Pathogenic variability, inheritance of virulence and host-pathogen interaction with temperature in Ascochyta fabae f. sp. lentis on lentil
Kemal, Seid Ahmed
The frequency and distribution of Mating types 1 and 2 of Ascochyta fabae f. sp. lentis were studied by controlled crosses of 223 isolates from western Canada and 14 other countries with tester isolates. Both mating types were recovered from local and foreign sources. Mating type 1 was the most frequent and Mating type 2 was not recovered from a large collection of isolates from the 1991 Canadian crop. However, no pseudothecia were observed on overwintered infected lentil stems from a field where both mating types had been identified. Incompatibility was observed between some domestic isolates and known testers but no self fertility was observed. The pathogenic variability of 84 isolates (domestic and foreign) was determined by inoculating seedlings of 10 lentil differentials (Chilean '78, Eston, Brewer, ILL358, ILL5588, ILL5684, Indianhead, Laird, Spanish Brown and Precoz) at 20°C in growth chambers. Isolates collected in 1978 and 1985 from western Canada were weakly virulent compared to 1992 collections. Although the 1992 isolates caused higher disease severity on all differentials, the greatest increase was observed on the widely grown cv. Laird. The foreign isolates varied from weakly to highly virulent but no geographic relationship was detected. Both ANOVA and cluster analysis showed a general lack of specific differential X isolate interactions, suggesting specificity at the host rather than the cultivar level. In field experiments, the differentials maintained the relative reactions observed in the growth chamber, except for ILL358 which showed adult plant susceptibility but seedling resistance. Both isolate virulence and host susceptibility generally increased with declining temperatures. Significant interactions occurred between temperature and differential and temperature and isolates. However, the ranking of differentials did not change with temperatures. In vitro and growth chamber experiments showed that the optimum temperature for mycelial growth and host infection was between 15 and 20°C. However, no strong correlation was found between mycelial growth and virulence. In crosses of isolates, the F1 progeny included transgressive segregants with respect to virulence. This suggested that virulence is controlled by many genes with additive effects. The segregation of disease reaction in F2 progeny of crosses between lentil genotypes was generally similar when judged by seedling tests or seed infection.