Cotton diseases: the ‘big four’ in New South Wales

Abstract

Cotton (Gossypium hirsutum), a billion-dollar crop is mainly grown under irrigated conditions in regional areas of New South Wales (NSW) and Queensland. Diseases are of major constraints to production in both states. In NSW, the ‘big four’ diseases of concern are regional dependent. Black root rot (BRR) was a prevalent seedling disease across NSW. The average disease incidence varied from 15 to 90% in the last two survey seasons. However, BRR was more severe in the south, where lower temperatures were recorded during seedling establishment. The BRR pathogen, Berkeleyomyces rouxiae (syn. Thielaviopsis basicola) was proposed to be spread from northern NSW, where it was first detected, based on uniform sequences of ITS, TEF1 and RPB2 loci. Soil drench with myclobutanil (3ml/l) reduced BRR severity slightly in a pot trial. Alternaria leaf spot (ALS) has long been considered minor, but ALS outbreaks were recorded on cotton seedlings in the south in 2017/18 season. The ALS also remained prevalent on mature crops, especially before crop defoliation in the last two seasons. The main pathogen, Alternaria alternata was re-identified. Of 10 fungicides tested, all showed strong suppression of A. alternata in vitro, except for azoxystrobin which suppressed growth of A. alternata as little as 10%. Fusarium wilt associated with Fusarium oxysporum f. sp. vasinfectum (Fov) and Verticillium wilt (Verticillium dahliae) were mainly detected in northern NSW, where disease incidence was recorded up to 70% and 90%, respectively. F. oxyporum was also prevalent on seedlings exhibited Rhizoctonia-like rot; and the pathogen was well-clustered with both Australian Fov and non-Australian races based on TEF1 sequences. Of the V. dahliae population recovered in 2017/18 and 2018/19 seasons, 17.4% and 34% were molecularly characterised as defoliating pathotype, respectively. Continued efforts in surveillance of the ‘big four’ will provide insights into their etiology and epidemiology.