Development of Alternaria leaf blight on north Australian cotton (Gossypium hirsutum), species prevalence, and its control using mancozeb

Abstract

Alternaria leaf blight is the most prevalent disease of cotton in northern Australia. A trial was conducted at Katherine Research Station to determine ( i) the efficacy of the protectant fungicide mancozeb on disease suppression, and subsequent yield and quality of upland cotton; ( ii) the disease development at various canopy levels; and ( iii) the prevalence of the Alternaria pathogen during the cotton growing season. The protectant fungicide, mancozeb, significantly ( P < 0.05) reduced disease incidence ( 85% to <= 74%), severity ( 2 to <= 1.94) and leaf shedding ( 1.59 to <= 1.32). It also reduced disease severity on cotton bolls from 2.28 ( control) to <= 1.04 ( fungicide treated). No significant effects of Alternaria leaf blight were observed on the yield and quality of cotton fibre. Significantly ( P < 0.05) higher disease incidence, severity and defoliation were observed at the lower canopy ( 1-7 nodes) level irrespective of treatments. The middle canopy ( 8-14 nodes) had the next highest incidence, severity and defoliation. The least disease and defoliation were observed at the top canopy level ( 15+ nodes). Disease development at various canopy levels of cotton plants during the growing season were predicted by a logistic model ( R-2 ranges from 0.58 to 0.98). For all treatments, predicted disease development on control plots was faster than that on treated plots. A. macrospora was the most prevalent pathogen during the early growing season and A. alternata was more prevalent towards the end of the growing season. In total, 16-65% of lesions were colonised by both A. macrospora and A. alternata.