Plum bacterial spot resistance in leaves and stems measured by a selection index

Abstract

Resistance to bacterial spot was evaluated for 66 plum (Prunus salicina and hybrids) genotypes from breeding programs at California, Florida, and Georgia in the USA and Queensland in Australia. Clonally propagated trees were planted in a high density, disease nursery in Gainesville, Florida and inoculated with Xanthomonas campestris Pv. Pruni in 1990 and 1991. Stems were injected using a hypodermic needle and syringe, and leaves were inoculated by dipping. Spreader trees of the susceptible cultivar 'Gulfruby' were planted in the inter-rows to aid disease spread. Disease intensity on stems was measured by length of inoculated canker and canker appearance rating, and on leaves by number of lesions per leaf and percentage of infected leaves. A selection index that combined these variables was used to rank the genotypes. Among the most resistant genotypes were the cultivars 'Robusto', 'Segundo', 'Rubysweet' and 'Bruce'. The four breeding programs differed significantly in levels of bacterial spot on leaves and stems, with the lowest levels of disease from the United States Department of Agriculture (USDA) program at Byron, Georgia and the highest levels from the USDA program at Fresno, California.