Incidence and severity of frogeye leaf spot and associated yield losses in soybeans in agroecological zone II of Zambia.

Abstract

The incidence and severity of frogeye leaf spot of soybean (Glycine max (L.) Merr.) was studied in agroecological region II of Zambia during the 1997/98 crop growing season. A survey was conducted on farmers' fields on SCSI Kaleya, Magoye and Hernon-147 cultivars. Disease incidence and severity was assessed by monitoring disease increments at two weeks interval (beginning of January to April) from nine fields, three from each province. Soybean cultivars were evaluated for yield losses resulting from frogeye leaf spot. Field plots of each cultivar were either sprayed twice with benomyl (benlate) or not sprayed at all. The results showed that the incidence of frogeye leaf spot was highest in Southern province (5.1), followed by Lusaka province (4.9) while Central province had the lowest disease incidence (1.8). Values for area under disease progress curve (AUDPC) were significantly greater (P < 0.05) for Lusaka and Southern provinces than for Central province. Yields in benomyl protected plots ranged from 1444 kg ha-1 to 2320 kg ha-1 and were significantly different among the cultivars. Average yields of non protected plants were reduced by 30.5% for Kaleya, 35.6% for Hernon-147 and 37.2% for SCS1. Incidence and severity increased with time and varied depending on weather parameters and susceptibility of cultivars to the disease. Yield losses due to frogeye leaf spot occurred through a reduction in seed size. Differences in weather conditions and amount of inocula are believed to contribute to the observed variation in incidence and severity of the disease at different locations.