Genetic and molecular analyses in crosses of race 2 and race 7 of Albugo candida

The inheritance of avirulence and polymorphic molecular markers in Albugo candida. the cause of white rust of crucifers, was studied in crosses of race 2 (Ac2). using isolates MiAc2-B1 or MiAc2-B5 (metalaxyl-insensitive and virulent to Brassica juncea cv. Burgonde) with race 7 (Ac7), using isolate MsAc7-A1 (metalaxyl-sensitive and virulent to B. rapa cv. Torch). Hybrids were obtained via co-inoculation onto a common susceptible host. Putative F-1 progeny were selfed to produce F-2 progeny. The parents and F-1 progeny were examined for virulence on the differential cultivars B. juncea cv. Burgonde and B. rapa cv. Torch. Segregation of avirulence or virulence of F-2 populations was analyzed on cv. Torch. Putative F-1 hybrids were confirmed by random amplified polymorphic DNA markers specific for each parent. Avirulence or virulence of F-2 progeny to B. rapa cv. Torch suggested 3:1 in each of three populations, supporting the hypothesis of a single dominant avirulence gene. Amplified fragment length polymorphism markers also segregated in regular Mendelian fashion among F-2 progeny derived from two F-1 hybrids (Cr2-5 and Cr2-7) of Cross-2. This first putative avirulence gene in A. candida was designated AvrAc1. These results suggest that a single dominant gene controls avirulence in race Ac2 to B. rapa cv. Torch and provides further evidence for the gene-for-gene relationship in the Albugo-Brassica pathosystem.