First report of Stemphylium solani associated with leaf spot of cotton in New South Wales, Australia

Leaf spot of cotton caused by Alternaria species has been long considered a minor disease in Australia, especially when only highly resistant Upland cotton (Gossypium hirsutum) are grown in commercial fields lately. However, leaf spot outbreaks were reported when crop was grown under favourable conditions for the disease development. In this study, we for the first time fully characterised the association of Stemphylium solani with leaf spot of cotton in New South Wales, Australia. A total of 32 isolates were recovered from leaf-spot-diseased leaves collected in the 2021/22 growing season. Leaf spot symptoms often occur in small circular to irregular necrotic lesions, often surrounded with purple halo, and were indistinguishable to those caused by Alternaria species. Sequences of the ITS, GAPDH and calmodulin genes of the three representative isolates revealed 99.86 to 100% sequence identity to the type isolate of Stemphylium solani CBS 116586. In a Neighbor-Joining analysis of the concatenated sequences of the ITS, GAPDH and calmodulin loci, the three cotton isolates were well clustered within the S. solani with a bootstrap support of 100%, thus, confirming the identification. On two-true-leaf cotton seedlings, the three S. solani isolates were able to incite spot symptoms that were similar to those observed in the field. The same fungal pathogen was recovered from inoculated diseased leaves, thus, confirming Koch’s postulations.