Atrazine Constrains Conidial Germination and Morphological Transformation of the White Leaf Spot Pathogen Neopseudocercosporella capsellae in Atrazine-Tolerant Canola ( Brassica napus )

White leaf spot (Neopseudocercosporella capsellae) poses a significant threat to canola (rapeseed, Brassica napus) production globally, particularly in Australia. Histological studies were undertaken under controlled-environment room conditions on triazine-tolerant (TT) cultivars Crusher TT with moderate resistance and Thunder TT with high susceptibility to white leaf spot to determine how atrazine and the timing of its application affect (i) the disease severity as measured by the percentage of leaf yellowed area, (ii) the rate of conidial germination, (iii) the rate of conidial collapse, (iv) the germ tube length in germinating conidia, and (v) the morphological transformation from multicelled hyphae or conidia into numerous single-celled blastospores (yeast phase). For each cultivar, two atrazine treatments were used, which resulted in the highest and lowest levels of disease suppression under hyphal inoculation and infection as previously determined. Across 21 days, the percentage of leaf yellowed area of both cultivars was significantly reduced by atrazine application; the rate of conidial germination was reduced; both germ tube length of germinating conidia and subsequent morphological transformation into single-celled blastospores were also reduced; and the rate of collapse of conidia was increased. Notably, in the absence of atrazine, conidial infection resulted in greater percentage of leaf yellowed area as compared with hyphal infection and was strongly associated with more extensive morphological transformation of conidia into single-celled blastospores. It was evident that the formation of single-celled blastospores through morphological transformation is a determining factor for the comparative pathogenicity differences between conidial versus hyphal inoculum, highlighting the significance of this morphological transformation in the pathogenesis of N. capsellae. These findings not only provide new understanding of atrazine–pathogen interactions under controlled conditions but have laid the foundation for future studies and opportunities to further explore potential dimorphism-targeted fungicidal strategies and to utilize this system as a model for herbicide–pathogen interactions.