Etiology, Epidemiology and Disease Forecasting

Knowledge of host resistance, inoculum levels, and weather conditions favorable for disease development is necessary to optimize a disease forecaster. A group of plant pathologists from five land-grant universities (North Dakota, Ohio, Pennsylvania, Purdue, and South Dakota) have collaborated to develop and improve performance of a disease forecasting system for Fusarium Head Blight (FHB). The main goal of this study was to determine the effect of host resistance, inoculum levels, and fungicide on FHB development in spring wheat. The experiment was conducted at the NDSU Agricultural Experiment Station, Fargo, ND. The experimental design was a split-split plot with three replications; inoculum levels (n = 2), fungicide treatment (1), and cultivars (3) as main plots, sub-plots, and sub-sub plots, respectively. The previous year crop was soybean. The plots with inoculum were created by distributing corn kernels infested with F. graminearum in the plots at the 6-leaf stage. Two FHB susceptible cultivars, Argent (hard white spring wheat and early flowering) and Granite (hard red spring wheat and late flowering), and one FHB resistant cultivar, Alsen (hard red spring wheat), were selected and planted on April 29, 2005. Alsen also was planted between main plots and sub-plots at 20 ft wide to serve as buffers. The buffer strips were free of inoculum. Trizole fungicide “Folicur” (@ 4 fl oz/acre) was applied to one sub-plot of each cultivar in each replicate when cultivars Alsen and Argent were at flowering (Feekes GS 10.51-10.52). The G. zeae population from each inoculum treatment was monitored daily from Feekes growth stage 8 (early flag leaf emergence) to Feekes GS 11.2 (soft dough) by collecting spores from air, and from Feekes GS 10 (boot stage) to Feekes 11.2 by head washings. Additionally, 90 wheat heads of each cultivar were monitored daily from Feekes scale GS 10 for growth synchrony. The disease incidence (number of infected head/total number of heads examined) and head severity (% of individual infected head) data were recorded in all treatments. FHB incidence was significantly (P<0.0001) different among the inoculum levels. The disease incidence and severity ranged from 21 to 51%, and 10 to 32%, respectively. Fungicide application significantly decreased disease severity and increased the seed test weight. The cultivars exhibited significant differences in FHB severity. The disease severity was significantly (P< 0.0001) lower (8 to 11%) in the FHB resistant Alsen than in the susceptible Argent (25 to 33%). Both fungicide application and cultivars had little or no effect on the disease incidence. As expected, air samples collected from high inoculum level and low inoculum level plots resulted in high number (range = 20-134) and low number (7-65) of G. zeae colony forming units (CFU) in 20 out 21 days of the samples, respectively. The majority (>97%) of the plants began and ended flowering in 3-4 days in both early and late flowering cultivars. These results indicate that favorable weather conditions for FHB, inoculum levels of G. zeae, level of host resistance, and fungicide application may have a significant role in disease development. Also, the fungus has a small window of opportunity to infect wheat heads, as the majority of the plants completed flowering within 3-4 days, a crucial stage for infection.