ARS | Publication request: Ascospore dimorphism-associated mating types of Sclerotinia trifoliorum equally capable of infecting chickpea

Submitted to: Australasian Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 15, 2011
Publication Date: June 30, 2011
Citation: Njambere, E., Chen, W., Frate, C., Temple, S. 2011. Ascospore dimorphism-associated mating types of Sclerotinia trifoliorum equally capable of infecting chickpea. Australasian Plant Pathology. 40:648-655. DOI 10.1007/s13313-011-0069-3.

Interpretive Summary: In many heterothallic pathogens, mating types are found to be associated with variation in virulence and some other ecological traits. Sclerotinia trifoliorum is unique from other Sclerotinia species in that it is heterothallic with two mating types, and the mating type gene also has effects on ascospore size. The possible association of variation in virulence with the two mating types and hence the ascospore size in S. trifoliorum is investigated using isolates collected from naturally infected chickpea plants and isolates generated from controlled crosses. Chi-square tests showed that isolates collected either from crown-infection, initiated by mycelium, or from stem infection, initiated by ascospores, had a distribution of the two mating types not significantly different from 1:1. Greenhouse tests of field and laboratory-derived isolates of the two mating types did not show significant differences in causing stem rot when inoculated on chickpea stems using mycelium-colonized agar plugs. Both field and laboratory isolates varied in virulence in greenhouse assays, but the variation in virulence was not associated with mating type. Chickpea cultivars showed significant differences in disease levels in the field, and some cultivars were highly susceptible. However, all the chickpea genotypes were equally susceptible to S. trifoliorum in greenhouse assays, suggesting that different types of resistance mechanisms were tested in the two approaches and that resistance in chickpea is complexly inherited.

Technical Abstract: Sclerotinia trifoliorum causes stem and crown rot of chickpea and other forage and grain legumes, and is one of the three important species of the genus Sclerotinia. S. trifoliorum is unique from the other two species in that it is heterothallic and has two opposite mating types required for completeing the sexual cycle. Mating types in many other heterothallic pathogens have been associated with secondary traits including virulence. This study investigates possible associations between mating type and variation in virulence in S. trifoliorum. Field-collected isolates showed a distribution of the two mating types not significantly different from 1:1. Strains of both mating types derived from controlled crosses did not show significant difference in virulence on chickpea. The two mating types are not associated with variation in virulence. Thus, we do not need to consider mating types of S. trifoliorum in selecting isolates for screening resistance in chickpea. Field tests showed significant differences among chickpea cultivars in susceptibility to S. trifoliorum. However, the cultivars were all equally susceptible in greenhouse trials, suggesting that field tests and greenhouse screening tested different resistance mechanisms. Greenhouse test procedures need to be modified to reflect field resistance.