Ash Yellows and Lilac Witches'-Broom: Phytoplasmal Diseases of Concern in Forestry and Horticulture.

Ash yellows (AshY) and lilac witches' -broom (LWB) are diseases of Fraxinus and Syringa species, respectively, believed to be caused by phytoplasmas (formerly called mycoplasmalike organisms. AshY and LWB in highly susceptible taxa are characterized by slow growth, progressive loss of vitality, dieback, and premature death (Figs. 1 and 2). These diseases are considered together in this article because evidence indicates they are caused by the same pathogen. Knowledge of AshY and LWB has accumulated primarily since 1983, but enough is now known for these diseases to serve together as a model for tentative understanding of lesser known phytoplasmal diseases of trees and shrubs in forests and horticultural landscapes. Objectives of this article are to summarize information about AshY and LWB, relate this information to knowledge of other phytoplasmal diseases of perennial plants, and identify needs for further research. Fraxinus and Syringa are closely related genera in the Oleaceae. Some ash species in Europe and North America are commercially important for wood products, including furniture, tool handles, and baseball bats; and some species are planted in horticultural landscapes (Fig. 2A). Three of the 12 North American species, white ash (F. americana), green ash (F. pennsylvanica), and velvet ash (F. velutina), receive emphasis in this article. Lilacs are flowering shrubs or small trees grown in temperate regions for their fragrant and attractive floral displays in spring. Lilac and ash taxa are generally graft-compatible, and some cultivars of lilac have been grown on ash rootstocks.