Field experiments were conducted to evaluate the hypothesis that johnsongrass control in corn causes increased maize dwarf mosaic virus (MDMV) and maize chlorotic dwarf virus (MCDV) disease severity because of increased movement of insect vectors from dying johnsongrass to the corn crop. Johnsongrass control treatments included 1) broadcast POST nicosulfuron, 2) directed POST imazethapyr, 3) mechanical control, and 4) no treatment. Disease severity in both a virus-susceptible and a virus-tolerant corn hybrid was evaluated. With the virus-susceptible hybrid, greater disease severity was observed where johnsongrass was controlled in the experimental area than where johnsongrass was not controlled. Increases in disease severity were independent of the method of johnsongrass control. Corollary studies conducted on the same site verified a double infection of corn with MDMV and MCDV and documented movement of blackfaced leafhoppers, the insect vector of MCDV, subsequent to treatment of johnsongrass.
