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Virulence Characteristics Accounting for Fire Blight Disease Severity in Apple Trees and Seedlings

First, second, fifth, sixth, seventh, and eighth authors: Department of Plant Pathology, The Pennsylvania State University, University Park 16802; second, third, fifth, and sixth authors: The Pennsylvania State University Fruit Research and Extension Center, Biglerville 17307; and fourth author: United States Department of Agriculture, Animal and Plant Health Inspection Service, Plant Protection and Quarantine, The Pennsylvania State University, University Park 16802.

The gram-negative bacterium Erwinia amylovora is the causal agent of fire blight, the most destructive bacterial disease of rosaceous plants, including apple and pear. Here, we compared the virulence levels of six E. amylovora strains (Ea273, CFBP1367, Ea581a, E2002a, E4001a, and HKN06P1) on apple trees and seedlings. The strains produced a range of disease severity, with HKN06P1 producing the greatest disease severity in every assay. We then compared virulence characteristic expression among the six strains, including growth rates in immature apple fruit, amylovoran production, levansucrase activity, biofilm formation, carbohydrate utilization, hypersensitive cell death elicitation in tobacco leaves, and protein secretion profiles. Multiple regression analysis indicated that three of the virulence characteristics (amylovoran production, biofilm formation, and growth in immature apple fruit) accounted for >70% of the variation in disease severity on apple seedlings. Furthermore, in greenhouse-grown ‘Gala’ trees, >75% of the variation in disease severity was accounted for by five of the virulence characteristics: amylovoran production, biofilm formation, growth in immature apple fruit, hypersensitive cell death elicitation, and sorbitol utilization. This study demonstrates that virulence factor expression levels account for differences in disease severity caused by wild isolates of E. amylovora on apple trees.

Additional keywords:disease incidence, DspA/E, HrpN, hypersensitive reaction, pCPP60, pEA29, type III secretion.