Control of Stewart's wilt in sweet corn with seed treatment insecticides

Corn flea beetles, Chaetocnema pulicaria, vector Erwinia stewartii (synamor ph Pantoea stewartii), which causes Stewart's bacterial wilt of corn (Zea m ays). A seed treatment insecticide, imidacloprid, killed flea beetles and r educed the number of feeding wounds and Stewart's wilt symptoms per leaf in greenhouse studies. The objective of our research was to evaluate the abil ity of imidacloprid and thiamethoxam seed treatments to control Stewart's w ilt on sweet corn hybrids under field conditions with naturally occurring p opulations of the corn flea beetle. Six field trials were planted at four l ocations in 1998. Eleven field trials were planted at nine locations in 199 9. The treatment design was a factorial of sweet corn hybrids and seed trea tments. Stewart's wilt incidence ranged from 0 to 54% in the 1998 trials. I ncidence of Stewart's wilt in nontreated plots of the susceptible hybrid Ju bilee ranged from 2% at the 8-leaf stage to 77% 1 week after mid-silk in th e 1999 trials. Seed treatment insecticides reduced the incidence of Stewart 's wilt by approximate to 50 to 85% relative to nontreated controls. The le vel of control was approximate to 75 to 85% in seven trials planted before 1 June 1999, when incidence of Stewart's wilt on nontreated Jubilee ranged from 4 to 71%. The level of control was approximate to 50 to 70% in the thr ee trials planted after 1 July 1999, when incidence of Stewart's wilt on no ntreated Jubilee ranged from 44 to 73%. Although comparisons varied, the le vel of control gained from seed treatment insecticides was similar to the n ext higher level of host resistance. Seed treatment insecticides appear to control Stewart's wilt during very early growth of corn plants, when foliar applications of insecticides are ineffective and the effectiveness of host resistance varies depending on the proximity of flea beetle feeding sites to the plant's growing point.