Category Archives: Disease – Broad Leafed Woody Ornamental

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.

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

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 or burying them. Always prune in dry weather, and after each cut, disinfest pruning shears by dipping them for at least 30 seconds in a 10% bleach solution, or alcohol (spray disinfectants that contain at least 70% alcohol can also be used).

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 UW Garden Facts XHT1015 for pruning tips). Properly fertilize, water and mulch shrubs to avoid stress that may predispose them to disease. Avoid overhead watering that may keep leaves wet. If you have had 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 insure that you use the fungicide in the safest and most effective manner possible.

For more information on bacterial blight: See UW-Extension Bulletin A2538, or 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 a 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 usual. 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 die-back 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 (i.e., 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 shrub roots and lower stems. It can affect almost any conifer or hardwood species, from seedling to maturity. Trees and shrubs stressed due to drought or defoliation can be particularly susceptible to Armillaria root disease.

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 root 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 fungus tissue called “mycelial fans” may be present within and beneath killed bark. Stem and root wood 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.

Can 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 pathogen 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 the 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 will reduce the risk of infection of other trees.

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

Anthracnose

What is anthracnose?  Anthracnose is the name of several common fungal diseases that affect the foliage of woody ornamentals in Wisconsin.  Trees that are most commonly and severely affected by anthracnose include ash, maple, white oak, sycamore, and walnut.  Anthracnose typically affects young leaf tissue.

Anthracnose - image 1
Anthracnose is very common on many types of trees and shrubs. It often occurs on the leaves of ash (left) and maple (right) trees, causing blotchy-brown, dead areas.

What does anthracnose look like?  Symptoms of anthracnose vary from host to host, but in general, include irregular spots, and dead areas on leaves that often follow the veins of the leaves.  Affected tissue can vary in color, but is often tan or brown.  Severely affected leaves often curl and may fall off.  In some tree species, such as sycamore, twigs can also become infected leading to twig dieback.

Where does anthracnose come from?  Anthracnose is caused by several fungi (many historically classified in the genus Gloeosporium) that survive in leaf litter.  These fungi are host specific.  The anthracnose fungus that infects one type of tree (e.g., ash) is not the same one that infects another type of tree (e.g., maple).  However, when anthracnose occurs on one tree, then weather conditions (typically cool and moist conditions) are favorable for development of the disease on many types of trees.

How do I save a tree with anthracnose?  DO NOT panic.  For many trees, anthracnose is a cosmetic disease, making a tree look a little ragged, but not killing the tree.  However, if a tree has been defoliated by anthracnose for several years, or it is a tree, such as a sycamore, where twig infections can occur, then you may want to use a fungicide for disease control.  Three treatments are typically needed for adequate control: one at bud break, one when leaves are half expanded, and one when leaves are fully expanded.  Fungicides containing copper, chlorothalonil, or mancozeb are registered for anthracnose control in Wisconsin.  DO NOT use the same active ingredient for all treatments.  Instead, alternate the use of at least two active ingredients to help minimize problems with fungicide-resistant strains of anthracnose fungi.  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.

Anthracnose - image 2
Anthracnose can be severe on some hosts. On white oak (left), it can cause extensive leaf browning and curling. On sycamore (right), anthracnose can cause twig dieback.

How do I avoid problems with anthracnose in the future?  You can reduce the number of spores that cause anthracnose infections by removing and disposing of fallen, infected leaves in the autumn.  Leaves can be buried, burned (where allowed) or composted.  When composting, make sure that your compost pile reaches high temperature (approximately 140°F).  Also, make sure that your compost pile is routinely turned 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 anthracnose fungi.  Also, maintain good tree vigor by watering and fertilizing trees appropriately.  Check with your local county Extension agent for details on how to properly care for trees.

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