Germination of Fusarium graminearum ascospores and wheat infection are affected by dry periods and by temperature and humidity during dry periods

The effects of temperature (temp) and relative humidity (RH) on germination of Fusarium graminearum ascospores, and of dry periods (DP) of different lengths and of temp and RH during DP on ascospore survival were studied both in vitro and in planta. Optimal temperatures for ascospore germination at 100% RH were 20 and 25°C; germination was ≤5% when ascospores were incubated at 20°C and RH≤93.5%. Viable ascospores were found at all tested combinations of DP duration (0 to 48 h) × temp (5 to 40°C) or RH (32.5 to 100% RH). Germination declined as DP duration and temp increased. Germination was lower for ascospores kept at 65.5% RH during the DP than at 76.0%, 32.5%, or 93.5% RH. Equations were developed describing the relationships between ascospore germination, DP duration and temp or RH during DP. Durum wheat spikes were inoculated with ascospores and kept dry for 0 to 48h at approximately 15°C and 65% RH; plants were then kept in saturated atmosphere for 48h to favor infection. Fungal biomass, measured as F. graminearum DNA by qPCR, declined as DP increased to 24 and 48h at 3 and 9 days post infection (dpi) but not in spikes at maturity.