Emergence of Cladosporium macrocarpum disease in canola (Brassica napus)

New disease symptoms were observed on canola (Brassica napus) crops late in the 2019 and 2020 growing seasons in Western Australia. Cladosporium macrocarpum was isolated from infected material, and the fungus responsible for the symptoms was demonstrated by fulfilling Koch’s postulates. One strain exhibited relatively high tolerance to prothioconazole and tebuconazole fungicides compared to other canola pathogens Sclerotinia sclerotiorum and Leptosphaeria maculans. In a 2019 field trial in a commercial canola crop, symptom incidence caused by Cladosporium was only significantly reduced by a double application of a foliar fungicide (active ingredients prothioconazole and tebuconazole) registered in canola for other diseases, while single applications gave no significant response. This new disease, caused by a ubiquitous fungal species, may be a consequence of changes to farm management strategies to reduce other fungal diseases of canola.