Atrazine Application Shapes White Leaf Spot ( <i>Neopseudocercosporella capsellae</i> ) Development on Triazine-Resistant Canola

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 the percent leaf disease incidence (%LDI), percent leaf area diseased (%LAD), and percent leaf area collapsed (%LAC) for canola cultivar, disease development period (days postinoculation), 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 the greatest disease suppression from atrazine application 0 or 7 days before inoculation, and Thunder TT showed the greatest from application 0 days before and 7 days after inoculation. In terms of %LAD, Crusher TT showed the greatest suppression from application at 0 and 7 days before inoculation, and Thunder TT showed the greatest from application 0 days before or 1 day after inoculation. In relation to %LAC, Crusher TT showed the greatest suppression from application 7 days before inoculation, and Thunder TT showed the greatest from application 0 days before or 1 day after inoculation. Due to the extent of suppression of WLS by atrazine application being substantial, its variability with cultivar susceptibility and application timing, and the strong interaction of cultivar × application timing, these together highlight 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.