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.
