A SYSTEMS-APPROACH TO ASSESS CROP HEALTH AND MAIZE YIELD LOSSES DUE TO PESTS AND DISEASES IN CAMEROON

Three surveys of maize fields were undertaken in 1993 in Cameroon, ran ging from low to high altitudes, and from forests in the south with bi modal rainfall(two cropping seasons) to the northern savanna with mono modal rainfall distribution (one cropping season). Field conditions, i nsects and diseases, plant growth and yield variables were assessed in 164 fields. Increased soil fertility (expressed as plant vigour) favo ured Bipolaris maydis, Physoderma maydis, and Busseola fusca. There wa s a positive relationship between soil organic matter and B. fusca, El dana saccharina and B. maydis. As organic matter decreased, percentage leaf area lost to pathogens, particularly Puccinia sorghi, increased. There was an inverse relationship between wild grasses around a field and stem borers in the field. Stem diameter, cob fill and weight incr eased with altitude reflecting the change from early maturing lowland cultivars to very late maturing mid-to high-altitude cultivars. Across ecological zones and surveys, B. fusca accounted for 95% of all the s pecies found on maize, followed by E. saccharina. In the first croppin g season, the mean percent borer infestation was similar in lowland an d highland with a mean of 43%. Borer incidence was higher during the s econd cropping season. In both low and mid-altitude fields, 52-56% of the plants were infested, resulting in a calculated cob weight loss of 9 g per plant At that time, the average plant loss from dead hearts a cross zones was 11%. B. maydis, Puccinia polysora, and Physoderma mayd is predominated in the lowlands, and E. turcicum, P. sorghi, and Phaeo sphaeria maydis occurred mainly in mid-to high-altitudes. Maize streak virus, stem diseases, B. maydis, Rhizoctonia sheath blight, and P. so rghi consistently had significant negative impacts on cob weight. The aetiology of stem lesions was not determined, but they significantly a ffected cob weight in the humid lowlands. Average reduction in cob wei ght from foliar and stem diseases in the different ecological zones an d seasons ranged between 10 and 12 g per plant. (C) 1997 Elsevier Scie nce B.V.