ISSUE 9 July 10, 2008
I’ve begun to receive calls on ash rust, a common disease of ash trees. This fungus is often noticed as small patches of orange lesions on the leaves. Sometimes the disease attacks the petiole or twig, occurring as a large misshapen mass. The lesions contain two types of disease spores. If the infection is severe enough, leaves will fall off the tree prematurely.
Ash rust fruiting bodies on the underside
of the leaves. Photo by Joe Zeleznik.
Ash rust on the twig of a green ash tree,
Logan County N.D. Photo by Shelley Feist,
Logan County Soil Conservation District.
Like many other rust diseases, ash rust has two hosts – ash trees and several cordgrass (Spartina) species. The disease is most serious near areas where the cordgrass is abundant, such as wet or saline soils. There are several types of rust diseases on cordgrass and they are difficult to distinguish from one another. Therefore, the presence of rust on cordgrass does not necessarily mean that ash trees in the area are threatened.
Most infection occurs in the spring, but new infections can still occur in early summer if conditions are right – wet weather with warm temperatures, about 55 to 75 F. The best way to control ash rust is to eliminate nearby cordgrass. If this is not possible, fungicides that contain the active ingredient myclobutanil may be used as a protectant fungicide on the trees whenever conditions for severe infection are high.