Levels of Stewart's wilt resistance necessary to prevent reductions in yield of sweet corn hybrids

Stewart's wilt reactions and yield of a total of 69 sweet corn hybrids were evaluated in trials in 1999 and 2000 in order to determine the level of Stewart's wilt resistance necessary to prevent reduction in yield of sweet corn hybrids. Plants at the 2- to 3-leaf stage were inoculated with Erwinia stewartii using the pinprick method. Stewart's wilt symptoms were rated from 1 to 9, and incidence of systemic infection was determined as a percentage for each experimental unit. Primary ears were harvested about 21 days after midsilk, and yield was measured as ear weight and number of marketable ears. Percent yield was calculated for each hybrid by dividing yield from inoculated treatments by yield from noninoculated treatments and multiplying by 100. Hybrid means for Stewart's wilt ratings in inoculated treatments ranged from 2.0 to 7.3. The relationships between percent yield and Stewart's wilt severity ratings were described best by curvilinear regressions, whereas percent yield decreased linearly with the incidence of systemic infection in 2000. Ear weights of hybrids with ratings below 3 or 3.5 were estimated from the regression equations to be within 95 or 92%, respectively, of those from noninoculated treatments of the same hybrid. The number of marketable ears from hybrids with ratings of 3 or below was estimated from the regression equations to be within 90% of those from noninoculated treatments of the same hybrid. A level of resistance that resulted in Stewart's wilt ratings below 3 or 3.5 corresponded to nonsystemic infection of most plants (i.e., incidence of systemic infection below 5 or 10%, respectively). The adverse effects of Stewart's wilt on ear weight and marketability appeared to be minor for sweet corn hybrids with levels of resistance that prevented or minimized systemic infection.