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.
Nomenclature: Glyphosate; nicosulfuron; johnsongrass, Sorghum halepense (L.) Pers. #3 SORHA; corn, Zea mays (L). ZEAMX.
Abbreviations: DAT, days after treatment; MCD, maize chlorotic dwarf; MDM, maize dwarf mosaic; MCDV, maize chlorotic dwarf virus; MDMV, maize dwarf mosaic virus; POST, postemergence; SS, Southern States; WAT, weeks after treatment; WF, weed-free.