Financial support: Fondecyt 1990785.

Keywords: anastomosis groups of Rhizoctonia solani, Bacillus lentimorbus (Paenibacillus lentimorbus), Bacillus subtilis, Lycopersicon esculentum.

Rhizoctonia solani is one of the most important soilborne fungal pathogens, which develops both in cultured and non-cultured soils, causing diseases in different crops such as rice, bean and tomato, among others. Fumigants, such as methyl bromide (MeBr), have been widely used to control this and other soilborne pathogens. The use of MeBr, which has proved to be effective in controlling the pathogens already mentioned, will be forbidden in few years because of its high toxicity towards human beings and animals, and because of the environmental pollution and reduction of the ozone hole caused by this soil fumigant. One of the alternatives to control soilborne pathogens is the use of bioantagonistic microorganisms (fungi or bacteria) naturally found in soils, that could control pathogen development and/or spread, thus preventing plant diseases. These microorganisms use different biocontrol strategies including secretion of antibiotics, secretion of enzyme systems that could degrade the pathogen cell wall or effective competition for nutrients. The selection of bioantagonistic microorganisms, in addition to the direct effect on pathogen development, must consider conditions where the bioantagonist should develop, i.e. salinity and pH of soils and different temperature or presence of metal ions, among others. In addition, it is important to determine the innoquity of the biocontrol agents towards the plant. Therefore, the objective of this work is the isolation of antagonistic bacteria that could control R. solani, both in vitro and in vivo, and their characterization in terms of antagonistic mechanisms used to control the pathogen, innoquity towards tomato plants and conditions for growth similar to those present in the field.

Bacteria were isolated from the rhizoplane and surrounding soil of healthy and R. solani diseased tomato plants, cropped in greenhouse and at the field level in the V region of Chile. Several strains were isolated and screened on three different R. solani isolates (identified as belonging to the anastomosis groups AG-2-1 and AG‑4), to establish whether they had the ability to control the development of the pathogen.  The screening allowed us to select three bacterial isolates that were identified as Bacillus subtilis (one isolate) and B. lentimorbus (two different isolates), and to determine that their biocontrol activity was based on the secretion of antibiotics that affected R. solani development. Both anastomosis groups of R. solani were controlled, although AG-2-1 was more sensitive to the use of any of the bacteria than R. solani AG-4. Nevertheless, this result implies that these bacterial isolates have a broad spectrum of action because of their ability to control R. solani from different anastomosis groups.

It is also important to test if the bacterial isolates are innocuous to tomato plants, because if they result to be pathogenic they cannot be used to control R. solani at least in this crop.No lesions in roots or in crowns of tomato plants were observed after treatment with any of the three bacteria, neither tomato seeds were affected by the presence of any of them. Therefore, these results suggest that B. subtillis and B. lentimorbus are not pathogenic for tomato seeds and seedlings, and that they could be used safely to control R. solani because they are innocuous to tomato plants. In addition, we determined that these bacteria showed a good capacity of tomato root colonization, a very important fact for an effective biocontrol activity.

Finally, it was important to determine if these bacterial isolates could develop in conditions similar to those found in soils at the field level, before they are formulated to be used at large scale. We determined their ability to develop under different pH, salinity, Fe³⁺ and temperature conditions, based on the analysis of soils where tomato is cultivated in the V Region, Chile. B. subtilis and B. lentimorbus developed well in conditions similar to those that can be found at the field level, also showing a good capacity of tomato root colonization in these different conditions. These results suggest that the B. subtilis and B. lentimorbus isolates selected from several bacterial isolated have an excellent potential to be used as biocontrol agents of R. solani in tomato greenhouses at the field level.

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