Influence of Host Resistance on Stewart's Wilt Forecasts and Probability of Exceeding Thresholds for Use of Seed-Treatment Insecticides on Sweet Corn

Many sweet corn (Zea mays) hybrids commercially available today have higher levels of resistance to Stewart's disease (caused by Pantoea stewartii subsp. stewartii) than the cultivars from which Stevens developed the first forecast of this disease in the 1930s. Incorporating levels of host resistance into forecasts of the seedling wilt phase of Stewart's disease (i.e., Stewart's wilt) could improve control decisions for sweet corn which are made prior to planting. Incidence of systemic infection of seedlings was assessed on 27 sweet corn hybrids with a range of reactions to P. stewartii. In total, 741 observations were collected from 1998 to 2009 in 79 field trials at 15 locations throughout Illinois and one each in Kentucky and Delaware. Relative frequency distributions of the incidence of systemic Stewart's wilt were developed for combinations of hybrids with different levels of resistance and ranges of winter temperature from Stewart's wilt forecasts. The probability of exceeding thresholds of 1 or 5% incidence that warrant the use of seed-treatment insecticides on sweet corn grown for fresh market or processing, respectively, was determined from these frequency distributions. Levels of host resistance affected the incidence of systemic seedling wilt within ranges of winter temperatures used by Stewart's wilt forecasts. For moderate and resistant hybrids, frequency distributions of Stewart's wilt incidence and mean incidence ranging from 0.7 to 1.8% did not differ among three winter temperature ranges above –2.8°C. Conversely, distributions of Stewart's wilt incidence on susceptible hybrids differed among each of the four ranges of winter temperature from the Stevens-Boewe forecast (i.e., >0.6, –1.1 to 0.6, –2.8 to –1.1, and <–2.8°C), with mean incidence ranging from 0.5 to 8.5%. Occurrence of Stewart's wilt also differed among trials varying in number of winter months above –4.4°C, the criterion used by the Iowa State forecast of this disease. Levels of host resistance to P. stewartii also affected the occurrence of Stewart's wilt as predicted by the Iowa State method. The probability of exceeding economic thresholds of 1 or 5% incidence of systemic Stewart's wilt depended on levels of host resistance and winter temperature. Stewart's wilt is unlikely to exceed economic thresholds when the mean winter temperature is below –4.4°C. When mean winter temperature was above –2.8°C, the probability of exceeding 1% incidence of systemic Stewart's wilt was 0.59 for susceptible sweet corn hybrids and 0.28 for moderate and resistant hybrids. When mean winter temperature was below –2.8°C, the probability of exceeding 1% incidence of systemic Stewart's wilt was 0.22 for susceptible hybrids and 0.04 for moderate or resistant sweet corn hybrids. The probability of exceeding 5% incidence was less than 0.1, except when the mean winter temperature was above –2.8°C and susceptible hybrids were grown.