Soft rot and blackleg reactions in diploid potato hybrids inoculated with Erwinia spp.

From 1993 to 1996 three groups of potato genotypes were evaluated for resis tance to Erwinia spp.: (1) 31 interspecific diploid hybrids (28 resistant a nd three susceptible), (2) five hexaploid or pentaploid somatic hybrids of Solanum tuberosum (tbr) x S. brevidens (brd), and (3) eight cultivars. Two evaluation methods were applied: tuber point inoculation with either Erwini a carotovora ssp. atroseptica (Eca) or E. chrysanthemi (Ech) to test tuber soft rot resistance and stem base inoculation with Eca to test blackleg res istance. Some resistant diploid hybrids and somatic hybrids of tbr x brd we re superior to cultivars for both tuber and stem resistance. Tuber and stem resistance to Erwinia spp. in the most resistant diploid hybrids were equa l to the highly resistant somatic hybrids of tbr x brd. Tuber resistance to Eca was highly correlated to tuber resistance to Ech (r=0.815***). In two years of evaluation for stem resistance, three diploid hybrids and a deriva tive of one of the somatic hybrids (USA M 264) failed to develop symptoms o f blackleg following inoculation with Eca. Analyses of variance for tuber a nd stem resistance indicated significant effects of genotype, year and geno type x year interaction. A positive relationship between tuber and stem resistance to Erwinia spp. h as been found, however the genetic control of resistance in tuber and stem is partially independent. In the case of Eca the correlation coefficient wa s r=6.725***. Therefore it should be possible to obtain resistant genotypes to both blackleg and tuber soft rot. Several resistant diploid hybrids wer e selected from among those tested, which also have several other character s desirable for potato breeders.