RELATION BETWEEN SUPPRESSIVENESS TO TOMATO FUSARIUM WILT AND MICROBIAL POPULATIONS IN DIFFERENT GROWTH MEDIA

Tomato Fusarium wilt suppressiveness of three growth media, with respect to peat and vermiculite, and the relationship between microbial populations of growth media in the severity of this pathogen were examined. The growth media evaluated were olive oil husk + cotton gin trash composted and mixed with rice husk (OC+R), spent mushroom composted and mixed with peat (SM+P) and coir fibre. To determine the role of the microflora in the suppression, the two composts were also heated to 60°C for 6 days. Three bioassays were carried out with these infested growth media and controls not infested. Disease severity was recorded during 25 days after tomato seedling transplantation. In these seven growth media the density of culturable groups of microorganisms was determined by dilution plating on semi-selective media before the bioassays. The two composts showed suppressiveness to Fusarium wilt with respect to vermiculite and peat while coir fibre was conductive. Furthermore, in OC+R its suppressiveness is due more to the growth media’s non heat-labile properties than in SM+P. Significant negative correlations were found between severity and Bacillus spp., cellulolytic actinomycetes and bacteria, oligotrophic actinomycetes and bacteria, copiotrophic bacteria, total actinomicetes (cellulolytic + oligotrophic + copiotrophic) and total bacteria (cellulolytic + oligotrophic + copiotrophic) populations in these growth media. Bacillus spp. are known antagonists, so they may account for the suppressiveness in our composts. Cellulolytic and oligotrophic actinomycete populations were associated with Fusarium wilt suppressiveness for another growth media. High microbial activity has been reported as a Fusarium wilt suppressiveness factor. Copiotrophic, celullolitic and oligotrophic bacteria contribute to this microbial activity.