Reproductive Potentials of Races 15b and 56 of Wheat Stem Rust

Abstract

Variations in the prevalence of races 56 and 15B-1 (Can.) of wheat stem rust (Puccinia graminis Pers. f. sp. tritici Erikss. and Henn.) have occurred that cannot be explained by changes in the wheat varieties grown in Western Canada. The reasons for the changes in prevalence were investigated by comparing in growth cabinets the reproductive potentials or aggressiveness of the two races and the factors that might influence them. When a mixture of urediospores of the two races was used to inoculate three susceptible varieties for successive uredial generations, race 56 predominated at 25 and 20 °C, and race 15B-1 (Can.) predominated at 15 °C. Experimentation showed that the urediospores of race 56 were more infective than those of 15B-1 (Can.) and that the incubation period of race 56 was shorter than that of race 15B-1 (Can.). Pustules of race 56 grew faster than those of race 15B-1 (Can.) during early stages of development but those of 15B-1 (Can.) ultimately became larger than those of race 56. Race 56 produced more urediospores per pustule than race 15B-1 (Can.) on the susceptible varieties Little Club, Red Bobs, and Marquis at 15 and 20 °C. Infection density markedly affected competitive abilities. Race 56 tended to predominate in light infections but race 15B-1 (Can.) tended to predominate in heavy infections. Pustule size decreased as infection density increased. The possible significance of these findings under field conditions is discussed. It seems likely that the rise of race 15B-1 (Can.) to predominance in 1950 resulted not only from the selection pressure of the widely grown varieties that were resistant to race 56 and susceptible to 15B-1 (Can.) but from the lower temperatures that prevailed and favored 15B-1 (Can.) over 56. The rise to predominance of race 56 from 1956 to 1961 may have resulted from the greater aggressiveness of that race under the moderate or warm conditions that prevailed.

Physiologic races of wheat stem rust in Canada from 1919 to 1969

“…Displacement of the widely virulent race ClO (15B-1 (Can.)) by the relatively avirulent race C17 (56) after 1955 can be explained by altered environmental condiditions, mainly temperature (10). Race C17 (56) was succeeded by the new more widely virulent race C9 (1 5B-1 L (Can.))…”

“…Many of these varieties were resistant to race 56. During the early 1950's 15B was favored also by low temperatures which were unfavorable for race 56 (10). In contrast to race 56, 15B has many subraces that were easily separated with the supplementary differential hosts used.…”

Gene-for-gene relationships in the Avena: Puccinia graminis host–parasite system in Canada

“…1960 1961 1962 1963 1964 1965 1966 1967 1968 isolates total 1960 1961 1962 1963 1964 1965 1966 1967 1968 isolates total British Columbia Alberta, Saskatchewan, and Manitoba (2,24) have shown an inverse relationship between the number of virulence genes possessed by the pathogen and competitive ability although in P. graminis it appears to be complex and dependent on environment (11). This has led to the conclusion that unnecessary genes for virulence are somehow disadvantageous and that their decline and possible elimination is to be expected.…”

Variation in aggressiveness is detected among Puccinia triticina isolates of the same pathotype and clonal lineage in the adult plant stage

“…For such clonal species, the pathotype is sometimes considered as a basic genetic unit in population studies, given the large selection pressure exerted by the qualitative resistance genes, and the diversity for pathogenicityrelated traits at a lower scale is often ignored or neglected. Pathotypes were sometimes compared for quantitative traits on the basis of a single isolate per pathotype (Katsuya and Green 1967). Quantitative variation in pathogenicity among individuals can be the consequence of a change from avirulence to virulence (Vera Cruz et al 2000), or related to the number of qualitative virulence factors (Thrall and Burdon 2003) but it is likely that aggressiveness variations also occur independently of qualitative virulence (Villaréal and Lannou 2000;Pilet et al 2005;Lannou 2012).…”