SEVERITY OF FIRE BLIGHT ON APPLE CULTIVARS AND STRAINS IN MICHIGAN

During the 1991 growing season, 84 apple (Malus domestica) cultivars and strains in an experimental planting were evaluated in July and again in September for susceptibility to fire blight; a standard rating system was used. All cultivars were infected, but the severity of the disease varied significantly among cultivars. Among cultivars currently planted in Michigan, Jonathan, Rome, Idared, Braeburn, and Fuji were very susceptible; Golden Delicious, Gala, and Jonagold were moderately susceptible; Granny Smith, Empire, and McIntosh were intermediate; and Red Delicious and Liberty were moderately resistant. Correlation of lesion length data for 16 cultivars tested in New York State and USDA severity scores for naturally infected trees in Michigan was r = 0.67 (P = 0.004). Planting intermediate to moderately resistant rather than very susceptible cultivars is likely the best means of controlling fire blight in areas infested with streptomycin-resistant strains of Erwinia amylovora.