European Journal of Plant Pathology 110: 119–128, 2004. © 2004 Kluwer Academic Publishers. Printed in the Netherlands. Interactive effects of host, pathogen and mineral nutrition on grey leaf spot epidemics in Uganda Patrick Okori 1 , Patrick R. Rubaihayo 2 , Ekwamu Adipala 2 and Christina Dixelius 1 1 Department of Plant Biology and Forest Genetic, Swedish University of Agricultural Sciences, P.O. Box 7080, S-750 07 Uppsala, Sweden (Phone: +46(18)673248; E-mail: Patrick.Okori@vbiol.slu.se); 2 Department of Crop Science, Makerere University, P.O. Box 7062, Kampala, Uganda Accepted 16 June 2003 Key words: Cercospora zeae-maydis, fertilisers, maize, parasitic fitness, resistance Abstract Grey leaf spot incited by Cercospora zeae-maydis is a new devastating foliar disease of maize in East Africa. For effective control, elucidation of the most critical elements of the grey leaf spot disease pyramid is important. This study investigated the role of mineral nutrition, pathogen variability and host resistance in the epidemic. Trials were conducted under field and controlled environments. The 28 isolates used in the controlled environment varied significantly (P ≤ 0.05) in parasitic fitness measured indirectly as disease efficiency, but no infection pattern could be attributed to known C. zeae-maydis pathotypes. Data from field trials showed that host resistance and mineral nutrition significantly (P ≤ 0.05) affected disease efficiency, with highest disease development occurring in nitrogen-augmented plots. Exclusive phosphorus application had no clear effect on grey leaf spot epidemics but combined application with nitrogen significantly (P ≤ 0.05) reduced the predisposition effects of nitrogen to the disease. Overall, treated plots had less disease than unfertilised plots. Fertiliser application had no effect on sporulation capacity, while cultivars significantly affected it. Geographic differences in amount of disease were observed, suggesting environment influences on grey leaf spot incidence. The results suggest that the current grey leaf spot epidemics in East Africa are due to favourable cultivars, poor mineral nutrition and environmental interactions. Introduction Maize is Uganda’s and East Africa’s most important cereal. As a consequence, deliberate efforts to increase production by use of elite cultivars and fertilisers are being promoted by East African governments (GOU, 2000). However, a major obstacle to increased maize production in East Africa is a new foliar disease, grey leaf spot caused by Cercospora zeae-maydis Tehon and Daniels (Bigirwa et al., 1999; Okori et al., 2001; Asea et al., 2002). In the United States, reduced or non-tillage maize production system has increased grey leaf spot severity (de Nazareno et al., 1993). In Africa, where such a system is rare or absent, incidence and severity of grey leaf spot is similarly high. This is presumably due to continuous cropping of maize all year round in response to high demand and the relatively longer growing season of up to 300 days in Africa compared to 120–150 days in the US (Ward et al., 1999). Maize is intolerant to nutrient deficiency. Nevertheless, soil fertilisation is reported to influence grey leaf spot epi- demics (Smith, 1989; Ward, 1996). Thus, it is clear that like other plant disease epidemics, grey leaf spot is a product of interactions between host, pathogen and environment as impacted upon by man (Zadoks and Schein, 1979). In the US, grey leaf spot has spread slowly, spanning over 50 years before reaching epi- demic proportions (Latterell and Rossi, 1983; Ward et al., 1999). Whereas in Africa, the disease has spread faster since it was first reported in South Africa in 1990–1991 and is now endemic in most sub-Saharan countries (Ward and Nowell, 1998; Ward et al., 1999). There are, however, differences in the levels of dis- ease severity between the two continents, being higher