Postemergence Control of Johnsongrass and its Effect on Maize Dwarf Mosaic Virus Incidence and Vectors in Corn. M. J. Vangessel, Former Graduate Research Assistant, Department of Crop Science, North Carolina State University, Raleigh 27695-7620. H. D. Coble, Professor, Department of Crop Science, North Carolina State University, Raleigh 27695-7620. Plant Dis. 77:613-618. Accepted for publication 6 January 1993. Copyright 1993 The American Phytopathological Society. DOI: 10.1094/PD-77-0613.

The number of alighting aphids and maize dwarf mosaic virus (MDMV, johnsongrass-strain) incidence were determined when johnsongrass was controlled with nicosulfuron or primisulfuron applied at different growth stages of corn. In 1990, more alighting aphids were found in alighting traps in weed-free plots than in weedy plots. Maize dwarf mosaic (MDM) incidence, calculated as area under the disease progress curve (AUDPC), was more prevalent when either nicosulfuron or primisulfuron was applied at the eighth compared to the fifth leaf collar stage. In 1991, more corn plants were infected with MDMV when johnsongrass remained throughout the season than when it was removed with a postemergence herbicide. Correlations between cumulative alate aphids and AUDPC were nonsignificant in both years. The proximity of infected johnsongrass to corn appeared to have a much greater impact on MDMV incidence than did the number of vectors in aphid traps. MDMV-infected johnsongrass can remain a source of inoculum for up to 7 days after the application of nicosulfuron or primisulfuron. In an aphid–host preference study (species × presence of MDMV × nicosulfuron application), apterous (wingless) Rhopalosiphum maidis continued to increase on johnsongrass not treated with nicosulfuron. However, when johnsongrass was treated with nicosulfuron, apterae numbers peaked 3–4 days after treatment, then declined. More winged (alate) R. maidis were found on corn than on johnsongrass in 1990. In 1991, fewer alate R. maidis were found on johnsongrass treated with nicosulfuron than in other treatments. The presence of systemic MDMV symptoms in corn or johnsongrass had no impact on the numbers of alate or apterous R. maidis.

Keyword(s): corn leaf aphid, senescence, Sorghum halepense (L.) Pers., and Zea mays L.