Use of geospatially-referenced disease and weather data to improve site-specific forecasts for Stewart's disease of corn in the US corn belt

Stewart's disease of corn, caused by the bacterium Pantoea stewartii (syn. Erwinia stewartii), is important because its presence within seed corn fields can prevent the export of seed corn due to quarantine (phytosanitary) restrictions. Phytosanitary field inspection reports for 1972-1999 (500 to over 1300 fields/year), and mean monthly temperatures for December, January, and February preceding each inspection year, were geospatially referenced by county using ArcView geographic information systems (GIS) software. Warmer winter temperatures during December, January, and February generally allow greater numbers of the insect vector (corn flea beetles, Chaetocnema pulicaria) to survive, thereby increasing the risk of Stewart's disease epidemics due to higher levels of initial inoculum (infested beetles) that will be present during the ensuring growing season. Weather-based models to accurately predict the risk of Stewart's disease in the U.S. Corn Belt, however, have been developed. Using GIS software, risk maps were created to graphically depict seasonal, regional, and county risks for Stewart's disease. Using this approach, we found that counties with mean monthly temperatures < -4.4 °C (24 F) for at least 2 of the 3 winter months had zero to low risk of Stewart's disease (< 2% prevalence), while counties with mean monthly temperatures ≥ -4.4 °C for at least 2 or 3 winter months had a moderate to high risk, respectively. Using individual monthly mean temperatures for December, January, and February (Iowa State Method), we accurately predicted that 1998 and 1999 would be high risk years for Stewart's disease in Iowa. This study provides the basis to more accurately assess the seasonal and site-specific risks associated with the occurrence of Stewart's disease several months prior to planting. This advance warning will help seed corn producers to make more informed disease management decisions, such as the choice of (low risk) planting sites (which will minimize or eliminate the use of insecticides). Just as importantly, advanced warnings for high risk planting sites would alert seed corn producers that insecticides will be required to minimize the risk of Stewart's disease for high risk seasons and sites.