Efeito da compostagem na sobrevivência de Ralstonia pseudosolanacearum em restos culturais de tomateiro e qualidade do composto orgânico

Abstract

The composting process is a microbial biotransformation used as an alternative for the adequate destination of the crop residues. It is a simple, economic and efficient technique for the treatment and stabilization of these residues and minimizes the environmental impacts in soil, air and water besides eliminates plant and human pathogens. The goals of this work were to investigate the survival of a mutant of phytobacteria Ralstonia pseudosolanacearum (CRMRS74Rif) in tomato crop residues during composting process and analyze the quality of the organic compound in relation to physical, chemical and microbiological variables. Three composting piles were made of tomato and napier grass crop residues plus ovine manure. Then, nine mesh bags with 10 g of infected tomato plants were placed in each pile, three at each depth of 20, 40 and 60 cm. Piles were turned over after 10, 20 and 30 days when evaluations of bacterial survival were made. At the end of the composting process, we quantified microbial population, antagonism against R. pseudosolanacearum, total phenolic content and compost quality. At the first evaluation (10 days) CRMRS74Rif were not recovered in culture medium, indicating that it was already eliminated of the infected material. Total bacteria were present in high populations in the compound, ranging from 7.0 x 1012 to 2.0 x 1016 CFU/g of compound, and the fluorescent Pseudomonas were the most frequent. Sixty-two isolates showed antagonistic activity against R. pseudosolanacearum CRMRS74Rif, detected by the inhibition halo formation. However, only 32 isolates maintained the antagonism in a second in vitro assay. The fungal population ranged from 3.2 x 102 to 1.2 x 103 CFU/g of compound. Based on morphological structures 40 isolates were identified at genus level: Aspergillus spp., Cladosporium spp., Curvularia spp., Fusarium spp., Paecilomyces spp., Trichoderma spp., and the Oomycete Pythium sp. Among these isolates, nine produced toxic metabolites and inhibited the pathogen growth with halos ranging from 15 to 23 mm. The total population of actinomycetes in compound ranged from 3.8 to 9.5 x 104 CFU/g of compound and based on cultural characteristics 33 isolates were selected for the antagonistic tests. Among them, four produced toxic metabolites against R. pseudosolanacearum CRMRS74Rif with halos ranging from 23 to 33 mm. Total phenolic content found in composting pile averaged 0.0065%. The high temperatures generated in the piles during the termophylic phase (media 50oC), the presence of antagonistic microbial populations and the phenolic compounds might be the cause of the quick elimination of R. pseudosolanacearum from the compound. The results of the physico-chemical and microbiological analysis attended the established limits of the current Brazilian legislation for using and comercialization of an organic compound, indicating that the obtained product has good quality.