Epidemiology and management of grey leaf spot : a new disease of maize in South Africa.

Abstract

Grey leaf spot is a relatively new fungal disease of maize in South Africa. It has become well established in the province of KwaZulu-Natal, and is capable of reducing grain yields by 20 to 60%. The disease is spreading to neighbouring provinces and countries. This study was conducted to establish solutions to the problem that could be easily implemented by maize farmers. Available literature was reviewed to establish the most appropriate epidemiologically based control measures that might be applicable in South Africa. Field trials were conducted to determine the effects of stubble and conventional tillage practices, cultivar susceptibility, fungicides, the correct time and frequency of fungicide treatment. and the financial benefits of fungicide treatment on grey leaf spot severity. The trials were evaluated for disease severity and grain yields. No commercial hybrids were identified to be resistant to grey leaf spot in the maize hybrid response to grey leaf spot trial. However, subsets of high-yielding hybrids less-susceptible to disease were identified - including PAN 6480, CRN 3584, SNK 2154 and PAN 6578. The most susceptible hybrids were identified to include RS 5206, PAN 6552, A 1849, PAN 6528 and PAN 6140. Fungicides containing carbendazim/flusilazole, were found to be most effective in controlling disease and increased maize yields. Hybrids such as RS 5206 and RS 5232 highly susceptible to disease and showed the highest grain yield response to fungicide treatment, whilst least-susceptible hybrids, such as PAN 6480, had the lowest response. The tillage trial aimed at management practices to reduce grey leaf spot indicated fungicides to be more effective in managing disease than tillage practices aimed at a reduction of initial inoculum. Trials on chemical control of grey leaf spot identified fungicides of the triazole and benzimidazole chemical groups to be effective in controlling disease, but only combination products of these chemical groups, were registered, in support of the pathogen resistance strategy. Products registered were carbendazim/flusilazole, carbendazim/flutriafol and carbendazim/difenoconazole. The frequency and timing of fungicide applications for the control of grey leaf spot in maize studies identified spray treatments initiated when disease had progressed to the basal five leaves and, before the exponential phase of the epidemic, provided the most effective disease control and concomitant high grain yields. Further spray treatments were necessary with early disease infections, in order to provide disease control until crop physiological maturity. The final study on the economic benefits of fungicide treatment of grey leaf spot in maize in KwaZulu-Natal indicated that the highest added yield response was not necessarily the best parameter to justify fungicide treatment. Rather, the expected added profit was a better parameter. In this study the highest added profits were R1 400 ha(-1) from the triple-spray programme in 1993/94 and R439 ha(-1) from a single-spray in 1992/93. The optimum treatment choice depended on the individual's risk-return preferences, which reflect his level of risk-aversion. An integrated approach using tillage practices, crop rotations, hybrids less- susceptible to the pathogen and the judicious use of fungicides is likely to be the most successful in controlling the disease. In the long term, the cornerstone of the integrated approach will be the development and use of hybrids resistant to the disease.