Evaluation of Pathogenic Isolates in Ethiopia for the Control of Chocolate Spot in Faba Bean

Faba bean (Vicia faba L.) is one of the earliest domesticated food legumes in Ethiopia and is now cultivated on large areas in many countries. Production of the crop is, however, constrained by several disease infections including fungal diseases. The objectives of this work was to find out natural prevalence of Bacillus isolates on faba bean leaves in Amhara Regional state of Ethiopia; and to identify potential biocontrol agents for the management of chocolate spot (Botrytis fabae). Thirty eight distinct groups of isolates of Bacillus species out of a total 110 isolates obtained from 12 districts of north-west Ethiopia were found to occur on faba bean leaves. They differed in morphological and cultural characteristics. Thirty isolates of Bacillus spp. were tested for their effects on Botrytis fabae pathogen by dual culture technique on potato dextrose agar. Sixteen isolates produced 5 mm or higher inhibition zone and out of these isolate 43y and 56y were the most effective having inhibition zone of 8 and 7 mm, respectively. Isolates reduced the growth of the pathogen colony in dual culture by 2364%. The highest reduction was caused by isolate 115y (64%), followed by 114-3y (52%) and 29y and 67y (50% each). Isolates of Bacillus were placed on actively growing colonies of B. fabae and were found to cause lysis of B. fabae mycelium. Eleven isolates caused 8 mm or higher lytic area of mycelium. Maximum lysis of mycelium was caused by isolates 111-1y (16.8 mm) and 116y (11.3 mm), and they were confirmed from CABI Global Plant Clinic as Bacillus and not pathogenic to plants and animals. Further evaluation of promising antagonistic isolates by the detached leaf technique showed that most of them reduced the disease development on leaves. However, the degree of disease reduction varied with cultivar. Isolates 108-2y, 20-2y, 47-2y and 36-1y proved most effective in retarding disease development on two susceptible and one tolerant cultivar and can be further explored for commercial use for management of chocolate spot disease of faba bean.