Studies on genetical recombination and variation in Puccinia striiformis Westend

Taylor, Elaine Christine (1982) Studies on genetical recombination and variation in Puccinia striiformis Westend. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

Variants of yellow rust (Puccinia striiformis Westend.) with new combinations of virulence characteristics appear frequenlly in the population and cause significant yield losses by attacking wheat cultivars which were previously resistant to yellow rust. Experiments were conducted to investigate the methods by which variants can be formed. Selected virulence characteristics and certain aspects of pathogen growth of new variants were studied and compared to isolates of the races from which they arose. Single spore cultures of a white-spored isolate of race 104E137 and a yellow-spored isolate of race 37E132 were grown together on the wheat cultivar Sappo and the resulting urediosporos were screened for changes in spore colour and virulence characteristics as distinguished by seedlings of cultivars from the world and European differential sets. Single spore cultures of the variants, all yellow, were grouped into two races (105E137 and 36E132) by their reactions on the differential sets. The isolates of race 105E137 were unusual in that, although they were virulent to Vilmorin 23, they were avirulent to Cappelle Desprez. Isolates of the two new races were then mixed and white isolates sought. White isolates with virulence characteristics of the white isolate of race 104E137 were identified and one white isolate resembling one of the new yellow races, race 36E132, was also obtained from the mixture. A white isolate of race 105E137 arose in a pure culture of this race. During an experiment using ultra-violet radiation, a variant virulent on the cultivar Compair was isolated. The mechanisms which may have led to the formation of these variants are discussed. It is concluded that the results are consistent with the theory that mutation and hyphal fusion followed by the reassortment of whole nuclei, restricted by mating type, occurring between isolates cause variation in yellow rust. However, this does not preclude the possibility that the isolates were formed by alternative mechanisms. An experiment was conducted to compare the ability of the two new yellow races isolated in the first experiment, the yellow 'parental' race and a yellow isolate of race 104E137, to compete in mixtures with the white 'parental' isolate of race 104E137, on seedlings of cultivars apparently equally susceptible to the components of the mixtures, in the glasshouse. The germination, spread of a sporulating region and sporulatinn of the isolates on seedlings of selected cultivars were compared. Under certain conditions the new variants reproduced as successfully as the 'parental' isolates. The significance of these results is discussed in relation to the production and subsequent success of new genotypes and to the work of other researchers.

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