Responses of 11 Fraxinus cultivars to ash yellows phytoplasma strains of differing aggressiveness

Five cultivars of Fraxinus americana (white ash) and five of F. pennsylvani ca (green ash) were graft-inoculated with three strains of ash yellows phyt oplasmas at Ames, IA, and with three other strains at Ithaca, NY. A sixth g reen ash cultivar was tested only in New York. Trees were allowed to grow i n field plots for 3 years. Infection was detected via the DAPI (4',6-diamid ino-2-phenylindole.2HCl) fluorescence test. Incidence of witches'-brooms on infected trees was greater on white ash than green ash and varied signific antly among phytoplasma strain treatments at both locations. Volume growth of infected ash, averaged across cultivars over 2 years in Iowa and 3 years in New York, was 49 and 59%, respectively, as great as that of noninfected trees. Foliar greenness was reduced significantly by infection at both loc ations, and this reduction was positively correlated with growth reduction. Cultivars at each location varied significantly in growth of noninfected t rees and in growth of diseased trees relative to that of noninfected trees (a measure of phytoplasma tolerance), but cultivar means for these variable s in Iowa were not significantly correlated with those in New York. Green a sh cvs. Bergeson, Dakota Centennial, and Patmore and white ash cv. Autumn A pplause were above average in tolerance at both locations. Phytoplasma stra ins at each location varied significantly in aggressiveness as indicated by host growth suppression.