Thermal time and time of sowing impacts disease development of Brassica napus inoculated with diverse Sclerotinia sclerotiorum isolates

Sclerotinia stem rot (SSR), caused by the necrotrophic fungal pathogen Sclerotinia sclerotiorum, is a severe disease of broadleaf crops including canola/rapeseed (Brassica napus), leading to significant yield loss in conducive years. Replication of field conditions is challenging in variety disease resistance screening with testing required under a wide range of environmental conditions and at different plant growth stages. We investigated the role of thermal time in disease progression using three sowing times in the growing season, with six commonly grown Australian varieties of canola inoculated with four diverse West Australian isolates of S. sclerotiorum at 30% flowering. Area Under the Disease Progression Stairs (AUDPS), sclerotia production and weight, and seed production were measured. Time of sowing was found to be a crucial factor in explaining differences in AUDPS, stem width, seed production and sclerotia number according to the analysis of variance (P < 0.05), influencing isolate aggressiveness and disease progression. Linear mixed-effect models, regression decision tree models and principal components analysis were also conducted to determine the importance of a range of variables being included in variety screening for resistance. For all these analyses, both thermal time from sowing to 30% flowering when plants were inoculated, as well as thermal time over the 28-day inoculation period, in both canola and S. sclerotiorum, were important in explaining the variation. The study concludes by recommending that thermal time should be included in future SSR prediction risk models.