Spread of maize chlorotic dwarf virus in maize fields by its leafhopper vector, Graminella nigrifrons

Adult leafhoppers of Graminella nigrifrons, given a 2-day acquisition access period to maize chlorotic dwarf virus (MCDV), were released in the center of maize plots planted in early May (1985 and 1986) or in late June to early July (1984-1986). Disease incidence (y) was assessed at least twice after insect release and represented as the proportion of plants infected by MCDV in successive 80-cm wide annuli from the source. Disease gradients were best described by the log-logistic model, i,e., logit of y versus 1n(distance) was a straight line. The model indicated that the rate of spread was proportional to y, 1 - y, and 1/distance. The spread parameter (b), a measure of the gradient steepness and slope of the linearized model, ranged from 1.3 for the early planting in 1985 to 2.0 for the late planting in 1984. In 1984 and the early plantings of 1985 and 1986, there was little change in b over time. In the late plantings, however, b increased (indicating steeper gradients) between 14 and 21 days after release. At approximately 21 days after release, the distance at which y declined to 0.10 (10%) ranged from 124 to 525 cm. The rates of increase in y over time for the entire plots and at selected distances from the release point were measured using the apparent infection rate (r). There was no discernible effect of distance from the source on r. The r parameter consistently declined over time. Results indicate that MCDV spread can be substantial when viruliferous leafhoppers are introduced into a field of susceptible maize