A quantitative method for inoculating strawberry leaves withXanthomonas fragariae, factors affecting infection, and cultivar reactions

Abaxial surfaces of strawberry (Fragaria × ananassa) leaves were quantitatively inoculated with the causal agent of angular leaf spot, Xanthomonas fragariae, using a rigidly mounted spray gun device to examine factors affecting infection. Lesions were uniformly distributed within the inoculation sites, and their numbers increased as the distance between the spray nozzle and leaf surface decreased and the duration of the inoculation burst increased. Leaves of plants exposed to darkness before inoculation had low numbers of lesions, but numbers increased markedly after only 0.5 h of light and became maximal after 4 h, demonstrating the importance of inducing stomatal opening prior to inoculation. Exposing plants to a high or low preinoculation humidity did not affect lesion numbers, but incubating plants at a high postinoculation humidity resulted in greater numbers of lesions than those incubated at low humidity. Exposing plants to mist for 3 days following inoculation did not further increase lesion numbers. Plants exposed to frost (–1.5 ®C for up to 1.5 h) before inoculation had fewer lesions than those exposed to no frost, a result of frost-induced closure of stomata. Nineteen isolates obtained from diverse geographic regions were assessed for their aggressiveness on five strawberry cultivars, but no isolate × cultivar interaction was observed. The isolates exhibited a continuum of increasing aggressiveness, but no substantial differences were observed. Twenty-two strawberry cultivars were evaluated for their resistance to X. fragariae. A continuum of increasing resistance was observed except for the cultivar 'Tristar', which appeared to be substantially more resistant than the others. In a separate test, two clones, US 4808 and US 4809, which have been released for resistance breeding purposes, were more resistant than the standard cultivar 'Honeoye', but they were not completely resistant. The inoculation procedure described here will be useful for quantitatively assessing angular leaf spot of progeny in disease resistance breeding programs.