Atrazine application shapes white leaf spot (Neopseudocercosporella capsellae) development on triazine-resistant canola

Abstract

Studies were undertaken on two atrazine-tolerant canola (Brassica napus) cultivars (moderately resistant Crusher TT and highly susceptible Thunder TT) to white leaf spot (WLS) (Neopseudocercosporella capsellae), to determine how atrazine application affects WLS development. There were significant main treatment effects in relation to percent leaf disease incidence (%LDI), percent leaf area diseased (%LAD) and percent leaf area collapsed (%LAC) for canola cultivar, disease development period (days-post-inoculation), atrazine spray (before or after inoculation), and %LDI for atrazine spray timing (0, 1, 3, 7 days), along with significant two- and three-way interactions between treatments. For %LDI across a 21-day period, Crusher TT showed greatest disease suppression from atrazine application 0 or 7 days before inoculation, and Thunder TT from application 0 days before and 7 days after inoculation. In terms of %LAD, Crusher TT showed greatest suppression from application at 0 and 7 days before inoculation, and Thunder TT from application 0 days before or 1 day after inoculation. In relation to %LAC, Crusher TT showed greatest suppression from application 7 days before inoculation and Thunder from application 0 days before or 1 day after inoculation. That the extent of suppression of WLS by atrazine application was substantial and that it varied with cultivar susceptibility, application timing, and that there was a strong interaction of cultivar x application timing, highlights potential opportunities for farmers to exploit better cultivar choices in conjunction with manipulating the timing of atrazine application to maximize WLS suppression and consequent canola yield.