Bacillus isolates as potential biocontrol agents against chocolate spot on Faba beans

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Abstract
Chocolate spot caused by Botrytis cinerea and Botrytis fabae is a serious disease of Vicia faba L. beans. Aggressive infection of that disease can result in severe yield losses (up to 100%). Genetic resistance to chocolate spot in V. faba L. is weak. Chemical control of the disease gives only partial crop protection because of widespread fungicide resistance in both chocolate spot agents. Biological control with applications of bacteria antagonistic to Botrytis may provide a useful alternative to chemicals. Of 270 Bacillus isolates tested, 54 (20%) were found to cause necrosis and (or) chlorosis when placed on leaves of V. faba L. Only 14 strains (6.5%) were able to prevent chocolate spot symptoms from developing in vivo and all of these antagonised Botrytis cinerea and Botrytis fabae in vitro by antibiosis. Bacillus macerans BS 153 was selected for biocontrol experiments in both the greenhouse and field in which plants were sprayed with bacterial suspensions. Detached leaves were inoculated with Botrytis at intervals after Bacillus macerans BS 153 application. The beneficial effect of the biocontrol agent persisted much longer in greenhouse-grown plants. The estimated ED50 was 1.3 times less in the greenhouse spray than in the field spray. From the high number of bacteria required and the rate of decline of introduced Bacillus population in field-grown plants, it was concluded that biocontrol of chocolate spot by Bacillus is impractical unless improved formulations and spray application techniques are developed.Key words: chocolate spot, Bacillus isolates, biocontrol.