Atividade de rizobactérias antagonistas a Fusarium oxysporum F. sp. Passiflorae em solos cultivados com milho, sorgo e milheto

Abstract

CAPÍTULO 1 = Although Brazil ranks as the world's largest producer of passion fruit, fruit productivity in the country is low due the occurrence of disease in the crop. The fusarium wilt, caused by Fusarium oxysporum f. sp. passiflorae (Fop), is one of the most feared diseases lead to death of the plant reducing the productive period of culture. Currently, the joint use of cultural practices and biological control have outstanding importance in the integrated management of plant diseases. Information monocots effect on the activity of rhizobacteria antagonists in the soil can be used for optimization of an integrated management program of fusarium wilt, allowing biocontrol agents remain active in the area in which the protection should be taken. In addition, monocotyledons may serve as an extra source of income to the producer. Thus, the objective of this study was to evaluate the activity of rhizobacterial antagonists to Fop in soils cultivated with maize, sorghum and pearl millet, in the greenhouse. In this sense, rhizobacteria were isolated from the rhizosphere soil of passion fruit and subjected to tests for detection the production of diffusible compounds, volatile compounds and chitinases.167 isolates were evaluated, 21 of them were positive for the production of diffusible compounds, two for volatile compounds, 32 to produce chitinases and eight had at least two mechanisms of action. A test mutual antibiosis and growth at different pH levels was carried out with these eight isolates verifying the incompatibility isolated 76 with others; incompatible isolated 77 with isolated 89 and growth of all isolates in wide pH range. Microbial activity was evaluated in soils cultivated with maize, sorghum and millet, in addition to fallow, in the greenhouse, through the metabolic quotient (qCO2), obtained by the ratio between the basal soil respiration (BSR) and microbial biomass carbon (MBC), and the enzyme acid phosphatase activity. For this, three combinations of rhizobacteria, based on mutual antibiosis test, were applied to pots with crops of monocots and after 47 and 63 days of application, soil samples were collected and sent to the laboratory for analysis. The CBM and RBS were determined by methods of chloroform fumigation-extraction and chloroform-fumigation-incubation adapted, respectively. The activity of acid phosphatase was analyzed only after 63 days by measuring the concentration of liberated ρ-nitrophenol resulting from the enzymatic hydrolysis of ρ-nitrophenyl phosphate. There were significant differences (p <0.05) in the treatments tested only the second time of sampling, to 63 days. By analysis of variance, the interaction between species and rhizobacteria combinations was significant (p <0.05) only for the qCO2 attribute, whose values tend to be higher under millet cultivation in combination 3 (isolates R77 + R95 + R104 + R120) (0.03 mg C-CO2 kg-1 CBM h-1). The enzyme acid phosphatase activity showed higher values for the pearl millet crop (66.87 μg of ρ-nitrophenol cm³ soil h-1) confirming greater rizobacteriana activity in this culture. CAPÍTULO 2 = One of the main diseases affecting the passionflower in northeastern Brazil is fusarium wilt, caused by Fusarium oxysporum f. sp. passiflorae (Fop). This disease leads to irreversible death of the plants causing great economic loss. The rhizobacteria have demonstrated their potential as in vitro biocontrol agents, but methods are needed to ensure the activity of these microorganisms in the field, since not always control is obtained under these conditions. The effect of root exudates on soil microbial activity is a well reported mechanism and varies according to the species of the plant, on this, the objective of this study was to evaluate the activity of rhizobacterial antagonists to Fop in soils cultivated with maize, sorghum and pearl millet, in the greenhouse. For this, rhizobacteria were isolated from the rhizosphere of passion fruit and tested for potential inhibition of Fop by production of volatile compounds, diffusible and chitinase in vitro. Among the tested rhizobacteria, 26.95% demonstrated antagonist activity presented at least one of the tested mechanisms of action. Of the antagonists tested, two produced volatiles compounds, 21 produced diffusible compounds, 32 produced chitinases and eight produced at least two of these substances. With isolates that have at least two mechanisms of action against Fop was performed a reciprocal antibiosis growth test and a test in a wide pH range. It was verified by mutual antibiosis test the incompatibility between isolated 76 and the others, except the isolated 77, and of this with isolated 89. In the pH test revealed that all isolates grew in a wide pH range, ranging from 5.0 to 7.0 on the third day of incubation. The following combinations of rhizobacteria: 1) R76 + R77; 2) R77 + R97 + R116 and 3) R77 + R95 + R104 + R120 were applied to soils cultivated with maize, sorghum and millet, plus the fallow, in the greenhouse, to determine the microbial biomass carbon (MBC), basal soil respiration (BSR) and metabolic quotient (qCO2), after 47 and 63 days from the application of rhizobacteria, and determination of the enzyme acid phosphatase activity after 63 days of the application of the same. There was a significant difference between the cultivated species by Tukey test (p <0.05), with regard to MBC levels, RBS, qCO2 and acid phosphatase activity only in second time of sampling, to 63 days. The interaction between the cultivated species and combinations rhizobacteria, as variance analysis, was significant (p <0.05) only for qCO2 attribute, whose values tend to be higher under millet cultivation in combination 3. The higher acid phosphatase activity was also obtained with the millet crop. The metabolic quotient and acid phosphatase activity were the biological parameters used to determine the millet culture such as that provided greater activity of the antagonists in the soil.