White leaf spot (Neopseudocercosporella capsellae) can be severe and problematic worldwide across both horticultural and oilseed Brassicaceae, including susceptible rapeseed. In this study, 82 isolates from 2015 and 106 isolates from across Australia in 2016 were first assessed for their virulence against three different rapeseed (Brassica napus) cultivars. For both years there were significant (P < 0.001) differences. Also, there were significant (all P < 0.001) differences between isolates in each year, and between cultivars. For 2016 isolates, there were also significant differences (P < 0.001) between isolates across three different Australian states, and a significant interaction (P < 0.001) between isolates with cultivars. Of the three Australian states, isolates from Victoria were most virulent. Among tested cultivars, cv. Scoop was most susceptible. Subsequently, phylogenetic analysis of 114 of these same 2015 and 2016 isolates showed current isolates clustered separately from the majority of 2005 N. capsellae isolates collected from Western Australia a decade earlier, confirming significant genetic change within N. capsellae populations over the past decade. However, isolate clusters showed no association with geographical location. The results suggest that phylogenetic association among 2005 and 2015–2016 N. capsellae isolates is complementary with pathogenicity variations explained by geographically different N. capsellae pathogen populations. Neopseudocercosporella capsellae populations are evolving rapidly, challenging management through host resistance at a time of increasing incidence and severity of white leaf spot disease over the past decade. The outcome is well illustrated by cv. Scoop, previously resistant to 2005 isolates but moderately susceptible to 2015 and highly susceptible to 2016 isolates.

White leaf spot (Neopseudocercosporella capsellae) is a persistent and increasingly important foliar disease for canola (Brassica napus) across southern Australia. To define the role of plant growth stage on development of the disease epidemic, we first investigated the response of different canola cultivars (Scoop and Charlton) at five Sylvester-Bradley growth stages against N. capsellae. White leaf spot disease incidence and severity was dependent upon plant growth stage and cultivar (both P < 0.001), with plants being most susceptible at plant growth stage 1,00 (cotyledon stage) followed by plant growth stage 1,04 (4th leaf stage). Then, second, to quantify the impact of this disease on canola yield, we investigated the in-field relationship of white leaf spot disease incidence and severity with seed yield loss following artificial inoculation commencing at growth stage 1.04 (4th leaf stage). White leaf spot significantly (P < 0.001) reduced seed yield by 24% in N. capsellae inoculated field plots compared with non-inoculated field plots. We believe that this is the first time that serious seed yield losses from this disease have been quantified in-field. The current study demonstrates that N. capsellae disease incidence and severity on canola is determined by host growth stage at which pathogen infestation occurs. Emerging seedling cotyledons were highly susceptible, followed by less susceptibility in first true leaves to emerge but then increasing susceptibility as plants subsequently age towards the 4th leaf stage. This explains field observances where white leaf spot readily establishes on emerging seedlings and subsequently becomes more prevalent and severe as plants age.

Rapeseed (Brassica napus) production in Australia relies heavily on triazine‐or glyphosate‐tolerant cultivars. For 14 triazine‐tolerant cultivars, disease development of Neopseudocercosporella capsellae (white leaf spot), Alternaria brassicae and A. japonica (Alternaria leaf spot), and Hyaloperonospora brassicae (downy mildew) were all dependent upon herbicide application timing (p < 0.001), with significant differences between cultivars (p < 0.001) and a significant interaction (p < 0.001) between herbicide application timing and cultivars. Atrazine applied preinfection by N. capsellae, A. brassicae, or A. japonica enhanced disease incidence, severity, and leaf collapse, while atrazine application postinfection for these same pathogens reduced all three disease parameters. However, for H. brassicae, application of atrazine after, and especially prior to, infection resulted in lower disease incidence, severity, and leaf collapse. Application of glyphosate on five glyphosate‐tolerant cultivars for N. capsellae resulted in significant differences (p < 0.05) between glyphosate application treatments, and between host cultivars in terms of incidence and consequent leaf collapse. For A. brassicae, A. japonica, and H. brassicae, glyphosate resulted in significant differences (p < 0.001) across application timings between cultivars, and a significant interaction (p < 0.001) between herbicide application timings and cultivars. Glyphosate applied on glyphosate‐resistant rapeseed after, and especially prior to, attack by H. brassicae, reduced downy mildew. These are the first studies to highlight how the timing of application of triazine or glyphosate in relation to pathogen infection is critical to the susceptibility of rapeseed to white leaf spot, Alternaria leaf spot, and downy mildew. This new understanding offers fresh possibilities for improved management of these diseases in herbicide‐tolerant rapeseed crops.