Spread of Maize Dwarf Mosaic Virus from Johnsongrass to Corn

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Abstract
Spread of maize dwarf mosaic virus (MDMV) from introduced virus-infected johnsongrass to adjacent susceptible corn in experimental plots was evaluated during 1979 and 1980. The relationship between MDM incidence in corn and distance from the source was adequately described by the model Y = a(exp(-bD)), where Y is disease incidence at distance D from the source, b is the spread coefficient, and a is the scaling factor. In both years, b was significantly greater than zero, demonstrating that MDMV spread to the corn test plots. In control plots with no intentionally placed virus source, b values and disease incidence were lower than in test plots. For a single planting in 1979, the steepness of the gradient of disease incidence from the source (quantified by b) decreased with time as fever plants remained uninfected. For 2 successive plantings in 1980, no significant difference in b values was observed. MDMV spread also was not related to prevalent wind direction. No spread of maize chlorotic dwarf virus (MCDV) to corn was observed by sympatomatology even though 90 and 50% of the johnsongrass plants were infected in 1979 and 1980, respectively, as indicated by enzyme-linked immunosorbent assay. The leafhopper vector of MCDV, Graminella nigrifrons, was also present during both years. The presence of MDMV in the control plots indicates that there were virus sources other than johnsongrass or that MDMV moved > 400 m from johnsongrass to the control plots.

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