Chemical control of maize grey leaf spot

Grey leaf spot, Cercospora zeae-maydis, has, in a relatively short period, caused significant annual grain yield losses in the maize industry of KwaZulu-Natal, South Africa. No commercial hybrids are resistant to the disease, and because alternative measures of control, such as crop rotations and tillage practices, have limited effects, fungicides are the main option available for control of the disease. Benomyl was initially registered for the control of grey leaf spot (GLS), but, because of the possible development of fungicide-resistant strains of C. zeae-maydis to the benzimidazole chemical group of fungicides, alternative fungicides, with different modes of action, were investigated. This study was initiated to establish which fungicides and fungicide mixtures, with different modes of action, would control grey leaf spot effectively, and delay the possible development of pathogen resistance. Fungicides belonging to the triazole chemical group, and combinations of fungicides of the benzimidazole and triazole group, were highly effective. These combinations provided effective disease control, longer duration control and higher grain yield responses. The combination of the two fungicide groups, with their different modes of action, not only provide excellent control of GLS, but also offer the benefit of slowing down the development of pathogen resistance to fungicides. Lower than recommended rates of fungicides tested resulted in higher disease and lower grain yields. Recommended rates for difenoconazole and carbendazim were optimum for the control of GLS. However, recommended rates tested for benomyl, flutriafol and flusilazole/carbendazim were less than optimum for the control of GLS. (C) 1997 Elsevier Science Ltd.