Quantifying the aggressiveness, temporal and spatial spread of <i>Pantoea stewartii </i> in sweet corn

Stewart’s disease, caused by Pantoea stewartii (Smith), can cause severe economic damage to seed and sweet corn crops due to phytosanitary regulations that prevent the export of seed corn, as well as causing direct reductions in yield. To date, there are no quantitative data on the aggressiveness of P. stewartii isolates at the plant scale, nor on the temporal and spatial spread of this pathogen in the field. To quantify and compare the effect of temperature, leaf position (age) and isolate on the aggressiveness of P. stewartii isolates, growth chamber experiments were conducted using the sweet corn variety Jubilee. The aggressiveness of 13 P. stewartii isolates was quantified and compared by measuring incubation period, rate of lesion expansion/day, and time to leaf death. To study the effect of temperature, plants at the V7 growth stage were inoculated with the following isolates of P. stewartii: Rif 9A, a rifampicin-naldixic acid resistant isolate; and either ES 9211 or ES 9245 (both wild types). This was done by inoculating single leaves with 2 of the 3 isolates, one isolate on each side of the leaf midrib. There were 24 plants/ replication, and experiments were performed twice at each of 5 temperatures (21° C, 24° C, 27° C, 30° C, and 33° C). To study the effect of isolates, growth chamber experiments were conducted at what was found to be the optimal temperature (30 ̊ C). Sweet corn plants (variety "Jubilee") were inoculated at the V8 growth stage with 12 wild-type P. stewartii isolates and Rif 9A. Both sides of the midrib of 4 leaves per plant were inoculated with a single isolate of P. stewartii. There were