Herbicides Constrain Hyphal Growth, Conidial Germination, and Morphological Transformation in a Dimorphic Fungal Pathogen

White leaf spot disease [Neopseudocercosporella capsellae (Ellis & Everhart) S.I.R.Videira & P.W.Crous] poses a significant threat to rapeseed production globally. The herbicides atrazine and glyphosate are widely applied to herbicide-tolerant major crops, including rapeseed. Herbicides can affect disease levels directly and indirectly by stressing host plants, influencing pathogens, and altering abiotic and biotic stress levels in the environment. The specific effects of herbicides on the dimorphic pathogen N. capsellae regarding hyphal growth, conidial germination rate, and the morphological transformation from multi-celled hyphae or conidia into numerous single-celled blastospores remain unknown. Hence, studies were performed on two agar media [malt extract agar (MEA) and water agar (WA)] to determine how atrazine and glyphosate, each applied at 1 g a.i. L−1 or the commercial recommended concentrations of 10 and 7.8 g a.i. L−1, respectively, affect these characteristics in four highly pathogenic isolates of N. capsellae. Across a 32-day assessment period, the hyphal growth of all four isolates subcultured individually on MEA or WA was significantly restricted by both concentrations of atrazine and glyphosate. For both atrazine and glyphosate, restriction of hyphal growth was much greater at the higher commercial recommended concentration. Glyphosate restricted hyphal growth more than atrazine for each comparative concentration. Using a mixture of all four isolates, a similar trend of suppression by atrazine or glyphosate occurred in relation to conidial germination and the morphological transformation from multi-celled hyphae or conidia into numerous single-celled blastospores. These new insights into how herbicides constrain hyphal growth, conidial germination, and morphological transformation suggest their potential as a control measure in herbicide-tolerant crops to limit the epidemic spread and development of not only N. capsellae in rapeseed but other dimorphic fungal pathogens as well.