LAUSR

Abstract Recent sunflower disease surveys in the major production fields of South Africa reported Alternaria alternata (Fries) Kiessler as the main causal agent of Alternaria leaf blight (ALB). The economic impact of ALB caused by A. alternata on sunflower in South Africa is unknown and there are currently no fungicide treatments registered for its control. The aim of this study was to determine the efficacy of five fungicides and one biological control agent (Integral® Pro, Bacillus amyloliquefaciens) as seed treatments, and three spray fungicides to control ALB on sunflower over three sunflower growing seasons (2013/14, 2014/15 and 2015/16). Alternaria leaf blight developed as early as three weeks after emergence in field trials, suggesting seed transmission of the pathogen. Alternaria leaf blight severity negatively correlated with 1000-seed weight (r = −0.33). Seed-borne A. alternata was significantly reduced in the field by treating seeds with Celest® XL and Dynasty® CST. Integral® Pro showed potential for future use as part of an integrated control measure for ALB of sunflower. Abacus® and Amistar® TOP were the most effective spray fungicides in reducing ALB disease severity. As ALB of sunflower has numerous possible sources of inoculum such as wind-blown inoculum, infected seed and plant debris, control should include various approaches. In this study the use of strobilurin-based fungicide seed treatments and spray fungicides both reduced ALB disease severity and also delayed sunflower leaf senescence.