Variation in aggressiveness of ash yellows phytoplasmas

Twelve strains of phytoplasmas belonging to the ash yellows (AshY) group, f rom across the known range of AshY and representing six host species, were assessed for differences in ability to suppress growth and cause chlorosis in graft-inoculated Fraxinus pennsylvanica (green ash) and Catharanthus ros eus (periwinkle). In each of two experiments with ash and one with periwink le, different strains caused significantly different degrees of growth supp ression and loss of foliar greenness. These growth and color impacts were p ositively and significantly correlated among experiments and between ash an d periwinkle, indicating strain variation in aggressiveness. After two stra ins that differed in aggressiveness were coinoculated to periwinkle plants, polymerase chain reaction/restriction fragment length polymorphism (PCR/RF LP) assays of DNA from leaves remote from the inoculation sites revealed th e presence of the aggressive strain sooner and more frequently than that of the less aggressive strain. Thus, aggressiveness was associated with more rapid multiplication and/or movement than was achieved by the less aggressi ve strain. When either strain was inoculated 11 weeks before the other into the same plant, only the initial strain could be detected after a further 12 weeks of incubation. Thus, the initial strain or its effect on the host may have interfered with multiplication and/or long-distance movement of th e second strain. A concept of preemptive dominance is proposed to account f or detection by primary PCR of only single phytoplasma strains in plants th at may harbor two or more strains.