Stalk rots are among the most serious diseases of corn, causing yield loss through reduced grain fill, premature death, and lodging. Because of the interactions between stalk-boring insects and stalk rot fungi, Bt hybrids have the potential to provide a side benefit in terms of reduced stalk rot. But stalk rot fungi enter corn plants through several different pathways, so the influence of European corn borers on stalk rot can range from none to great, depending on the specific field conditions. For the last 2 years we have evaluated stalk rot in Bt hybrids and near-isogenic non-Bt hybrids in Ames and other locations. In December, Dale Farnham reported yield performance for Bt hybrid trials conducted with ISU Extension field specialists throughout the state. In several of those locations, data also were collected on stalk rot. Corn borer infestations in those plots were naturally occurring and were not severe. In Ames, we conducted a trial designed to measure the influence of Bt hybrids on ear rot and stalk rot diseases. This experiment had much smaller plots, and we manually infested half of them with European corn borer larvae (with the cooperation of USDA entomologist Rick Hellmich). We included five pairs of hybrids representing five Bt events (CBH351, DBT418, MON810, BT11, and 176): Garst 8600 and 8600Bt, DeKalb DK566 and DK566Bt, Pioneer hybrids 3489 and 34R06, NK 4640 and 4640Bt, and NK 4349 and MAX21.

In the Ames experiment, most of the Bt hybrids had a lower incidence of stalk rot than their near-isogenic hybrids, but the differences were not always statistically significant (see graph). Stalk rot incidence was greater in the plots that were manually infested, and the differences between Bt and non-Bt hybrids were greater in these plots. Yields, measured as average ear weight, were not significantly greater for any of the Bt hybrids in this experiment compared with the near-isogenic hybrids.

In the other plots, stalk rot was measured as the average inches of stalk discoloration. Stalk rot was mild in most of these plots, and there were no significant differences between Bt and non-Bt hybrids (Table 1). Yield data from these plots were presented in the December 1998 ICM newsletter, and yield differences were very small as well. With light corn borer pressure in most of the locations, large differences between Bt and non-Bt hybrids would not be expected.

Obviously the relationships among stalk rots, European corn borers, and Bt hybrids are not clear-cut. We are conducting further experiments to evaluate the influence of different Bt events on stalk rot development, including possible influences of Bt transformation on the fungal species composition in stalk rot infections.

Table 1. Average stalk rot (inches of discoloration) from Bt and non-Bt corn hybrids in 10 counties.