NEWS for North Dakotans
Agriculture Communication, North Dakota State University
7 Morrill Hall, Fargo, ND 58105-5665

May 6, 1999

Diseases May Threaten Green Ash, Lilacs

Bacteria that can cause diseases in green ash trees and lilacs are present throughout central North America, a regional survey coordinated through North Dakota State University shows. However, an NDSU forestry specialist says it's too early to tell if the diseases are causing any damage to trees or shrubs.

The specialized bacteria, called phytoplasmas, cause "ash yellows" and "lilac witches' brooms," explains Marcus Jackson, forester with the NDSU Extension Service. The phytoplasmas survive only in the food-conducting tissues of host plants or in certain sap-sucking insects, such as leafhoppers, that have fed on infected plants. The diseases are transmitted as infected insects move and feed.

"Ash yellows phytoplasmas have been shown to significantly reduce growth and cause dieback of white ash and green ash in the northeastern United States," says Jackson. "Although ash yellows can cause dramatic changes in tree functions over one or two years, performance of established trees is usually reduced very gradually over several years."

The most characteristic symptom of ash yellows is the proliferation of branches from a single point (witches' brooms). These witches' brooms develop on large limbs or trunks and appear bushy, Jackson says. Leaves on witches' brooms tend to be small and yellowed and may develop as simple leaves, with one leaflet per leaf stem (ash trees normally have compound leaves comprised of five to seven leaflets).

Most infected trees do not have witches' brooms, but there are no other diagnostic symptoms. If a tree is performing poorly, ash yellows should be considered as a possible cause. Infection can be verified through the NDSU Plant Diagnostic Lab for a fee. Jackson recommends that samples should be taken from witches' brooms, but samples also may be taken from roots if no witches' brooms are present. Contact the Diagnostic Lab for cost and packaging information at (701) 231-7854.

Ash yellows phytoplasmas were first confirmed in North Dakota green ash trees in 1993 and lilacs in 1997. Beginning in 1996, NDSU Plant Pathologist Jim Walla coordinated a regional project including six states and three Canadian provinces to determine the incidence and impact of ash yellows in Great Plains and Rocky Mountain green ash trees. Community forests, rural plantings and native woodlands were all sampled during the project, which has determined the incidence level of disease in this region.

"We do not know yet if ash yellows is causing damage in North Dakota," Jackson stresses. "An association was found between the presence of ash yellows phytoplasmas and increased dieback in green ash, but we do not know if the phytoplasmas caused the dieback."

Walla is currently investigating the effect that ash yellows has on natural regeneration of green ash. In addition, he will soon be screening cultivars to identify tolerance. If found, tolerant trees may be used for landscape plantings and as parents for future windbreak trees.

Where possible, good plant-care principles such as proper irrigation, fertilization and pest control may prevent infected trees from declining, Jackson says. The presence of these bacteria is a concern since ash and lilac are among the best and most important woody plants in this region.

Until researchers can pinpoint what effect the diseases are having on trees in this region, they continue to recommend the use of multiple species in new plantings and the reduction of the ash component (if high) during windbreak, woodland or landscape renovation. They stress that ash and lilac should continue to be planted as they are important components of this region's native forests, windbreaks and urban landscapes.

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Sources: Marcus Jackson (701) 231-8478 and James Walla (701) 231-7069

Editor: Dean Hulse (701) 231-6136