Severity of Chocolate Spot Disease on Faba Bean (Vicia faba L) and Characterization of Botrytis fabae Isolates in Southwest Ethiopia

Abstract

Chocolate spot caused by the fungus Botytis fabae Sard. is the major disease threatening faba bean (Vicia faba L.) production in Ethiopia. However, the intensity and importance of this disease is not well studied in southwest Ethiopia. The present study was conducted to determine the severity and to characterize B. fabae isolates in major agro-ecologies of South west Ethiopia. A total of 44 faba bean fields were surveyed in 11peasant associations (PAs) of south west Ethiopia and all of the fields were infested with chocolate spot. The disease severity indices (DSI) varied among PAs, altitude range and crop management practices. For surveyed fields, the mean DSI ranged from 33.4% to 69.4 % in PA. Logistic regression analysis showed that PA, crop variety, crop history and altitude were significantly associated with DSI in a multiple variable model. Higher DSI was significantly associated with high altitude (>2200m.a.s.l). Significant differences were observed in the frequency of isolates among PA with colony color (X²=35.94, df = 2, p<0.05) and colony growth rate (X²=38.7, df = 2, p<0.01). According to morphological characteristics all isolates were identified as B. fabae species (11-14 × 7-10 µm, mean 12.5×7.8 µm). In greenhouse, all isolates showed typical chocolate spot lesions and differed in their aggressiveness (27% more, 64% medium and 9% less aggressive). The study revealed high occurrence and importance of chocolate spot in the major faba bean growing areas located at high altitudes and integrated disease management options like use of tolerant and high yielding varieties with appropriate cultural practices like timely  weeding, optimum seeding rate, repeated ploughing, fallow cropping or crop rotation with cereals are recommended.

Keywords: Faba bean, chocolate spot, Botrytis fabae, severity, aggressiveness