Insights Into How Glyphosate Constrains Conidial Germination but Stimulates Morphological Transformation of the White Leaf Spot Pathogen <i>Neopseudocercosporella capsellae</i> in Glyphosate‐Tolerant Rapeseed ( <i>Brassica napus</i> )

ABSTRACT To investigate the effect of glyphosate on white leaf spot ( Neopseudocercosporella capsellae ) epidemics in glyphosate‐tolerant (RR) rapeseed (canola, Brassica napus ), studies were conducted using cultivars Hyola RR 500 (moderately disease‐resistant) and Hyola RR 504 (highly disease‐susceptible) to determine how glyphosate application and timing impact (i) disease severity as measured by percentage of leaf yellowing area (%LYA), (ii) conidial germination rate, (iii) germinating conidia germ tube length and (iv) morphological transformation from multicelled hyphae or conidia into yeast‐like single‐celled blastospores. Two glyphosate treatments used were based on prior studies identifying those inducing greatest or least disease stimulation under hyphal inoculation and infection. Across 21 days, glyphosate application stimulated both white leaf spot disease and conidial and hyphal morphological transformation into blastospores but suppressed conidial germination and germ tube extension. There was a strong association of morphological transformation with leaf yellowing area. Notably, increased blastospore formation from glyphosate application appears to be a key driver for the success of both conidial and hyphal infections and probably provides an alternative pathway for enhancing N. capsellae infection and disease development in the presence of glyphosate. Moreover, that increased %LYA occurred with glyphosate alone suggests glyphosate can facilitate plant senescence that also favours the disease. These findings provide a new mechanistic understanding of the interaction between glyphosate, N. capsellae and host plant, and also open the way for using this system as a herbicide–pathogen–host plant model to explore the strategic use of recommended glyphosate application in managing dimorphic fungal diseases in glyphosate‐tolerant rapeseed and other herbicide‐tolerant crops.