Comparisons of Tolerance of Ash Yellows Phytoplasmas in Fraxinus Species and Rootstock-Scion Combinations.

Abstract

Growth responses of different ash (Fraxinus) species and rootstock-scion combinations to ash yellows (AshY) phytoplasmas were compared in greenhouse experiments by expressing each measurement as a proportion of the final average value of the variable in noninoculated, own-rooted control trees. Phytoplasmal infection suppressed shoot growth of white ash (F. americana) and green ash (F. pennsylvanica) beginning when buds opened, but did not suppress velvet ash (F. velutina) until after 60 days of growth. AshY-associated growth losses in height, stem diameter, and root volume, averaged across two experiments, were 80, 93, and 98%, respectively, in white ash; 60, 57, and 79% in green ash; and 23, 0, and 12% in velvet ash. Growth in height, but not in stem diameter or root volume, of diseased white ash on velvet ash rootstock was significantly greater than growth of diseased own-rooted white ash. White ash witches'-brooms grafted onto healthy velvet ash continued to grow but did not produce vigorous, dominant shoots. Growth of diseased velvet ash on white ash roots was severely suppressed in comparison with that of diseased own-rooted velvet ash. Management of AshY through the use of tolerant genotypes may require tolerance in both scions and rootstocks.