By: Jackie Carroll
Ash yellows is a devastating disease of ash trees and related plants. Find out how to recognize the disease and what you can do to prevent it in this article.
Ash yellows is a newly discovered plant disease, first detected in the 1980’s. It probably existed long before that, but wasn’t detected because the symptoms are so similar to those of other plant diseases. In many cases, you won’t be able to get a firm diagnosis without laboratory tests. A tiny, mycoplasma-like organism that we call the ash yellows phytoplasma causes the infection.
A disease that infects members of the ash (Fraxinus) family, ash yellows only exists in North America. The symptoms are similar to those of environmental stress and opportunistic fungi. Although we see it most often in white and green ash trees, several other species of ash can also become infected.
Ash yellows doesn’t discriminate about location. We find it in commercial woodlots, natural forests, home landscapes and urban plantings. Dieback may be rapid or very slow. Although it may be several years before the tree deteriorates to the point where it is unsightly or a danger to your landscaping and buildings, it’s best to remove it promptly to prevent the spread of disease. Replace it with trees that aren’t members of the ash family.
It may be as long as three years after infection before the symptoms of ash yellows appear. An infected tree usually grows at about half the rate of a healthy tree. Leaves may be smaller, thinner, and pale in color. Infected trees often produce tufts of twigs or branches, called witches’ brooms.
There is no effective ash yellows disease treatment. The disease is spread from plant to plant by insects. The best course of action if you have a tree with ash yellows is to remove the tree to prevent the spread to other trees.
Does this mean that you have to give up ash trees and lilacs in the landscape? If you know there is a problem with ash yellows in the area, don’t plant ash trees. You can plant lilacs as long as you choose common lilacs. Common lilacs and hybrids of common lilacs are known to resist ash tree yellows.
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WHAT 'S AILING YOUR ASH TREES?
Jill D. Pokomy, Plant Pathologist
Forest Health Protection, S&PF, USDA Forest Service
It could be a recently discovered disease called ash yellows. Ash yellows, a disease caused by mycoplasmalike organisms (MLOs), inhibits growth and causes decline of ash (Fraxinus) species. Although this disease may have been present in the northeastern USA since the 1930s, it was not described apart from the broader problem of ash decline caused by environmental factors until the 1980s This article will provide practical imformation about ash yellows, its relationship to premature decline in ash species, how to identify symptoms, and outline management prescriptions for rural and urban forests.
Premature Decline of Ash: A Historical Perspective
Since the 1930s, an unexplained progressive decline of white ash, and to a lesser extent green and black ash, has been observed in the northeastern United States. This decline syndrome is commonly referred to as ash decline. Ash trees of all age classes in forest, hedgerow, roadside and home landscape sites are affected and commonly die 2-10 years after the onset of symptoms. Early studies linked ash decline to adverse environmental factors such as drought, shallow soils, flooding, or parasitism by opportunistic fungi. It was not until the 1980s that a distince disease, ash yellows, was discovered to be frequently associated with ash trees exhibiting symptoms of slow growth and branch dieback.
To date, no single factor has been proven to be the cause of ash decline. Ash yellows and environmental stressors may, in fact, be co-conspirators in the development of ash decline. Field studies demonstrate that ash yellows is often detected on sites where environmental stressors such as water shortage, competition with neighboring trees or insect damage are present. In a recent survey of 4 midwestern states, widespread growth reduction and crown dieback observed in green ash forest stands was found to be independent of MLO (ash yellows) infections. The region wide drought in 1980 and 1981 and the moderate to extreme drought in 1988 and 1989 may have been the cause of the reported decline in the Midwest. More recently in the Midwest, crown dieback has been reported in green and white ash, following the severe cold winter temperatures of 1994. Clearly, current knowledge supports the theory that ash decline can result from various causes, and that ash yellows can be, but is not always, a causal factor.
The impact of ash yellows on ash populations is not well documented. Individual infected trees are likely to show declining radial and apical growth and, often, dieback. The disease occurs in woodlots and forests, home landscapes, and urban plantings.
Ash species play a significant role in the ecology of rural and urban northeastern forests. Their decline could have a serious impact on the health of aquatic and terrestrial plant and animal communities. Native green ash, found along streams and lakes, floodplains and in riparian corridors, provides nesting sites and roosts for several species of birds and other wildlife. White ash occurs mainly on upland slopes and ravines. Black ash usually occurs along rivers and small streams, and on the edges of bogs. Many insects and fish thrive in the cool water temperatures maintained by the shade of stream-side ash trees. In the urban setting, green and white ash are in high demand for landscaping and reforestation efforts. Their decline will result in reduced property values and wildlife habitats, coupled with large expenditures in community tree planting and maintenance programs.
The ashes represent a valuable hardwood resource, with an estimated 275 million board feet of ash lumber sawn annually. In the northeastern USA, approximately 33% of the commercial forest area has ash as a component, with green and white ash being the most important ash species. Black ash is an important timber species and is highly prized by Native Americans for use in basketmaking. Ash wood, tough and resistant to shock, is used for handles, oars, baseball bats and furniture.
To help clarify the incidence of ash yellows in the urban forests of the upper Midwest, the Forest Service is funding a 2 year project to survey green ash trees in 9 Iowa and Wisconsin communities. The project is a cooperative effort with Iowa State University, and the Iowa and Wisconsin Departments of Natural Resources, municipal and state foresters and county conservation districts in the communities surveyed. The results of this survey will help to assess the magnitude of the ash yellows threat to urban areas in the upper Midwest. The project also sponsors educational workshops for urban foresters that provide information on recognizing field symptoms and management options. According to Mark Gleason, project coordinator from Iowa Sate University, "the level of cooperation between agencies has been exemplary."
Ash yellows is caused by wall-less microbes called mycoplasmalike organisms (MLOs) which invade the tree's vascular system (phloem sieve tubes). Little else is known about the disease cycle of ash yellows. The causal MLOs are presumed to be spread by insect vectors such as leafhoppers, because these insects are the most common vectors of MLOs causing other plant diseases. Symptoms develop 0-3 years after MLOs are detected in ash phloem.
DISTRIBUTION AND HOST RANGE
Ash yellows has been reported only in North America. The main range of the disease includes parts of 16 northeastern and Midwestern states and the southernmost portions of the Canadian provinces of Ontario and Quebec (Figure 1). Ash yellows has also been found in two southwestern locations. In addition to white ash (F. americana) and green ash (F. pennsylvanica), ten other ash species including blue ash (F. quadrangulata), black ash (F. nigra), and velvet ash (F. velutina) are also reported hosts.
Symptoms of ash yellows vary with species. White ash sustains permanent and often rapid decline in apical and radial growth. Slow twig growth and short internodes can cause foliage to appear tufted at the tips of twigs and the crown to appear thin and sparse. Small leaf size and light green leaf color, upturned leaf margins, and premature fall coloration are common. Abnormal branching may also occur. Witches'-brooms, clusters of upright spindly shoots, may develop. Witches'-brooms form most often on trees with severe dieback, usually at the root collar but sometimes on the trunk, and commonly at basal injuries made by mechanical equipment or frost. Green ash exhibit symptoms similar to white ash but often appear to sustain less dieback and sometimes produce witches'-brooms without other distinctive symptoms.
Field diagnosis of ash yellows can be difficult. Reduced growth, abnormal branching, and progressive decline are typical symptoms but can be caused by other factors such as poor site conditions, drought stress, freezing and flooding damage, mechanical or chemical injuries, insect attack, and parasitism by opportunistic fungi. Witches'-brooms are diagnostic, but only a small percentage of infected trees display this symptom at a given time.
To assess a stand for the presence of ash yellows, inspect suppressed saplings and stumps, particularly along the edges o the stand, for the witches'-brooms, if ash yellows is present, careful inspection will usually reveal at least one tree or stump with a broom. If brooms are found, then trees showing abnormal branching may be assumed to have ash yellows.
A standard microscopic test is available to detect MLOs in ash phloem tissue. Microscopic testing is required for diagnosis of the disease in most individual or landscape trees, but careful field survey is nearly as effective as microscopic testing for detection of the disease in a stand or woodlot.
There is no known way to prevent or cure ash yellows. White ash that becomes infected when young do not grow to merchantable size. Most merchantable-sized diseased ash trees live for at least 5-10 years. Management prescriptions that promote species diversity and reduce plant stresses from water shortage and competition should minimize growth losses associated with ash yellows.
Management of stands where ash yellows occurs should be aimed at gradual replacement of white ash with other species. Where ash yellows occurs in merchantable white ash stands, trees of this species that exhibit slow growth and dieback should be removed during regular harvests as follows:
Managers of shade and ornamental trees should consider management strategies which:
In the future, ash cultivars or rootstocks resistant to or tolerant of ash yellows may become available.
The US Forest Service has published a new informational brochure entitled, "How to Identify and Manage Ash Yellows in Forest Stands and Home Landscapes." This color guide illustrates symptoms of ash yellows and provides forest managers and landowners with management prescriptions to minimize losses associated with ash yellows and preserve the resource. For a free copy, write or fax:
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There is no way to prevent or cure ash yellows phytoplasma infections. Remove infected trees with severe dieback. Avoid planting ash trees in areas with active ash yellows. Grow trees best suited to soil, rainfall, drainage, and other vital conditions at the site. For established ash trees, provide irrigation during extended dry periods, fertilize as needed, prune to sustain a sound branching structure, and maintain 2-3 inches of composted mulch over as much of the root zone as possible. In the future, rootstocks and ash cultivars that are tolerant of ash yellows may be available.
Written by: Dan Gillman
Photos: C. L. Ash, Diseases of Woody Ornamentals and Trees. APS Press.
Emerald Ash Borer is the most common of all the diseases in ash trees. It is caused by an invasive wood-boring beetle named emerald ash borer. This beetle attacks the nutrient-carrying vessels of the tree, and has infested millions of trees in the United States alone.
This infestation is caused due to day-flying wasp-like moths called Banded Ash Clearwing. They attack only ash trees, especially the green ones. They cause the bark of the tree to become rough and damage the tissues that are responsible for the flow of food and water. Feeding by some species of these insects may also result in the weakening of branches or may even kill the entire tree.
Treatment: Apply pesticides in the month of August as the pests appear during this time of the year.
This disease is characterized by the formation of galls caused by insects or mites. Galls are abnormal plant growths that look like green or brown clusters formed on the branches. A tiny mite called Eriophyid is responsible for infesting the ash tree. It causes male flowers to grow into small greenish round structures that turn dark brown in late summer. Once the galls begin to grow, the disease cannot be cured. Although these galls do not harm the tree, they look unsightly.
Treatment: The mites start growing in spring therefore, apply insecticides before the first blossom to avoid their growth. Ensure that the tree receives full sunlight to partial shade.
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This disease is caused when adult plant bugs and nymphs feed on the leaflets of ash trees when they start unfolding in early May. The main visible symptom of this disease would be the areas of the leaf that wilt and turn brown. If infested heavily, the premature drop of leaves take place. However, new leaves are formed by mid-summer.
Treatment: Maintain proper health of the tree by watering and mulching regularly. Use insecticides for killing the natural enemies of this tree however, make sure that your timing is right. Check for insects or any symptoms in early spring when the leaf buds start opening.
Leafcurl ash aphids are small green-colored insects that are covered in waxy threads. They feed on plant juices on the underside of the leaves, causing them to deform and curl. The leaves twist and turn yellow, which makes them look unsightly.
Treatment: Spraying insecticides may not prove that effective when you notice the damage as the insects are hidden in the foliage. However, water the tree thoroughly and apply insecticidal soap.
Oystershell scales develop in late May to early June when Lepidosaphes ulmi feed on the barks of the tree. Infestation by this insect causes the barks to crack. In case of heavy infestation, it may even weaken and kill the branches.
Treatment: Prune and destroy the affected branches. Apply insecticidal soap in every 7 days, but ensure that you time it properly according to the hatching of their eggs in spring.
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