ANEEN SCHOEMAN AND BRADLEY FLETT, ARC-GRAIN CROPS INSTITUTE
Diplodia ear and stalk rot can occur in areas where maize is planted season after season. The use of conservation agricultural practices where reduced tillage is used may increase the occurence of this disease.
The pathogenic fungus is Stenocarpella maydis also known as Diplodia maydis. In South Africa, Diplodia ear rot usually occurs in epidemics and is favoured by drought conditions early in the season followed by wet weather late in the season.
Infected ears are light in weight and can be totally rotted. Infected kernels will also receive a lower grading value and this can lead to economic losses. Due to the lightweight of the kernels, they can be easily discarded and broken up into pieces during the harvesting process and will affect the yield. Infected stalks can increase the lodging of maize plants and also affect yield.
Disease cycle and epidemiology
Diplodia maydis usually overwinters on stalk debris that can be either on the soil surface or it can be buried. When there are warm and moist conditions, pycnidia (spores) can be produced from the inoculum on the stalks and then the pycnidia can be disseminated by rain and the wind.
The maize stalks can be infected via the roots or crowns or behind the leaf sheaths. The maize ear can either be infected by pycnidia carried to the silks or through the nodes and infect between the ear and the leaf sheath.
Silk infections do not occur that often and it is usually between 0% - 3%. Infection of the ear usually occurs after three weeks of silking and when the green silks start to turn brown. Diplodia maydis has white to grey-brown fungal growth on the leaf basis of the maize ears. Symptoms usually develop from the base of the ear and gradually ramify toward the tip of the ear. Sometimes fungal growth will appear on the tip of the ear, but it does not occur that often. The maize kernels will be shrunken and appear to be glued together due to the white growth of the fungus (Photo 1).
The kernels itself might have a grey or dull brown colour. Black pycnidia will form late in the season on the nodes, kernels, ears (Photo 2) and on the infected stalks. These black pycnidia will then overwinter and be the primary inoculum for the following season.
The infected stalks must be buried or tilled into the soil in order to reduce inoculum for the following season. If possible, crop rotation should be practiced. Soybean is one of the best crops to use for crop rotation, otherwise groundnuts, grain and drybeans can also be considered.
Choice of cultivar
Maize cultivars do differ in their susceptibility for Diplodia infection. Planting a more resistant cultivar may reduce Diplodia occurrence.
Harvest and storage
Diplodia is a problem before harvest and is not really a storage problem. If it is a season where epidemics of Diplodia occur, it might be advantageous to harvest early en dry the kernels artificially to 11% moisture content. Diplodia maydis can infect kernels up to 11% moisture content, thus if the infected maize remain on the land, Diplodia rotting might be more severe.
The ARC-GCI plant pathologists can be contacted at (018) 299-6100 or you can email Aneen Schoeman at BelgroveA@arc.agric.za
or Bradley Flett at FlettB@arc.agric.za