Chocolate spot epidemics on different faba bean varieties and characterization of some Botrytis fabae isolates collected in dawuro zone, southwest Ethiopia

Abstract

Chocolate spot caused by the fungus Botrytis fabae is the major disease threatening faba bean (Vicia faba L.) production in Ethiopia. However, the intensity and importance of this disease are not well studied in the faba bean production areas of southwest Ethiopia. The present study was conducted to determine the distribution, epidemics of chocolate spot on different faba bean varieties and to characterize some B. fabae isolates in major agro- ecologies of Dawuro Zone. A total of 44 faba bean fields were surveyed in 11peasant associations (PAs) of the zone, 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% in Dali to 69.4 % in Waka 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 (>2200). The field experiments carried at three locations and experimental plots arranged in RCBD with three replications. The progress of the symptoms evolved slowly on certain varieties but at much greater rates on other varieties depending on location. DSI scores almost for all assessment dates and the AUDPC values varied considerably among the varieties and significant genotypic differences were observed at all locations. The varieties Hachalu, Kuse, Tumsa, Mesay, Moti, Gebelcho, Walki, Nc-58 and local cultivar were suffered from the disease (highest AUDPC). Conversely, CS20DK, Degaga, Bulga-70, Tesfa and Kasa were rated moderately resistant and developed the least symptoms at all locations. Highly significant differences were observed for number of pods per plant, seed yield and hundred seed weight among varieties. CS20DK (2100kg/ha), NC-58 (2027kg/ha), Moti (1973kg/ha) and Degaga (1910kg/ha) under Tocha conditions where as Degaga (1327kg/ha), CS20DK (880kg/ha) and NC-58 (870kg/ha) under Mari conditions were the top yielding varieties. The correlation between seed yield and DSI for all assessment dates were negatively and highly significant (p <0.01) having correlation values ranging from - 0.592 to -0.361 at Tocha and Mari. Significant differences were observed in the frequency of isolates among PA with colony color (X 2=35.94, df = 2, p<0.05) and colony growth rate (X 2=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 (B. fabae) 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.