Effect of inoculum concentration, isolates and leaf age on bacteria canker disease development in sweet cherry (Prunus avium L.) cultivars

Bacterial canker, caused by Pseudomonas syringae pv. syringae, is an economically important disease of sweet cherry (Prunus avium L.). The most viable and cost effective method of control is the use of resistant rootstock and scion cultivars. However, a screening method to distinguish resistant from susceptible cultivars in sweet cherry populations poses a challenge. In this study, we examined the effect of inoculum concentration, virulence of pathogen isolates and leaf age on disease development. Excised young and old leaves from 'Bing' and 'Rainier' were inoculated with 10 mu L of six pathogenic P. syringae pv. syringae strains including Pss 422, 424, 425, 426, 427 and 428 at 10(8) cfu mL(-1). Also, attached and detached young leaves from 'Bing', 'Rainier' and 'Regina' were inoculated with 10 mu L of 10(8), 10(6), 10(4) and 10(2) cfu mL(-1) dilutions. Disease was scored on a scale of 0-4 (0 indicates no necrosis and 4 = total necrosis). Newly emerging leaves of both 'Bing' and 'Rainier' were more susceptible (P < 0.05) than older leaves. The isolates exhibited different levels of virulence on the cultivars, with Pss 422, 424, and 425 being more virulent on 'Bing' and Pss 425 on 'Rainier' (P < 0.05). Higher inoculum concentrations (10(6) to 10(8) cfu mL(-1)) showed more necrosis in 'Bing' and 'Rainier' (P < 0.05) in both assays, although detached leaves had more rapid symptom development and disease progression than attached leaves (P < 0.05). The effect of inoculum concentration on 'Regina' was minimal (P < 0.05). The mean score in the detached leaf assay was significantly greater than the attached leaf assay. These data will provide a foundation for screening a wider sweet cherry germplasm for resistance or susceptibility to bacterial canker.