Incidence and severity of frogeye leaf spot of soyabeans[Glycine max(L)merr]in agroeconomical zone II of Zambia

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Mwase, Fredrick Weston
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Soybean (Glycine max (L.) Merr.) is an important crop in Zambia but many foliar diseases pose a serious threat to its successful production in the country. During the last three years frogeye leaf spot disease caused by Cercospora sojina Hara has become the most prevalent disease in most parts of Zambia. A survey on farmers' fields under SCSI, Kaleya, Magoye and Hernon-147 cultivars in agroecological zone II of Zambia was carried out during the 1996/97 growing season to determine the incidence and severity of frogeye leaf spot. Disease incidence and severity was studied by monitoring disease progress at two weeks interval from the beginning of January to April. A total of nine fields were surveyed in three provinces namely Central, Lusaka and Southern. Soybean cultivars were evaluated for yield losses resulting from frogeye leaf spot. Field plots of each cultivar were either sprayed twice with benomyl or not sprayed at all. Nine samples of frogeye leaf spot of soybean, collected from Central, Lusaka and Southern provinces revealed variability among the isolates of Cercospora sojina (Cs- 01 to Cs-09) in morphology, physiology and virulence. Isolate Cs-08 had the greatest conidial length and Cs-07 had the smallest conidial length. On the basis of conidial width, isolates were grouped in four categories; and included Cs-04, Cs-05 and Cs- 06 (small < 5.0/^m) ; Cs-08 (broad 6.0-6.5^) ;Cs-01, Cs-02, Cs-03, Cs-07 and Cs- 09 (very broad > 7.0^ ) . After the third day of growth on Carrot leaf decoction agar isolates Cs-04, Cs-07, Cs- 08 and Cs-09 produced the largest colony diameters while isolates Cs-02 and Cs-06 vi had the slowest rate of growth. Growth rate of isolates on Potato dextrose agar and Sabouraud dextrose agar did not show any significant differences (P <0.05) throughout the period of culturing. Pathogenicity and virulence of the isolates tested in green house grown plants at four weeks showed that isolates Cs-02 and Cs-08 were the most virulent and produced lesions with a mean diameter of 5.1 mm fourteen days after inoculation. The least virulent isolates were Cs-04, Cs-05 and Cs- 05 which produced lesions of 2.0 mm diameter or less. Results obtained from these studies 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 (0.3) basing on the 1-9 disease rating scale. Incidence and severity increased with time and varied depending on weather parameters and susceptibility of cultivars to the disease. Yield reduction was greatly influenced by the severity of the disease. Yield losses due to frogeye leaf spot occurred through a reduction in seed size. It is believed that differences in virulence of the isolates contributed to the observed variation in incidence and severity of the disease at different locations
Leaves -- Diseases and pests--Leaves -- Diseases and pests