Efficacies of bacterial and fungal isolates in biocontrol of Botrytis cinerea and Pseudomonas syringae pv. tomato and growth promotion in tomato do not correlate

Abstract

There is a need to develop more biological control agents to fulfil the increasing demand for biological crop protection. Testing for consistent efficacy in disease control under the relevant range of environmental conditions is one of the most demanding steps during screening programs. Bioassays were conducted to target three major diseases of tomato, stem canker caused by Botrytis cinerea, leaf spot caused by B. cinerea, and bacterial spot caused by Pseudomonas syringae pv. tomato, and to assess possible growth promotion of tomato seedlings. Nine quantitative screening approaches were analyzed for a test panel of approximately 100 isolates of bacteria and fungi, all obtained from tomato, and several known antagonists as reference isolates. Even with such a limited number of isolates promising antagonists, partly not yet described as antagonists, could be selected for control of the targeted diseases when labor and resource demanding in planta bioassays had been applied. Also some promising isolates enhancing seedling development could be identified. Independent screening assays for the different traits were needed since no correlation between the different traits were found. Attempts to simplify screening assays to high-throughput systems failed since there were no positive correlations with in planta bioassays. In conclusion, the often suggested first screening rounds using in vitro tests for huge numbers of isolates followed by in planta testing of a selected group of candidates, e.g. those with high in vitro production of certain secondary metabolites or biosurfactants, may not exploit the entire potential of antagonists. Especially antagonists combining various modes of action may be excluded by in vitro screening with a bias on a specific mode of action. Therefore, independent in planta assays are proposed to screen against different pathogens and for growth promotion.