The Verticillium wilt problem in Australian cotton

© 2021, Australasian Plant Pathology Society Inc. Verticillium dahliae is a soil-borne phytopathogen and the causal agent of Verticillium wilt. It affects many agriculturally important crops around the world, including cotton. In Australia, the billion-dollar cotton industry is increasingly impacted by Verticillium wilt. Internationally it has been reported that the defoliating V. dahliae Vegetative Compatibility Group (VCG) 1A causes severe damage to cotton. In Australia however, the non-defoliating VCG2A is causing more severe damage to crops in fields than the defoliating VCG1A. This review examines the current research to understand the Australian V. dahliae situation, including current classification systems, genetic analyses and management strategies. It appears that virulence cannot be defined solely by VCG in Australian Verticillium dahliae isolates causing disease in cotton, and that the industry must continually adapt their practices in order to keep the disease under control

Virulence not linked with vegetative compatibility groups in Australian cotton Verticillium dahliae isolates

Verticillium dahliae, the causal agent of Verticillium wilt, is a soil-borne ascomycete that infects numerous agriculturally important crops globally, including cotton. As a billion-dollar industry, cotton is economically important to Australia and the management of disease such as Verticillium wilt is key for the success of the industry. Internationally, defoliating V. dahliae isolates belonging to Vegetative Compatibility Group (VCG) 1A cause severe damage to cotton, while non-defoliating VCG2A isolates result in significantly less disease. However, in Australia, VCG2A is causing more severe damage to crops in the field than the defoliating VCG1A. This study aimed to replicate field observations in controlled greenhouse conditions. We examined and compared disease symptoms on a range of Australian commercial cotton varieties when inoculated with different V. dahliae VCGs. Seedlings were root dipped in conidial suspensions and assessed over seven weeks. The final disease score, disease over time and root length were analysed. Plant mortality resulted from both V. dahliae VCG1A and VCG2A isolates across all cotton varieties used, confirming that there are virulent VCG2A isolates present in Australia. To our knowledge, although virulent on other plant hosts, V. dahliae VCG2A has not previously been reported to be highly virulent in cotton. We infer that virulence cannot be defined solely by VCG in Australian V. dahliae isolates causing disease in cotton

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