Transgenic insect protection and maize stalk rot: a comparison of stalk rot occurrence and species composition of the stalk rot complex in Bt and non-Bt hybrids

A chronic and widespread threat to maize production, stalk rots are characterized by premature death of plants, lodging and reduced grainfill. The stalk rot complex is comprised of several fungal pathogens, including Gibberella zeae, Colletotrichum graminicola, and Fusarium verticillioides. Larvae of the European corn borer (ECB) (Ostrinia nubilalis) promote stalk rot by creating entry points for fungi, serving as vectors of certain pathogens, and causing physiological stress that predisposes plants to infection. Field experiments were conducted in 1998, 1999, and 2000 to determine whether the use of transgenic Bt hybrids expressing insecticidal proteins could influence stalk rot occurrence and fungal species composition of stalks. Five hybrids representing different Bt events were paired with their non-Bt counterparts and subjected to treatments of manual and natural infestation with ECB larvae. Stalks were evaluated for discoloration, pith disintegration (stalk rot), and lodging. Stalk tissue samples were obtained for species composition analysis. We found significant effects of Bt hybrids on stalk discoloration, pith disintegration, and lodging, but these effects depended upon the infestation treatment and the specific Bt event. In 1998, significant reductions in stalk rot symptoms and lodging were observed in some Bt hybrids compared to their non-Bt counterparts. Similar effects of Bt event were observed in 1999. In 2000, the amount of stalk discoloration and lodging was higher in some Bt events compared to their non-Bt counterparts, while the amount of pith disintegration remained lower in event BT11 compared to its non-Bt counterpart. These results suggest that whether Bt hybrids contribute to or hinder stalk rot management depends on the specific Bt event used. The species composition of the stalk rot complex differed significantly between Bt and non-Bt hybrids. As predicted by the vector relationship between ECB larvae and Fusarium species, the proportion of stalks infected with F verticillioides was lower in Bt hybrids. In 1999 and 2000, the proportion of stalks infected with G. zeae was higher in some Bt hybrids than in their non-Bt counterparts. By removing an infection pathway (ECB injury) that favors certain pathogens, Bt hybrids may be altering the fungal species composition in maize stalks