Virulence ofErwinia amylovoraStrains toMalussp. Novole Plants Grown in Vitro and in the Greenhouse

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Abstract
A rapid, efficient method to determine the virulence of strains of Erwinia amylovora to Malus sp. Novole has been developed and evaluated. The method uses plantlets of Novole propagated in vitro. Plantlets are inoculated by cutting one or more leaves with scissors dipped in a suspension of E. amylovora (5 .times. 107 colony-forming units per millilitter). Fourteen days later, those plantlets inoculated with strain E4001A (virulent to Novole) showed typical fire blight symptoms including systemic necrosis and watersoaking; plantlets inoculated with strain Ea 273 (avirulent to Novole but virulent on most other apple cultivars) showed no systemic fire blight symptoms. When 39 strains of E. amylovora were evaluated for virulence to Novole, there was a significant associations between data obtained from the plantlet assay and from inoculation of greenhouse-grown Novole plants. The plantlet assay was used to evaluate the virulence of 142 field strains from North America, Europe, and Egypt. Twelve strains from the eastern and central United States and Canada were virulent to Novole. Although there was a good correlation of symptom development in plantlets and greenhouse-grown plants inoculated with a standard virulent and avirulent strain, the growth of these E. amylovora strains after inoculation differed in the plant materials grown in vitro and in the greenhouse. In greenhouse-grown Novole plants, populations of both virulent and avirulent strains decreased 6 hr after inoculation. Between 6 and 72 hr after inoculation the virulent strain increased by 102, whereas the avirulent strain increased very little. In in vitro plantlets, there was no decline in population after inoculation; instead cells of both virulent and avirulent strains increased by 104 and 102, respectively, between 0 and 96 hr after inoculation.