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

Abstract

<p>Stewart's disease, caused by <i>Pantoea stewartii </i> (Smith), can cause severe economic damage to seed nd 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 <i>P. stewartii</i> 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 <i>P. stewartii</i> isolates, growth chamber experiments were conducted using the sweet corn variety Jubilee. The aggressiveness of 13 <i>P. stewartii</i> 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 <i>P. stewartii</i>: 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^y C, 24^y C, 27^y C, 30^y C, and 33^y C). To study the effect of isolates, growth chamber experiments were conducted at what was found to be the optimal temperature (30^y C). Sweet corn plants (variety "Jubilee") were inoculated at the V8 growth stage with 12 wild-type <i>P. stewartii</i> isolates and Rif 9A. Both sides of the mid-rib of 4 leaves per plant were inoculated with a single isolate of <i>P. stewartii</i>. There were 5 corn plants per isolate and 65 plants per replication. Experiments were performed twice. Lesion expansion (both acropetally and basipetally), were measured beginning on the day that lesions were first visible, and continued at 24-hour intervals until leaves were 100% senesced. The incubation period was shortest at 30^y C, and rate of lesion expansion (averaged over isolates) was also fastest at this temperature. There were only small differences among isolates on components of aggressiveness. Of the 3 leaf positions (leaf ages) tested, the pathogen was least aggressive on the fifth true leaf. Isolate ES 0601 expanded significantly faster than other isolates. In these experiments, <i>P. stewartii</i> was most aggressive on the seventh and eighth leaf positions and least aggressive on the fifth leaf. To quantify the temporal and spatial spread of this pathogen, field experiments were conducted in 2007 in Ames, IA. Seeds (variety Jubilee) treated with insecticide (Cruiser) and seeds not treated with insecticide were planted in field plots (8 rows x 91 cm), with 25 plants per row planted at 30 cm spacing. The center six plants in each plot were inoculated with a double-marked rifampicin and nalidixic acid resistant isolate of <i>P. stewartii</i> to (Rif 9A). Feeding scars caused by the primary insect vector (corn flea beetle) were sampled from each field plot every 6 to 10 days, beginning 37 days after inoculation and continuing until 16 August. The double-marked isolate of <i>P. stewartii</i> was first detected in field samples on 13 July. Incidence of <i>P. stewartii</i>-infected plants was 4% in seed-treated plots compared to 4% to 6.7% in non-treated plots. By the end of the season, mean incidence for the insecticide-treated plots was 14.2 y 3.15%, and mean incidence within non-treated plots was 18.5 y 1.92%. The exponential model best fit the <i>P. stewartii</i> progress curves, but there was no statistical difference between the mean rate of plant-to-plant spread over time in nontreated versus insecticide seed-treated plots (0.0105y 0.0010 log units per day versus 0.0089 y 0.0019 log units per day, respectively). The spatial pattern of <i>P. stewartii</i>-infected plants (based on ordinary runs analysis) was random for all plots and all sampling times.</p>