Inheritance of Resistance to Downy Mildew (<i>Peronospora destructor</i> [Berk.] Casp.) from <i>Allium roylei</i> Stearn in the Backcross <i>Allium cepa</i> L. × (A. roylei ×<i>A. cepa</i>)

Abstract

Abstract Allium roylei Stearn and both reciprocals of the interspecific hybrid between Allium cepa L. and A. roylei displayed no symptoms of Peronospora destructor (Berk.) Casp. after artificial or natural inoculations, whereas A. cepa was susceptible. In the offspring from the backcross A. cepa × (A. roylei × A. cepa ) resistant and susceptible plants segregated after artificial inoculations, fitting a 1: 1‐ratio. This suggests that resistance is controlled by a single, dominantly‐inherited locus (designated Pd 1 ) from the nuclear genome of A. roylei. During a severe epidemic in the field, plants from the same backcross segregated resistant and susceptible individuals in a 3: 1‐ratio. Escapes may explain the deviation from the 1: 1‐segregation, but the presence of a second resistance locus segregating independently from Pd 1 cannot be excluded, implicating possible differences in virulence between populations of P. destructor. The occurrence in the backcross offspring of plants having a morphology similar to A. cepa without showing symptoms of downy mildew opens perspectives for breeding P. destructor ‐resistant onion varieties.