Studies on variation within races and competion and genetical recombination between races in Puccina hordei

Falahati-Rastegar, Mahrokh (1977) Studies on variation within races and competion and genetical recombination between races in Puccina hordei. University of Southampton, Doctoral Thesis.

Abstract

Brown rust of barley Puccinia hordei Otth is an importantfoliage disease in the warm season in Southern Britain, causing considerable losses. A sexual stage is known for this fungus, but the appearance of new races is not apparently due to sexual hybridization, because the aecidial stage is very rare in Britain. Changes in virulence are more likely to arise by mutation and asexual recombination. Whether a newly developed race is capable of competing with other races in the field population depends upon its virulence and aggressiveness under the prevailing environmental condition: In order to obtain a colour mutant for recombination andcompetition studies, uredospores of race A were treated with ultra-violet radiation or Ethyl Methane Sulphonate. Following UV radiation one yellow coloured pustule appeared, but attempts to multiply it failed. The use of EMS resulted in production of three orange isolates all with the same pathogenicity as the parent race. After a time those orange isolates designated as A1 started segregating somatically, producing brown pustules and it proved imposible to obtain a completely stable line. To study vegetative recombination as a possible mechanism for the appearance of new races, the orange race (A1) was mixed successively with races F,J,K, and D and cultured on a susceptible host. A total of 564 cultures were screened but no detectable change in pathogenicity was observed. A population study was carried out; again the orange race was paired with races F, J, D and K in equal proportions and inoculated on to a range of susceptible barley cultivars at both the seedling and adult stages using two temperatures and two levels of inoculum density, heavy and light infection. A separate experiment with races F and A1 was carried out in the field. The effects of density of initial inoculum, time interval of successive spore collection, and location of the leaf on competition were also studied. In this experiment the predominating race at low temperatures with heavy inoculation was different from that at lower inoculation or higher temperature. In the majority of cases the race with the wider host range became the prevalent race regardless of its initial ratio in the mixture; contrary to expectation on current the cry. Histological studies provided a reason for the failure to detect asexual recombination and the prevalence of one race rather than another in mixed populations. Puccinia hordei has a very limited rate of mycelium spread, a small sporulation zone, a short latent period and secondary pustule formation does not occur after heavy inoculation. The collective effect of these factors is that opportunities for the production of new variants as the result of asexual recombination are few. The rates of spore germination, approssorium formation andpenetration were affected differentially by temperature in differentraces, and the race germinating rapidly under specific environmental conditions came to predominate in the population in heavy infection where there is competition for stomatal entry. Following light infection, the race which had higher spore viability, a larger number of spores per pustule, larger pustule size and higher infectivity became-the prevalent race. The relavance of these factors in relation to the competitive ability of a race in the field is discussed.

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