Reaction of Terminal Shoots of Apple Cultivars to Invasion by Erwinia amylovora

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Abstract
Young trees of 92 apple (Malus pumila) cultivars were trained to single shoots in a field nursery. After 8 wk growth the shoot tips were inoculated with a virulent isolate of E. amylovora by injection. Resultant fire blight lesions were measured 6 wk after inoculation when infection had ceased to extend visibly. Statistically significant differences in cultivar reaction were observed, especially in resistant cultivars. In 79 apple cultivars tested similarly in the greenhouse, statistically significant differences in cultivar reaction also occurred, but with greater separation of the susceptible cultivars. There was a strong correlation between the results of the field and greenhouse tests. Some of the resistant cultivars also possessed other desirable characters and should be considered for use in apple breeding. Discolored streaks were observed in the xylem extending in advance of the visible cortical lesions, particularly in many of the resistant cultivars. Gram-negative, rod-shaped bacteria were observed in the xylem streaks, and pathogenic E. amylovora was isolated from them. Length of xylem streaks from the point of inoculation was not correlated with cortical lesion length but the length of xylem streaking in advance of the cortical lesion was correlated with the standard error of the lesion length. Xylem streaking may indicate a potential for occasional severe infections in normally resistant cultivars.

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