria root disease.
White mats of fungal tissue called mycelial fans (arrow) may be present within and beneath the bark of stems and roots affected by Armillaria root disease.

Where does Armillaria root disease come from?  Armillaria root disease results from colonization of trees and shrubs by fungi in the genus Armillaria.  These fungi produce tough, cord-like strands called “rhizomorphs” that grow from decaying stumps and roots through the soil.  Infection of other trees or shrubs can result from penetration of intact roots by rhizomorphs.  In late summer or early fall, honey-colored mushrooms of Armillaria fungi develop near the bases of colonized plants and produce spores that are distributed by wind.  Infection also can occur after these spores germinate in wounds on stems or roots.

What does Armillaria root disease look like?  Above-ground symptoms of Armillaria root disease may include slow growth, yellowing and dwarfing of foliage, and thin crowns.  Dieback of twigs and branches also may occur as the disease progresses.  These symptoms may develop slowly and intensify over many years.  However, trees and shrubs also may be rapidly killed, with leaves or needles suddenly wilting or browning on a plant that appeared healthy just days or weeks earlier.  Bark on lower stems or roots may be killed and crack, with flow of resin common on conifers.  Thin white mats of fungal tissue called “mycelial fans” may be present within and beneath killed bark.  Stem and root tissue decayed by Armillaria fungi is often water-soaked, creamy to yellow in color, and spongy or stringy in texture.  Rhizomorphs are commonly seen on or beneath the bark and growing from decayed stumps and roots.

How do I save a tree affected by Armillaria root disease?  There is no practical way to eliminate Armillaria from trees that are already colonized by the fungus.  The useful life of an affected tree might be prolonged however, by supplemental watering during dry periods and appropriate fertilization to improve overall host condition.  In very vigorous trees, the Armillaria fungi may be “walled off” and confined to just a portion of the root system or root collar.  There are no chemical treatments that can effectively target Armillaria fungi within diseased trees.

How do I avoid Armillaria root disease in the future?  Practices that maintain trees in vigorous condition are the best means of preventing Armillaria root disease.  Watering and fertilization to avoid stress will help trees resist infection.  Because Armillaria root disease often develops in response to defoliation, suppression of both insect and leaf pathogen defoliators will indirectly reduce the occurrence and severity of Armillaria root disease.  Because stumps and root systems of previously colonized trees can serve as “food bases” supporting rhizomorph growth for many years, thorough removal of stumps and root systems will reduce the risk of infection of other trees.

For more information on Armillaria root disease:  Contact your county Extension agent.