Effective use of biological control agents (BCAs) is a potentially
important component of sustainable agriculture. Recently, there has
been an increasing interest among researchers in using combinations
of BCAs to exploit potential synergistic effects among them. The
methodology for investigating such synergistic effects was reviewed
first and published results were then assessed for available evidence
for synergy. Correct formulation of hypotheses based on the theoretical
definition of independence (Bliss independence or Loewe additivity)
and the subsequent and statistical testing for the independence–synergistic–antagonistic
interactions have rarely been carried
out thus far in studies on biocontrol of plant diseases. Thus, caution
must be taken when interpreting reported “synergistic” effects without
assessing the original publications. Recent theoretical modeling
work suggested that disease suppression from combined use of two
BCAs was, in general, very similar to that achieved by the more
efficacious one, indicating no synergistic but more likely antagonistic
interactions. Only in 2% of the total 465 published treatments
was there evidence for synergistic effects among BCAs. In the
majority of the cases, antagonistic interactions among BCAs were
indicated. Thus, both theoretical and experimental studies suggest
that, in combined use of BCAs, antagonistic interactions among
BCAs are more likely to occur than synergistic interactions. Several
research strategies, including formulation of synergy hypotheses in
relation to biocontrol mechanisms, are outlined to exploit microbial
mixtures for uses in biocontrol of plant diseases.