A closer look at fusarium ear rot of maize

EDSON NCUBE, ARC-GRAIN CROPS INSTITUTE

Fusarium ear rot (Photo 1) is primarily caused by Fusarium verticillioides, but F. subglutinans and F. proliferatum are also important.

Fusarium ear rot occurs in all areas where maize is grown because of the pathogen (Fusarium verticillioides) which can live within the maize plant without causing apparent disease. This disease is most severe under hot, dry weather conditions that occur after flowering.

The most detrimental effect of F. verticillioides, however, is that it produces fumonisin mycotoxins that have been associated with diseases of humans and livestock.

Symptoms

Symptoms for this disease vary depending on the maize genotype, environment and disease severity. Individual or groups of infected kernels are usually scattered randomly on the entire ear and they appear as whitish and pink kernels (Photo 2).

Fungal growth is frequently found on the tip of the ear as a result of stalk borer damage (Photo 3). Symptoms also occur where seed coats are broken due to the non-infectious condition known as silk-cut. In severe cases of Fusarium ear rot, the entire ear may be whitish on and between kernels.

Disease cycle

Fusarium verticillioides, F. proliferatum and F. subglutinans overwinter in maize debris. These Fusarium species, particularly F. verticillioides, survive in maize stalks as thickened hyphae in moist soils that have poor aeration and little or no competition with other fungi and bacteria.

The soil borne hyphae germinate and infect the germinating seed and roots and move up the plant through systemic growth. The fungus also produces airborne spores from sporulation on the tassels of the previous crop residue.

F. verticillioides can infect both through the mode of systemic infections from contaminated seed, as well as through the silk channel by airborne spores. Silk colonisation by F. verticillioides starts from the tip of the ear downward. Infection is enhanced by late-season rainfall and 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 through infected stalks or infected seed.

Control

Maize hybrids differ in their susceptibility to Fusarium ear rot. For example, studies currently underway at the ARC-GCI show that genetically modified maize hybrids that contain the insecticidal proteins for the control of maize stalk borer (Bt maize) have significantly less Fusarium ear rot symptoms compared to their non-Bt isohybrids.

Agricultural practices such as planting hybrids that are adapted to local climatic conditions; the use of hybrids with tight husks; control of ear feeding insects; avoiding excessive plant populations; maintaining adequate levels of nitrogen and other essential growth nutrients; crop rotation and subsoiling in compacted soils to minimise plant stress, are some of the possible means that can be followed to reduce Fusarium ear rot.

Standard grain storage procedures that prevent development of fumonisin mycotoxins in stored grain, such as drying maize kernels to moisture levels below 16% after harvest, are recommended. Stored grain should be aerated regularly to lower moisture content and temperature to desired levels. Adjusting the combine harvester to avoid kernel damage during harvesting, reduces mycotoxin contamination.

For more information on Fusarium ear rot, contact Edson Ncube at
(018) 299-6100.

Reference

Payne, G.A. 1999. Ear and kernel rots. In: White, D.G. editor. Compendium of corn diseases. APS Press, Illinois, USA.