Spring 1995

WHAT 'S AILING YOUR ASH TREES?

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 1930’s, it was not described apart from the broader problem of ash decline caused by environmental factors until the 1980’s 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 1930’s, 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 1980’s 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."

DISEASE CYCLE

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

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.

DIAGNOSIS

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.

Laboratory

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.

MANAGEMENT

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.

Forests

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:

• Harvest trees with greater than 50% crown dieback within 5 years.
• Remove other affected ash during subsequent harvests.

Home Landscapes

Managers of shade and ornamental trees should consider management strategies which:

• Remove trees with severe dieback, because they cannot be rehabilitated.
• Promote species diversity in tree planting programs, and avoid monocultures of ash along city streets.
• Select tree species suitable to planting sites, and avoid planting ash in drought-prone sites
• Encourage tree care practices that reduce plant stresses. Watering during drought and periodic fertilization to promote general tree health may be useful.

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:

USDA Forest Service

State and Private Forestry

1992 Folwell Avenue

St. Paul MN 554108

Phone: (612) 649-5262

Fax: (612) 649-5238

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