The process of infection by strains 21 Anz-2, 7 and 21 Anz-2, 3, 7 of Puccinia graminis tritici was quantitatively investigated to determine whether differences existed between them at any stage of the infection process and whether any such differences were related to the relative survival ability of the two strains in mixtures. The frequency of urediospore germination, the frequency of formation of appressoria, the period between inoculation and the eruption of uredia, the proportion of infection courts developing into uredia, and the visible characteristics of the uredia, did not differ between the two strains. However, penetration occurred more frequently from appressoria of strain 21 Anz-2, 3 , 7 than from those of strain 21 Anz-2, 7. Uredia of strain 21 Anz-2,3,7 increased more rapidly in size than did thoseof 21 Anz-2,7 and were ultimately larger in area. Similarly, more urediospores were produced per uredium by strain 21 Anz-2, 3, 7 than by 21 Anz-2, 7. It is suggested that these differences in frequency of penetration and in number of urediospores per uredium could account, at least in part, for the difference in relative survival ability between the two strains.
Abstract. Pathotypes of Cochliobolus sativus have been reported overseas but variation in the Australian population has not been investigated nor have the optimum conditions for the identification of variation been established. Preliminary experiments showed that maximum separation of infection responses of seedlings to infection by C. sativus conidia was obtained at 21°C using inocula containing 10 4 conidia/mL and a dew period of 40 h. Under these conditions, the existence of pathotypes in the Australian population of C. sativus was demonstrated. Using 20 lines, six pathotypes were identified among 34 isolates of the fungus. A differential set consisting of 12 barley cultivars is proposed.
A model describing the population trends for any number of races of rust in the urediospore stage during a single growing season is presented. The general case for the field is subsequently modified to describe trends in a population of rust races under constant conditions. A term which gives an estimate of the relative survival ability of each race in the population is finally obtained. This term allows the trends in the composition of the mixture to be predicted. The theoretical proportion of strain 21 Anz-2,3,7 in amixture of strains 21 Anz-2,7 and 21 Anz-2,3,7 of P. graminis trifici over a number of generations was compared with the proportion of strain 21 Anz-2,3,7 in an experimental population of the same two strains over the same period. The model was found to be a good first approximation to the results obtained with the experimental population.
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