New approaches to suppress Rhizoctonia solani

Rhizoctonia solani KÜHN is one of the most important soil-borne pathogens responsible for diseases on more than 500 host plants including many economically important crops worldwide. For example, against late sugar beet rot caused by R. solani no efficient control strategies are currently available. Preliminary studies have shown that it is possible to suppress the pathogen using antagonistic bacteria as well as fungi. However, inconsistent results under field conditions are one reason that translations into practical approaches failed. The objective of this project is to develop an optimal combination of antagonistic microorganisms on the basis of already evaluated endophytic bacteria and Trichoderma strains to suppress the pathogen on sugar beet. The use of biocontrol agents with different modes of action and ecological behaviours should minimize the problems under field conditions. Furthermore, we will analyze the microbial interaction in vitro and ad planta as well as the biocontrol effect in pot and field experiments using new molecular tools to develop an optimal biocontrol strategy.

Rhizoctonia solani Kühn is one of the most important soil-borne pathogens responsible for diseases and yield losses on more than 500 host plants including many economically important crops worldwide. For example, against late sugar beet rot caused by R. solani no efficient control strategies are currently available. In this project we developed an antagonistic cocktail based on an optimal combination of microorganisms antagonistic towards Rhizoctonia to suppress late root rot on sugar beet. This cocktail contains fungal as well as bacterial strains mainly from other host plants or alternative bio-resources like survival structures of the pathogen: Trichoderma velutinum G1/8 from Rhizoctonia sclerotia, Pseudomonas fluorescens L13-6-12 from the geocaulosphere of potato, and Serratia plymuthica 3Re4-18 from the endosphere of potato. Only Pseudomonas trivialis RE*1-1-14 is originated from sugar beet themselves. The selection based on in vitro and ad planta activity in field and greenhouse trials. New molecular and microscopic tools were integrated in this evaluation. For example, confocal laser scanning microscopy using fluorescently labelled strains resulted in a new understanding of the antagonistic interaction in the rhizosphere. Using this technique, we also found that all our selected strains showed an endophytic behaviour - they colonise the same habitat than the pathogen. Surprisingly, the antagonistic strains do not interact morphologically on the plant. The use of biocontrol agents with different modes of action and ecological behaviours can minimise the problems under field conditions. Other hurdles to translate successful studies into biocontrol strategies are the formulation and registration of biocontrol agents. Therefore, we developed a new formulation procedure: the microorganisms are introduced in alginate beats, where they can propagate and survive for a long time. The dried alginate beats allowed a technical application into the pelleting process of sugar beet seeds. This is the first approach that microorganisms survive this procedure, a requirement to an industrial production. Furthermore, we analysed possible risk of our biocontrol combination: none of the agents showed toxicity against eukaryotes (analysed in a new Caenorhabditis elegans assay) or induced turbulences in the natural microbial communities. The strategy developed was patented and can be used for sustainable and environmentally friendly plant protection against Rhizoctonia diseases. Although the strategy was developed for sugar beet as model plant, it can be transferred to other Rhizoctonia host plants.