Pathogenesis, Epidemiology and Disease Forecasting

Epidemiology researchers based at PA, OH, IN, ND, and SD land grant universities are collaborating to provide growers and agricultural industry with timely and reliable disease forecasts for Fusarium head blight (FHB). Knowledge about sources of inoculum, inoculum levels, and weather conditions favorable for FHB development is crucial in devising a handy and reliable disease forecaster. The effect of inoculum levels was studied in ND in 2003 and 2004. Two FHB susceptible hard red spring wheat cultivars, Oxen (an early flowering cultivar) and Granite (a late flowering cultivar) were sown on May 1 and 10 in 2003, and Argent hard white spring wheat (early flowering cultivar) and Granite were sown on April 30 and May 7 in 2004, in a field plot located at the NDSU Agricultural Experiment Station, Fargo. The previous years’ crops were dry bean in 2003 and crambe in 2004. Three inoculum levels (zero inoculum; low = 38g/m2; high =100g/m2) were applied, using G. zeae infested corn kernels, at the 6-leaf stage in all treatments. The experimental design was split plot, randomized complete block. Main plots were inoculum levels, sub-plots were planting date, and subsubplots were cultivar. Strips, 30 ft wide, of Alsen wheat (moderately resistant to FHB) were planted between subplots and main plots, to serve as buffers. The strips of Alsen were free of inoculum. In both years, the G. zeae population from each inoculum treatment was monitored daily from Feekes growth stage 8 (early flag leaf emergence) to Feekes 11.2 (soft dough) for air sampling, and from Feekes stage 10 (boot stage) to Feekes 11.2 for head washings. One hundred-fifty wheat heads from each inoculum treatment were monitored 3x a week 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 disease incidence was significantly different among the inoculum levels in both years. The disease incidence range was 5 to 9 % in 2003 and 19 to 40 % in 2004. The majority (>95%) of the plants began and ended flowering in 4-5 days on both planting dates and in all four flowering dates. In both years, high inoculum levels generally resulted in increased number of G. zeae colony units (CFU) recovered from both head washings and air sampling. The results indicate that, under favorable weather for FHB, inoculum levels of G. zeae 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, a crucial stage for infection, in 4 to 5 days. It appears that incorporating information about local sources and levels of pathogen inoculum may increase disease forecasting model performance.