Studies on ear rot and grey leaf spot of maize in South Africa.
In recent years there have been economically important epidemics of both Stenocarpella ear rot and grey leaf spot (GLS) in South Africa. These epidemics have adversely affected the grain yield and quality of the maize harvested. Maize researchers and breeders have had to re-assess the importance of maize disease in South Africa and make the necessary adjustments to their programmes. Literature reviews were undertaken on both Stenocarpella ear rot and GLS to provide the necessary background of technical information to conduct research under local conditions into these disease problems, and to assist in interpretation of results of experiments. A novel method of inoculating milled Stenocarpella infected ears into the whorls of maize plants (about 2 weeks before 50% anthesis) was developed to provide consistent inoculum pressure and increased ear rot. This inoculation method was practical, efficient, reliable, consistent and cheap to implement. Commercial organisations could use this inoculation method to inoculate a large number of plants per day, allowing for improved screening of breeding material and hybrids. Ear rot assessment methods, and researchers' ability to assess ear rot, were tested under South African conditions. The accuracy of the different methods tested varied considerably, particularly when there was a high level of ear rot that could not be seen without shelling the grain. Each method could be used in a maize breeding programme, depending upon the desired levels of accuracy and time taken using the given method. Researcher's ability to assess ear rot varied considerably and accuracy was correlated with the number of years experience in maize research. Grain colour affected the researcher's ability to accurately assess ear rot severity. Yellow-grained maize was more difficult to assess for ear rot than white-grained maize. Hybrid response to Stenocarpella ear rot infection was difficult to interpret owing to a significant interaction with the environment. Hybrid ear rot response was non-linear in nature. Normal methods of presenting disease data and classifying hybrids in resistance response categories were not successful. Non-linear regression analysis has to be used to do this. However, it is important that ear rot data be presented in a way that farmers can utilise the information. Pre-flower stress predisposes maize hybrids to ear rot infection. Hybrids that normally exhibited good levels of resistance to Stenocarpella ear rot may become severely colonised if drought stress occurs in the four weeks prior to flowering. This environmental interaction makes ear rot resistance breeding and the interpretation of results difficult. The incidence of maize ear rot was widely considered to increase with increased plant density. Experiments over three seasons in South Africa have shown that is not true under certain environmental conditions. In specific hybrids, plant densities of less than 50 000 plants ha"1 exhibited a higher incidence and severity of ear rot than plant densities greater than 50 000 plants ha(-1). The hybrids that usually responded in this manner were more susceptible to ear rot than the other hybrids. Generally, ear rot increased with increased plant densities over 50 000 plants ha(-1). The mechanisms and reasons for this could not be determined. Fungicide trials and regression analysis of hybrid yield trials over a two years period, at two locations in KwaZulu-Natal, showed that grain yield losses due to GLS infection were at least 13%. Severity of GLS was consistently higher at Cedara than at Greytown. Economic losses at Cedara ranged from Rl 919 - R2 278 ha(-1) and at Greytown from Rl 554 - Rl 726 ha(-1). Predicted hybrid losses ranged from R836 - R2 621 ha(-1) (13% - 37%), depending upon the level of inherent GLS resistance. Hybrid response to Cercospora zeae-maydis infection was linear in nature and hybrids could be categorised into response categories. Large differences in GLS resistance could be found between commercial hybrids. However, the current levels of GLS resistance in hybrids does not eliminate yield loss under high GLS inoculum levels, and fungicide application was economically justified on most hybrids. Newly released hybrids show increased levels of GLS resistance. The application of systemic fungicides to GLS-susceptible maize was highly effective in controlling GLS and increasing yield substantially. The most effective fungicides belonged to the triazole and benzimidazole group of fungicides. Protectant fungicides were not as effective as systemic fungicides. Copper-based fungicides were phytotoxic to maize in two seasons and at both locations. Fungicide mixtures of the two groups active against GLS are being used on commercially. The effectiveness of fungicides did not vary over location or hybrids, but was influenced by inoculum pressure. Effective control strategies have been implemented to control both Stenocarpella ear rot and GLS in South Africa. Crop rotation, the selection of the more ear rot and GLS-resistant hybrids, and the judicious use of fungicides has reduced the levels of both diseases to manageable levels. An integrated control strategy is needed to control these diseases and efforts are being made to educate farmers to this effect. Maize pathological research now enjoys a greater emphasis than it did in the early-1980s.