Glyphosate application affects white leaf spot (Neopseudocercosporella capsellae) development on glyphosate‐tolerant canola

Abstract

Canola (Brassica napus) production in Australia widely uses glyphosate‐tolerant (Roundup Ready [RR]) cultivars. White leaf spot (WLS; Neopseudocercosporella capsellae) significantly threatens canola globally, but particularly in Australia. Studies were undertaken on Hyola RR 500, with moderate resistance, and Hyola RR 504, which is highly susceptible to WLS, to determine how glyphosate application at various timings before and after N. capsellae infection affects WLS development. There were significant (p < 0.05) effects for leaf disease incidence (LDI), leaf area diseased (LAD), and leaf area collapsed (LAC) for both canola cultivars, for disease development period (days post‐inoculation), and for glyphosate application time (before or after inoculation). Although applying glyphosate before or after inoculation on both cultivars significantly (p < 0.05) increased LDI, LAD and LAC area under disease progress curve (AUDPC) values, glyphosate application before inoculation exerted a significantly greater effect on these values than glyphosate application after inoculation. On both cultivars across 21 days, glyphosate application 7 days before inoculation resulted in the greatest increases for LDI, LAD and LAC AUDPC values, while glyphosate application 7 days after had the least effect. This study highlighted potential significant benefit for farmers by withholding glyphosate applications until N. capsellae infections are well established in order to minimize predisposition to WLS by glyphosate applied prior to the main N. capsellae infection period.