Suppression of fungal pathogens

Suppression of fungal pathogens in lettuce is affected by soil type-dependent root colonization of beneficial bacteria, triggering root secretion of antifungal compounds

Published in Agronomy


Windisch et al. (2017) Agronomy 7:/44

Saskia Windisch 1,*, Sebastian Bott 1, Marc-Andreas Ohler 1, Hans-Peter Mock 2, Rico Lippmann 2, Rita Grosch 3, Kornelia Smalla 4, Uwe Ludewig 1 and Günter Neumann 1

  1. Department of Nutritional Crop Physiology, Institute of Crop Science (340h), University of Hohenheim, 70599 Stuttgart, Germany
  2. Leibniz-Institut für Pflanzengenetik und Kulturpflanzenforschung, 06466 Gatersleben, Germany
  3. Department Plant Health, Leibniz Institute of Vegetable and Ornamental Crops Großbeeren/Erfurt e.V., 14979 Großbeeren, Germany
  4. Julius Kühn-Institut-Federal Research Centre for Cultivated Plants, Institute for Epidemiology and Pathogen Diagnostics, 38104 Braunschweig, Germany

Already in 1904 the German phytopathologist Lorenz Hiltner postulated that secretions of plant roots can attract beneficial soil microbes but also pathogens as uninvited guests into the so-called “rhizosphere”, as soil compartment influenced by the activity of plant roots. He also speculated that the external application of plant-beneficial microbes with the ability to supress root-infection by pathogens could offer options for biological control of plant diseases (bio-control). In face of a declining availability and restrictions for application of chemical plant protection agents, to counteract detrimental environmental side effects, bio-control approaches are currently gaining increasing interest. However, high variability of the biocontrol responses, frequently observed particularly under real field conditions still remains a major challenge. This suggests an impact of additional factors determining the success of practical applications using beneficial microbes for disease control. In this context we investigated the role of different soils, providing the habitats for plant microbial interactions. It is well known that the soil type represents one of the major factors determining the composition and function of soil microbial communities. However, this effect is difficult to assess under real field conditions due to the simultaneous impact of site specific climatic factors (temperature, precipitation etc.).  To avoid these effects, we used a unique experimental field site providing three different soil types (sandy soil, loam, and loess loam) at the same location. Lettuce was selected as a highly sensitive model plant with respect to soil borne diseases, which was exemplarily treated with the pathogenic fungus Rhizoctonia solani (causing bottom-rot disease), with and without additional treatments with bacteria, providing pathogen suppressive properties (Pseudomonas sp. and Serratia plymuthica 3Re-4-18). Additional experiments were conducted under controlled conditions by use of so-called rhizoboxes equipped with root observation windows allowing the collection of root secretion by application of small sorption filters onto the root surface.

Main results:

  • The fungal pathogen inhibited lettuce growth in a soil type-specific manner in the order sandy soil > loamy soil > loess loam (no inhibition), both under field conditions and in the rhizobox experiment
  • The pathogen-supressive behaviour of the loess loam soil was associated with increased concentrations of root secretions with antifungal properties (benzoic acid, lauric acid) and high concentrations of soluble sugars in the lettuce rhizosphere which coincided with a high abundance of bacterial genera with documented pathogen antagonistic properties (e.g. Pseudomonas).
  • Accordingly, external application of pathogen-antagonistic bacteria (Pseudomonas sp. RU47; Serratia plymuthica) resulted in improved pathogen suppression also on the loamy soil and was able to stimulate the release of the antifungal root secretions, most likely involved in the suppression of the fungal pathogen.
  • Although, the introduction of high numbers of pathogen antagonists by simultaneous application of both bacterial strains was most effective in pathogen suppression, also negative effects on plant growth were observed associated with a reduced plant micronutrient status, suggesting competitive plant-microbial interactions in nutrient acquisition.