Screening for novel biocontrol agents applicable in plant disease management – A review

In current plant disease control, the divergence from chemical pesticide use has been fueled by the public concern about toxicity and environmental impact of these products, the development of pesticide-resistant pathogens, a rising ban of existing pesticides and, a decreased registration of new ones. Due its high potential as alternative/complement to these pesticides, biological disease control is now generally recognized and constitute an important tool in integrated pest management (IPM). However, one of the factors hampering the large-scale implementation of biocontrol is the lack of efficient, commercially available biocontrol agents (BCAs). The identification of novel BCAs thus forms a critical step in the development of commercial biocontrol products and requires rapid and robust screening methods suitable to evaluate high numbers of candidate BCAs. In this review, we present an overview and discussion of the screening systems reported to select novel BCAs for biocontrol of microbial plant diseases. Distinction is made between screenings for BCAs with direct antagonistic effect on the pathogen (via e.g. parasitism, antibiosis, or competition) and BCAs that exert their biocontrol activity indirectly by induction in the plant of an induced systemic resistance (ISR) to the pathogen. For both types, we further discriminate between phenotype- and marker-based approaches, which evaluate directly the intended phenotype (disease reduction) or the expression of a marker predictive for this phenotype, respectively. Finally, we discuss the importance of the type and origin of candidate BCA collections as a significant determinant of the screening success.