Alternaria leaf spot of cotton seedlings grown in New South Wales, Australia is predominantly associated with Alternaria alternata

Abstract

Alternaria leaf spot (ALS) of cotton, especially on cotyledons is generally considered a minor disease in Australia. However, severe disease outbreaks were recorded in multiple cotton crops in southern New South Wales (NSW) in the 2017/18 season. Due to ALS being considered a minor disease, causal species have not been formally characterised in Australia. In this study, putative small-spored Alternaria alternata was predominantly recovered from diseased cotyledons sampled from ALS hot spot fields in southern NSW as well as other locations across NSW. Species status of recovered isolates as A. alternata was confirmed through morphology and sequence analyses of the internal transcribed spacer (ITS) and translation elongation factor 1-alpha (TEF1) regions. Phylogenetic analyses of the two regions both individually or in combination resulted in a well-supported group of 69 isolates (83%) clustering with the ex-type specimen of A. alternata CBS 916.96, thus confirming the preliminary morphological identifications. In artificially inoculated assays on 15-day-old cotton seedlings, a subset of selected A. alternata isolates produced similar ALS symptoms at 7 days after inoculation (DAI) as seen in the field. Koch’s postulate was fulfilled by re-isolating A. alternata from infected ALS cotyledons at 21 DAI. This is the first main stream published report of A. alternata causing ALS on cotton cotyledons in NSW, Australia.