Ascospore dimorphism-associated mating types of Sclerotinia trifoliorum equally capable of inducing mycelial infection on chickpea plants

In many heterothallic fungal 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. The mating type gene has pleotropic effect on ascospore size; large ascospore isolates are phenotypically homothallic (L-type), and small ascospore isolates are heterothallic (S-type). 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 57 field isolates collected from crown lesions (infection initiated by mycelium) had a 1:1 distribution of L-type (29 isolates) and S-type (28 isolates), whereas 14 isolates from stem lesions (infection initiated by ascospores) had a distribution of 10 L-type isolates and 4 S-type isolates not significantly different from 1:1. Greenhouse tests using mycelial plugs as inoculum of field and laboratory-derived isolates did not show significant difference between the two mating types in causing stem rot of chickpea. The sample size of ascospore-initiated infection was small, and the controlled pathogenicity assays in the greenhouse only tested mycelial infection. Thus, whether the two types of ascospores have equal capability of infecting chickpea remains to be further investigated. Strong evidence of both field and greenhouse data showed that mycelia of both mating types of S. trifoliorum were equally capable of infecting chickpea.