Progeny tests to identify diploid potato clones homozygous at loci controlling resistance to PLRV

Using potato parental lines homozygous at a locus or loci controlling resis tance to potato leafroll virus (PLRV) can give advantages in the selection of resistant forms. In order to identify homozygous diploid clones their te st-cross families were evaluated. All the clones that were test-crossed exp ressed resistance in primarily- and secondarily-infected plants and etiolat ed sprouts, and were derived from mating genotypes highly resistant to PLRV . Genotypes from test-cross families varied in resistance to PLRV, and one family was found which had only resistant genotypes, suggesting that the re sistant parent of this progeny was homozygous at resistance loci. Evidence was gathered that resistance in some diploid clones may result from resista nce to virus multiplication as well as restricted virus transport from leav es to tubers.