Field and laboratory experiments were conducted to evaluate the effect of postemergence johnsongrass control on incidence of maize chlorotic dwarf virus (MCDV) and maize dwarf mosaic virus (MDMV) in corn and to confirm the presence and movement of the blackfaced leafhopper, the insect vector of MCDV. Corn plants surrounded by MCDV- and MDMV-infected rhizomatous johnsongrass were either treated or not treated with nicosulfuron at 35 g ai/ha. Corn tissue samples were taken at the time of treatment and 4, 9, 14, and 21 d after treatment and the presence of MCDV and MDMV was determined by enzyme-linked immunosorbent assay (ELISA). Virus incidence in treated experimental units was higher at the later sampling dates relative to the nontreated. Earlier differences in incidence of MCDV and MDMV double infection in corn were detected where johnsongrass was controlled. Movement of the insect vector of MCDV was observed within the experimental area after johnsongrass was controlled, but was not significantly different from that in nontreated areas.

Field experiments were conducted in 2000 and 2001 in Virginia to evaluate the incidence and severity of maize chlorotic dwarf virus and maize dwarf mosaic virus in response to postemergence (POST) johnsongrass control in two corn hybrids. Previous research demonstrated the increased disease severity in virus-susceptible corn hybrids as an indirect effect of POST johnsongrass control. The increased disease severity resulted from greater transmission by insect vectors, which moved from dying johnsongrass to the crop. Recent observations have indicated a lack of virus tolerance in glyphosate-tolerant corn hybrids commercially available in Virginia. A transgenic glyphosate-tolerant hybrid and a nontransgenic virus-tolerant hybrid, similar in growth characteristics and maturity, were subjected to POST treatments of nicosulfuron, whereas the glyphosate-tolerant hybrid was also treated with glyphosate. Both nicosulfuron and glyphosate, broadcast or directed, provided essentially complete johnsongrass control, although initial johnsongrass control was greater with glyphosate treatments. Little or no disease incidence occurred in the virus-tolerant hybrid. With the virus-susceptible hybrid, significant increases in disease incidence were observed in response to any herbicide treatment applied to johnsongrass-containing plots relative to the same treatment applied to weed-free plots. Johnsongrass control with nicosulfuron or glyphosate caused similar disease incidence and severity in this hybrid, regardless of application method. Results of these experiments indicated that growers' choice of hybrid genetics should focus primarily on disease resistance rather than on herbicide resistance in fields that are infested with johnsongrass.