DNA fingerprinting of Ascochyta blight of chickpea caused by Ascochyta rabiei

Chickpea (Cicer ariatium L.) is an important legume crop in Australia and all over the world. A major limiting factor of chickpea production is ascochyta blight, a devastating disease, caused by the fungal pathogen Ascochyta rabiei (Pass.) Labrousse. Molecular marker methods, including RAPD and ITS analysis, were applied to 33 fungal isolates collected from different regions in Australia. These methods, combined with pathogenicity tests, were able to distinguish true A. rabiei isolates from other nonpathogenic fungal isolates of similar morphology. Molecular markers specific to A. rabiei were detected which can be developed into probe(s) for future A. rabiei identification. A study of the genetic diversity among 33 A. rabiei isolates, sampled from the five chickpea-growing states in Australia, revealed a relatively low level of genetic diversity among the isolates. Two other isolates were shown to be relatively dissimilar at the 105 loci assessed, most likely due to their more recent introduction into Australia. The heterothallic nature of the mating system of A. rabiei is largely responsible for the increases in genetic diversity of this pathogen. Preliminary studies of the mating type distribution in Australia failed to detect mating type 2 (Mat-2) and hence provided evidence of only a single mating type in Australia (Mat-1). The genetic inheritance of the mating type character was investigated using an F I population developed from a single pseudothecium, from a sexual cross between a known Mat-1 isolate from Rosebery, Victoria, Australia and a Mat-2 tester isolate, supplied from the USA. Of the total 74 F1 progeny randomly assessed, 56 isolates were shown to be Mat-1, four isolates were able to mate with both tester isolates (Mat-1 and Mat-2) and the remaining 12 isolates did not mate with either. Due to the possibility of multiple nuclei within single ascospores, the Mendelian nature of the gene(s) involved in mating type was not determined. However, this study represents a first essential step towards elucidating the genetic mechanism controlling the mating type character of Ascochyta rabiei.