Growing Season Conditions and Isolate Are Important Determinants of Brassica Napus Susceptibility to Aggressive Sclerotinia Sclerotiorum Isolates

Management of Sclerotinia stem rot (SSR) disease in Brassica napus is heavily reliant on prophylactic fungicide applications at flowering, which often provides inconsistent control depending on timing of ascospore release in the field and environmental conditions. Understanding host response to Sclerotinia sclerotiorum infection is essential for sustainable disease management in the future. This study determined the host response of nine B. napus varieties to four aggressive S. sclerotiorum isolates across two years by measuring four disease variables; area under the disease progress stairs (AUDPS), seed production, sclerotia number and average sclerotia weight. Brassica napus varieties varied greatly in their response to the four measured variables, with varieties producing the highest AUDPS not the same varieties that had the lowest seed production, the highest numbers of sclerotia or heaviest sclerotia. Repeating the experiment over two years using the same varieties and isolates identified the impact of environment as the most influential factor on measured disease variables, highlighting the complexity of disease responses to diverse isolates and host genotypes under different environments. It was recommended that both long-term (such as inoculum production) and short-term (such as seed production) disease outcomes be combined with lesion length measurement (i.e. AUDPS) for future host screening studies.