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Genetic and Molecular Analyses in Crosses of Race 2 and Race 7 of Albugo candida

First author: Crop Production and Pest Control Research, U.S. Department of Agriculture-Agricultural Research Service, Department of Botany and Plant Pathology, Purdue University, 915 W. State Street, West Lafayette, IN 47907; second author: Pioneer Hi-Bred International, Inc., Research and Product Development, 7300 N.W. 62nd Ave., Johnston, IA 50131; third author: Pacific Agri-Food Research Centre, Agriculture and Agri-Food Canada, P.O. Box 1000, Agassiz, BC, V0M 1A0, Canada; fourth author: Monsanto Canada Inc., P.O. Box 250, 1885A Mitchell Road South, Listowel, Ontario, N4W 3H2, Canada; and fifth author: Saskatoon Research Centre, Agriculture and Agri-Food Canada, 107 Science Place, Saskatoon, SK, S7N 0X2, Canada

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 F₁ progeny were selfed to produce F₂ progeny. The parents and F₁ progeny were examined for virulence on the differential cultivars B. juncea cv. Burgonde and B. rapa cv. Torch. Segregation of avirulence or virulence of F₂ populations was analyzed on cv. Torch. Putative F₁ 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 F₂ progeny derived from two F₁ 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.

Additional keyword: specificity.