A nomenclature system for designating virulence combinations of oat crown rust (causal agent Puccinia coronata f. sp. avenae) isolates is proposed. Sixteen oat lines, with seedling resistance genes Pc38, 39, 40, 45, 46, 48, 50, 51, 52, 54, 56, 58, 59, 62, 64, and 68 occurring singly in each of them, are used as primary differentials. The host lines are arranged into groups of four (subset 1 = Pc40, 45, 46, 50; subset 2 = Pc38, 39, 48, 68; subset 3 = Pc51, 52, 58, 59; subset 4 = Pc54, 56, 62, 64). Avirulence and virulence of isolates on each line are indicated by low and high infection types, respectively. The consonants B through T are used to indicate the 16 possible infection type patterns on each subset (Table 1). Virulence combinations are assigned with a four-letter Pca code for races of P. coronata f. sp. avenae. Local differential series are separated from the Pca code by a slash, followed by a listing of the ineffective host genes in the local differentials on which the race was virulent.

The identification of virulence phenotypes has been and will continue to be an important part of the program to develop resistant host cultivars, as well as for determining regional virulence differences for the purpose of gene deployment in North America. The proposed Pca nomenclature should simplify the designation of complex virulence combinations. The new nomenclature also should facilitate the study of evolution of virulence in the oat crown rust populations. The previous use of frequencies of avirulence/virulence phenotypes made it difficult to follow trends involving complex virulence combinations. The proposed nomenclature permits the calculation of individual virulence frequencies as well as the determination of any virulence combinations in the oat crown rust population.

TABLE 1. Code for the 16 North American differential hosts for Puccinia coronata f. sp. avenae in ordered sets of four