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 St ewart's wilt resistance necessary to prevent reduction in yield of sweet co rn hybrids. Plants at the 2- to 3-leaf stage were inoculated with Erwinia s tewartii 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 afte r midsilk, and yield was measured as ear weight and number of marketable ea rs. Percent yield was calculated for each hybrid by dividing yield from ino culated treatments by yield from noninoculated treatments and multiplying b y 100. Hybrid means for Stewart's wilt ratings in inoculated treatments ran ged from 2.0 to 7.3. The relationships between percent yield and Stewart's wilt severity ratings were described best by curvilinear regressions, where as percent yield decreased linearly with the incidence of systemic infectio n in 2000. Ear weights of hybrids with ratings below 3 or 3.5 were estimate d from the regression equations to be within 95 or 92%, respectively, of th ose from noninoculated treatments of the same hybrid. The number of marketa ble cars from hybrids with ratings of 3 or below was estimated from the reg ression equations to be within 90% of those from noninoculated treatments o f the same hybrid. A level of resistance that resulted in Stewart's wilt ra tings below 3 or 3.5 corresponded to nonsystemic infection of most plants ( i.e., incidence of systemic infection below 5 or 10%, respectively). The ad verse effects of Stewart's wilt on ear weight and marketability appeared to be minor for sweet corn hybrids with levels of resistance that prevented o r minimized systemic infection.