Fusarium ear rot and fumonisins in maize kernels when comparing a Bt hybrid with its non-Bt isohybrid and under conventional insecticide control of Busseola fusca infestations

Maize production in South Africa is negatively affected by Fusarium verticillioides, an endophytic maize pathogen, as well as by Busseola fusca larval damage. Fusarium verticillioides causes ear, stem and root rot, and also produces fumonisin mycotoxins which are toxic to humans and livestock. The African stem borer (Busseola fusca) is a pest of economic importance in maize plants in South Africa. In this study, the interaction between F. verticillioides and B. fusca was investigated to elucidate its effects on Fusarium ear rot and fumonisin production in a Bt hybrid (MON810 event) and its B. fusca-susceptible non-Bt isohybrid. Field trials were conducted over three seasons using a randomised complete block design with six replicates per treatment. The effect of Beta-cyfluthrin (non-systemic, granular) and Benfuracarb (systemic, seed treatment) insecticide applications on the incidence of Fusarium ear rot and fumonisin production in maize was also determined in an unrelated conventional hybrid. For B. fusca infestations, larvae were dispensed into the whorl of each plant at the 12th leaf stage prior to tasselling, while a F. verticillioides MRC826 spore suspension was inoculated through the silks at the silking stage. Maize ears were harvested at physiological maturity and Fusarium ear rot, total fumonisin levels, stem borer damage and target DNA of fumonisin-producing Fusarium spp. quantified. Significantly less Fusarium ear rot and fumonisin were produced in the Bt maize hybrid compared to the non-Bt isohybrid under natural farming conditions, but fungal colonisation and fumonisin production under artificial F. verticillioides inoculation did not differ significantly between the Bt and non-Bt maize. Fumonisin production correlated moderately with the quantity of target DNA of fumonisin-producing Fusarium spp. extracted from maize plants. Benfuracarb application to control stem borer infestation resulted in a significant reduction in Fusarium ear rot and fumonisin production while Beta-cyfluthrin did not. Moreover, B. fusca damage to maize ears significantly increased when both insecticides were not applied to the B. fusca-infested plants. This study indicated that Bt maize and the application of Benfuracarb reduce B. fusca damage to maize ears thereby indirectly reducing Fusarium ear rot and fumonisin production. However, this was not consistent over seasons due to differences in climatic conditions.