Comparison of Rating Methods for Bacterial Spot Resistance in Japanese-Type Plum

Abstract

Rating methods were compared in field experiments to determine usefulness in breeding bacterial spot [ Xanthomonas campestrispv. pruni(Smith) Dye] resistant plums (Prunus salicinaLindl. and hybrids). Methods compared were measurements of number, size and incidence of leaf spots, percentage affected leaf area and stem canker length. Measurements of spot size and spot number were equal in repeatability and overall correlation with other rating methods but the correlations between these methods were moderately low (r = 0.54 to 0.59) indicating that they may be under separate genetic control and that ranking of cultivars will change with the rating method. The measurement of percentage affected leaf area by use of photographic standards was least repeatable (t = 0.42) and stem canker length was most repeatable (t = 0.58). The moderately high broad sense heritabilities (0.42 to 0.58) obtained for all rating methods indicates improvement of bacterial spot resistance should be possible.