Studies on sweet corn: Stewart's wilt forecasting, the effect of maize dwarf mosaic on foliar diseases, and herbicide sensitivity
Diseases and sensitivity to P450-metabolized herbicides can limit the production of high quality sweet corn. Separate studies were done to determine the probability of exceeding 1% and 5% incidence of Stewart’s wilt on sweet corn with different reactions to Pantoea stewartii, the effect of maize dwarf mosaic (MDM) on eight foliar diseases, and the effect of hybrid CYP genotype on sweet corn yield following postemergence applications of mesotrione or nicosulfuron. Stewart’s disease can be forecasted using the Stevens, Stevens-Boewe, or Iowa State forecasts. These forecasts assume a high degree of host susceptibility. Host resistance affects the incidence and severity of disease, and therefore can affect the accuracy forecasts of Stewart’s disease. Levels of host resistance affected the incidence of systemic seedling wilt within ranges of winter temperatures used by each of the forecasts. Frequency distributions of Stewart’s wilt incidence on moderate and resistant hybrids did not differ among the three temperature ranges above -2.8ºC (27ºF). 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., >0.6ºC, -1.1º to 0.6ºC, -2.8º to -1.1ºC, and <-2.8ºC). The probability of exceeding the 1% and 5% incidence thresholds that warrant the use of seed treatment insecticides also differed among hybrids with susceptible, moderate, or resistant reactions to Stewart’s wilt. The probability of exceeding 1% incidence of systemic Stewart’s wilt was greater than 0.5 on moderately susceptible to susceptible sweet corn hybrids when mean winter temperature was above -2.8ºC (27ºF). When mean winter temperature was below -2.8ºC (27ºF), using a seed treatment insecticide on moderate and resistant hybrids for Stewart’s wilt control was not economical because the probability of exceeding 1% incidence of systemic Stewart’s wilt was about 0.04. The probability of exceeding 5% incidence was less than 0.1 except when the mean winter temperature was above -2.8ºC (27ºF) and a moderately susceptible to susceptible hybrid was grown. A separate study also was done to evaluate the effect of MDM on eight foliar diseases of sweet corn. MDM-infection substantially increased the severity of five diseases including: southern corn leaf blight (SCLB), northern corn leaf spot (NCLS), gray leaf spot (GLS), Diplodia leaf streak (DLS), and eyespot. Among MDM-susceptible hybrids, mean severity ratings of SCLB, NCLS, GLS, DLS, and eyespot on MDM-infected plants typically were double those of virus-free plants. Three diseases not substantially increased by MDM-infection included: common rust, northern corn leaf blight, and Stewart’s wilt. MDM-infection appears to affect the severity of diseases caused by necrotrophic foliar fungi that colonize mesophyll tissue. MDM-infection did not appear to substantially affect diseases caused by pathogens which form haustoria or invade the vascular system. The extent to which SCLB severity is increased by MDM in terms of changes in level of host resistance also was determined. For MDM-susceptible hybrids, reactions to SCLB ranged from resistant to moderately-susceptible in virus-free treatments, but each of these hybrids was classified as moderately susceptible to susceptible when infected with MDM. Reactions to SCLB did not differ appreciably among viral treatments for MDM-resistant hybrids, ranging from moderately-resistant to moderately-susceptible. An additional study was done to determine if hybrid genotype at a major cytochrome P450 (CYP) locus on chromosome 5S affected yield following postemergence applications of mesotrione (HPPD-inhibiting herbicide) or nicosulfuron (ALS-inhibiting herbicide) at two growth stages. Yield of hybrids with two mutant cyp alleles (i.e., cypcyp) was reduced by applications of mesotrione. These hybrids were not evaluated for nicosulfuron because ALS-inhibiting herbicides kill these hybrids. Yield of hybrids heterozygous for mutant and functional CYP alleles (i.e., CYPcyp) were reduced by nicosulfuron but unaffected by mesotrione. Yield of hybrids with two functional CYP alleles (i.e., CYPCYP) were unaffected by mesotrione or nicosulfuron. Results of this experiment demonstrate that the genetic condition of hybrids at the CYP locus on chromosome 5S affects the yield of sweet corn following application of mesotrione or nicosulfuron at the V3 to V5 or the V5 to V7 growth stages.
Sweet corn, Zea mays, corn diseases, Maize dwarf mosaic, Stewart's wilt, herbicide sensitivity, yield, cytochrome P450 (CYP), CYP genes
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign