Occurrence and distribution of Peronosclerospora sorghi [Weston and Uppal (Shaw)] in selected countries of West and Southern Africa.

Surveys of sorghum and maize crops were undertaken in Nigeria, Zimbabwe, Zambia, Mozambique and Rwanda during 1991 and 1992. The occurrence and prevalence of sorghum downy mildew (SDM) caused by Peronosclerospora sorghi [(Weston and Uppal) Shaw] was assessed in regions of each country. In Nigeria only maize was systemically infected in the southern humid zone, where rainfall was 1200–1800 mm and the altitude 300–1000 m. This epidemic zone appeared to be geographically isolated from other areas of Nigeria where SDM was observed. Within the southern epidemic zone, yield loss was estimated to be 11.7%. Individual fields had up to 95% incidence of systemically infected plants. In the arid north of Nigeria (rainfall < 1300 mm, altitude 600–1200 m) both maize and sorghum were infected, and disease incidence was invariably low (<5%). Systemic SDM incidence on maize was negatively correlated with growth stage (r = −0.7746, P = 0.01). In Zimbabwe, Zambia, Mozambique and Rwanda sorghum and maize were infected with SDM in areas with an annual rainfall of 600–1200 mm and an altitude range of <300–1800 m. Incidence of infection within crops was generally low, and sites with infected crops were scattered in these countries. SDM local lesion infection was observed only on sorghum. Yield loss due to SDM in Zambia, Zimbabwe and Rwanda at the time of the survey was negligible. However, SDM is widespread in Africa and occurs in many different agricultural areas, and thus remains a threat to sorghum and maize production. Management of the disease using resistant varieties, cultural and chemical control should reduce the risk of future epidemics.