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FRITZ, Leila Lucia et al. Frequency of Bacillus spp. in the soils of several different irrigated rice cultivation systems, in Cachoeirinha, RS. Bragantia [online]. 2010, vol.69, n.2, pp.405-412. ISSN 0006-8705.

Entomopathogenic Bacillus are bacteria found mainly in the soil and are considered important agents in biological control of agricultural diseases and pests. However, there are fen studies describing the influence that different agricultural practices have on Bacillus spp. populations. This study proposes to determine the occurrence frequency of entomopathogenic bacteria from Bacillus genus in soil samples. These were collected during the agricultural year 2006/07 from different irrigated rice cultivation systems used in the EEA-IRGA plantations in Cachoerinha, State of Rio Grande do Sul, Brazil. We analyzed 36 soil samples taken from three irrigated rice plantation systems; (i) Pre-germinated (SPG); (ii) Conventional (SPC); and (iii) Direct plantation (SPD) and a fourth sample from a non-planted area (C). In each test one gram of soil was diluted, pasteurized and inoculated in Nutrient Agar (NA) and incubated for 24 hours at 30°C. All the bacteria colony-forming units (CFU) were transplanted to selective mediums and the B. thuringiensis, B. sphaericus and B. cereus species were identified using a phase contrast microscope. No significant differences in the occurrence frequencies were found in the soil samples of all plantation systems analyzed. On the other hand, the Bacillus spp frequency varied in the sampled period during the different growing phases of the irrigated rice in the EEA-IRGA plantations evaluated. Irrigation was the main factor contributing to variation of the Bacillus in the plantations.

Keywords : agroecosystems; entomopathogenic bacillus; cultivation systems; Bacillus thuringiensis; Bacillus sphaericus; Bacillus cereus.

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