COMPONENTS OF PARTIAL RESISTANCE TO POTATO BLACKLEG CAUSED BY PECTOLYTIC ERWINIA-CAROTOVORA SUBSP ATROSEPTICA AND ERWINIA-CHRYSANTHEMI

The various phases of the infection process in potato blackleg, caused by pectolytic Erwinia spp., were were interpreted as components of pa rtial resistance and studied in detail, Mother tubers of glasshouse-gr own plants of two potato cultivars were inoculated with antibiotic res istant marker strains of Erwinia carotovora subsp, atroseptica (Eca) o r E. chrysanthemi (EcH). Stem tissue samples of these plants were coll ected 3, 7, 14 and 21 days after inoculation and subsequently analysed for the presence of the bacteria. As soon as 3 days after inoculation , Erwinia bacteria were detected in the stem tissue of the plants. How ever, blackleg developed only when the rotting mother tuber tissue rea ched the sites where the stems were attached to the mother tuber. An i noculation method was developed for determining cultivar differences i n the incidence and rate at which rot in tuber cylinders proceeds to s prouts on inp of such cylinders. A set of 12 cultivars was screened fo r this putative component of resistance which was called 'stem base re sistance'. Significant differences for stem base resistance were found among the cultivars. These differences were clearer when inoculation was carried out with Ech in contrast to Eca. Glasshouse-grown plants o f the same set of cultivars were also screened for resistance of the a bove round part of tile stem. Cultivars differed significantly for thi s so-called 'stem tissue resistance', but results of some cultivars we re not consistent among years of testing. Multiple regression analysis revealed that stem base resistance, stem tissue resistance, tuber tis sue resistance and resistance of mother tubers in the field account fo r 63% (Eca) and 75% (Ech) of the Variance found after screening for re sistance to blackleg in the field. It was concluded that if results of this study are extrapolated to any set of potato clones, selection fo r resistance in breeding programmes under laboratory or glasshouse con ditions, would be mosi efficient when directed ra clonal differences f or stem base resistance.