A DNA test for routinely predicting mildew resistance in descendants of crabapple ‘White Angel’

Abstract

Apple mildew, caused by the fungus Podosphaera leucotricha, is an ongoing problem for apple growers in most world production regions. To manage apple foliar mildew with less fungicide, use of mildew-resistant cultivars is an ideal option. The mildew resistance locus of crabapple ‘White Angel’ was previously named Pl-w and genetically mapped, with the closest DNA markers at least 4.6 cM away on one side of the locus. The objective of this study was to develop a DNA test with markers flanking Pl-w that can predict mildew susceptibility in ‘White Angel’ descendants. Two families with ‘White Angel’ as a common parent and ‘Golden Delicious’ and ‘Fuji’ as the other parents were screened for mildew susceptibility in a growth chamber. These seedlings were genotyped with reported DNA markers in the genetic vicinity of Pl-w. Segregation ratios in the two families fitted 1:3 and 3:5 ratios for resistance:susceptibility of seedlings. The DNA test developed, Md-Plw8-SSR, contained two SSR markers, CH01e12 and CH02g09, estimated at 0.01 ± 1.0 cM and 5.0 ± 1.0 cM, respectively, flanking the Pl-w locus. Md-Plw8-SSR identified individuals resistant to foliar mildew with 99.9% accuracy. The segregation ratios suggested mildew resistance from ‘White Angel’ was controlled by two unlinked loci with an unknown frequency and distribution in elite germplasm of the resistance allele at the second locus. When used in seedling screening, the new DNA test can eliminate at least some susceptible individuals but cannot detect allelic variation at the second, unmapped locus to ascertain resistance. Combining Md-Plw8-SSR with DNA tests for other sources of mildew resistance is expected to improve the ability to breed for durable resistance in apple.