Control of Stewart’s Wilt in Sweet Corn with Seed Treatment Insecticides

Corn flea beetles, Chaetocnema pulicaria, vector Erwinia stewartii (synamorph Pantoea stewartii), which causes Stewart’s bacterial wilt of corn (Zea mays) (10,11). Flea beetles introduce E. stewartii into plant tissues that are wounded during feeding. Movement of E. stewartii in the vascular system of resistant plants is restricted to within a few centimeters of flea beetle feeding wounds. The bacterium may move systemically throughout the vascular system of susceptible plants. The seedling wilt phase of Stewart’s wilt can substantially reduce yield of susceptible or moderately susceptible sweet corn hybrids and dent corn inbreds (8–10,12). Yield is rarely affected when resistant or moderately resistant hybrids are infected after the 3to 5-leaf or 5to 7-leaf stages, respectively (9,12). When plants are infected prior to the 3-leaf stage (i.e., three fully open leaves with visible leaf collars), main stalks may be killed, causing tillers to grow profusely (8). Ears are not produced on these plants. Main stalk death probably results from movement of E. stewartii to the growing point, which may be only a few centimeters from infection sites during very early growth stages. Main stalk death occurs even in the most resistant sweet corn hybrids, although the incidence of main stalk death appears to decrease with increasing levels of resistance (8). The growth stage at which resistance begins to effectively control Stewart’s wilt by restricting the movement of E. stewartii is not known precisely. Resistance may not prevent systemic infection (i.e., movement of E. stewartii throughout the plant and occurrence of symptoms on new leaves) or main stalk death if flea beetles feed prior to the 2or 3-leaf stage on tissues close to the growing point. In preliminary greenhouse trials, a high proportion of moderately resistant plants inoculated in the whorl at the 1or 2-leaf stage became systemically infected (N. D. Freeman and J. K. Pataky, unpublished). Symptoms were restricted to within a few centimeters of inoculation wounds when moderately resistant and resistant plants were infected at later growth stages. Insecticides applied in-furrow at planting reduced flea beetle populations and controlled Stewart’s wilt better than insecticides applied to foliage (1,4). A seed treatment insecticide, imidacloprid (Gaucho), killed flea beetles and reduced the number of feeding wounds and Stewart’s wilt symptoms per leaf in greenhouse studies (7). If seed treatment insecticides kill flea beetles before E. stewartii is transmitted to plants, ABSTRACT Pataky, J. K., Michener, P. M., Freeman, N. D., Weinzierl, R. A., and Teyker, R. H. 2000. Control of Stewart’s wilt in sweet corn with seed treatment insecticides. Plant Dis. 84:11041108.