Effects of weather variables on ascospore discharge from Fusarium graminearum perithecia

Fusarium graminearum is a predominant component of the Fusarium head blight (FHB)\r\ncomplex of small grain cereals. Ascosporic infection plays a relevant role in the spread of\r\nthe disease. A 3-year study was conducted on ascospore discharge. To separate the effect\r\nof weather on discharge from the effect of weather on the production and maturation of\r\nascospores in perithecia, discharge was quantified with a volumetric spore sampler placed\r\nnear maize stalk residues bearing perithecia with mature ascospores; the residues therefore\r\nserved as a continuous source of ascospores. Ascospores were discharged from perithecia\r\non 70% of 154 days. Rain (R) and vapor pressure deficit (VPD) were the variables\r\nthat most affected ascospore discharge, with 84% of total discharges occurring on days\r\nwith R 0.2 mm or VPD 11 hPa, and with 70% of total ascospore discharge peaks ( 30\r\nascospores/m3 air per day) occurring on days with R 0.2 mm and VPD 6.35 hPa. An\r\nROC analysis using these criteria for R and VPD provided True Positive Proportion (TPP) =\r\n0.84 and True Negative Proportion (TNP) = 0.63 for occurrence of ascospore discharge,\r\nand TPP = 0.70 and TNP = 0.89 for occurrence of peaks. Globally, 68 ascospores (2.5% of\r\nthe total ascospores sampled) were trapped on the 17 days when no ascospores were erroneously\r\npredicted. When a discharge occurred, the numbers of F. graminearum ascospores\r\nsampled were predicted by a multiple regression model with R2 = 0.68. This model, which\r\nincludes average and maximum temperature and VPD as predicting variables, slightly\r\nunderestimated the real data and especially ascospore peaks. Numbers of ascospores in\r\npeaks were best predicted by wetness duration of the previous day, minimum temperature,\r\nand VPD, with R2 = 0.71. These results will help refine the epidemiological models used as\r\ndecision aids in FHB management programs.