Characterisation of Verticillium dahliae by inter-simple sequence repeats identified a virulent subpopulation affecting Australian cotton

Verticillium dahliae is an agricultural phytopathogen of notable importance around the world and is increasingly impacting the billion-dollar Australian cotton industry. In diseased Australian cotton, the VCG classification system of V. dahliae does not adequately reflect the disease symptoms observed. The non-defoliating VCG2A isolates have caused significant damage, a problem not readily observed overseas, and the defoliating VCG1A isolates have not always caused widespread severe symptoms as expected. This study examined a selection of V. dahliae isolates derived from diseased cotton plants in NSW, Australia, America and Israel and classified them into groups using Inter-Simple Sequence Repeats (ISSR). A subset of these isolates were tested in a pathogenicity assay using cotton plants. The combined results confirmed that Australia has VCG2A isolates capable of causing significant disease ("defoliating-like") and that when analysed using ISSR, they are distinct from the non-virulent VCG2A populations. Additionally, there is a group of VCG1A isolates that are able to cause significant damage to cotton plants. The ISSR analysis provides a better understanding of the V. dahliae populations circulating within Australia and is a useful tool for diagnostic use, with a potential use for diagnostic confirmation of a causative strain.