Pesticide compatibility of bacterial biological control agents with the potential to control aphanomyces root rot in peas

Aphanomyces euteiches Drechsler can cause severe root rot in peas. Previous research has identified two bacterial isolates, Bacillus subtilis LU1241 and Paenibacillus polymyxa LU1133, able to suppress aphanomyces root rot of peas. If these bacteria are to be used as biological control agents in the field, it is important to know whether their efficacy is affected by commonly used pesticides. An assay was developed to determine the effect of selected fungicides, insecticides and herbicides on populations (colony forming units) of the two potential biological control agents. None of the fungicides (cyproconazole, metalaxyl-M, azoxystrobin and sulphur) inhibited growth of P. polymyxa and all stimulated growth of B. subtilis compared with the non-fungicide control. The herbicides sethoxydim and bentazone had no effect on either bacterium and diquat significantly inhibited growth of B. subtilis but had no effect on P. polymyxa. The insecticides pirimicarb, lambda-cyhalothrin and diazinon had no effect on B. subtilis. Pirimicarb had no effect on P. polymyxa, but abnormal colony morphology with spreading growth was observed for both diazinon and lambda-cyhalothrin. Different sensitivity to the pesticides was observed with each bacteria species and this could influence the use of the biological control agents in a field environment.