Busseola fusca x Fusarium verticillioides interaction in maize

EDSON NCUBE AND BRADLEY FLETT, ARC-GRAIN CROPS INSTITUTE, POTCHEFSTROOM

Busseola fusca (B. fusca) (Photo 1) is the most injurious stem borer of maize in South Africa and occurs at altitudes ranging from sea level to 2 000 m above sea level. It is widespread throughout the maize production triangle of South Africa.

Life cycle of B. fusca

Busseola fusca can have three moth flights per season. Moths are pale brown (Photo 2) with a wingspan of 35 mm -
40 mm. Female moths lay eggs behind leaf sheaths that hatch within three to five days into larvae. Larvae pupate which result in the next generation of moths. The life cycle of B. fusca can be between seven and eight weeks depending on favourable conditions existing.

Fusarium verticillioides naturally occurs on maize in South Africa. Fusarium verticillioides causes seedling blights as well as ear, stalk and root rots of maize.

All these symptoms reduce yield and ear rots also reduce the grain quality of maize. Other Fusarium spp. which cause Fusarium ear rot and naturally occur in South Africa, include F. subglutinans and F. proliferatum. Fusarium verticillioides and F. proliferatum produce fumonisins, mycotoxigenic secondary metabolites that are probable cancer-promoting toxins. High levels of fumonisins have been associated with oesophageal cancer in humans, hole-in-the-head disease of horses and liver cancer in rats.

Life cycle of F. verticillioides

Fusarium verticillioides survives in buried maize stalks as thickened hyphae in moist soils. The soil borne hyphae germinate and infect the germinating seed and roots and move up the plant through systemic growth. The fungus also produces macro conidia and airborne micro conidia which infect maize ears during early silking.

The mode of kernel infection by airborne spores of F. verticillioides is both through systemic infections from contaminated seed as well as through the silk channel. Silk colonisation by F. verticillioides occurs and follows the silk down until the fungus infects the developing kernel.

Infection is enhanced by late-season rainfall as well as the physiological state of the silks after pollination. Direct invasion of kernels can also occur through weak points, such as stress cracks in the pericarp and through the pedicel. The fungus is released back to the soil via infected maize residues.

Stem borers damage maize stems and ears which provide another infection mechanism by airborne or rain-splashed F. Verticillioides spores. In addition, stem borers also act as vectors for F. verticillioides.

The B. fusca x F. verticillioides interaction is not adequately understood. It is unknown whether B. fusca is attracted to, or survive longer, on plants infected with F. verticillioides, and whether B. fusca infestations increase fumonisin contamination of grain under South African grain production conditions.

Genetically modified maize (Bt maize) has become an important tool to control stem borers over the past decade in South Africa. However, higher fumonisin concentrations in non-Bt (maize without Bt) hybrids have been associated with high levels of European corn borer (Ostrinia nubilalis) infestations on maize in the USA and Europe, which can also be the case with B. fusca in South Africa.

Studies are currently underway to elucidate the B. fusca x F. Verticillioides interaction and to investigate the effects this interaction mayhave on Fusarium ear rot and fumonisin production in both Bt- andnon-Bt maize hybrids in South Africa.

Preliminary results indicate that B. fusca interacts with F. Verticillioides resulting in an increase in Fusarium ear rot
(Photo 3 and Photo 4) andfumonisins in non-Bt maize compared to Bt maize. Results have alsoshown that the application of systemic and non-systemic insecticides,such as Bulldock® and Oncol® controls stem borers and indirectlyreduces Fusarium ear rot and fumonisin production.

Therefore, producers can benefit from making use of biotechnology, such as Bt maize or registered insecticides to control stem borers to prevent damage to maize plants that will indirectly decrease Fusarium ear rot and fumonisin production.

The authors of this article and/or other plant pathologist can be contacted at the ARC-Grain Crops Institute at
018 299 6100.