INFLUENCE OF TEMPERATURE ON THE Rhizoctonia solani (KUHN) STRAINS OBTAINED FROM COTTON FIELDS ON THE BRAZILIAN STATES

The temperature is important for studies of Rhizoctonia solani (Kuhn), since it is a cosmopolitan and polyphagous pathogen. Appropriate temperatures can favor the pathogen, which starts the infection process. On the other hand inappropriate temperatures can impose a delay to its growth and development. The objective were evaluate the influence of temperature on mycelial growth, sclerotia production and pathogenicity of R. solani strains. In the fields were obtained 18 strains from cotton seedlings on the States of Minas Gerais-MG (8), Bahia-BA (3), Goias-GO (2), Mato Grosso-MT (4) and Mato Grosso do Sul-MS (1), which were tested at temperatures of 15 degrees C, 18 degrees C, 21 degrees C, 24 degrees C, 27 degrees C and 30 degrees C. For mycelial growth, strains were placed in Petri dishes (9 cm diameter) containing potato-dextrose-agar (PDA). The dishes were placed into a germination chamber with a photoperiod of 12 hours. There were orthogonal daily measurements of the diameter of the colony during 8 days and the rate of mycelial growth was quantified afterwards. The dishes were kept for three months in the respective growth chambers for the sclerotia production analysis. For the pathogenicity determination and evaluation of disease severity the method described by Oliveira et al. (2008) was followed. The data were subjected to analysis of variance. There was significant interaction between isolates and temperatures. Among the eight strains of Minas Gerais, one had a higher rate of mycelial growth at 24 degrees C and three at 27 degrees C, adjusting to the quadratic model. Three strains showed better fit to a linear model and did not differ statistically for the temperatures. Strains from GO had a higher rate of mycelial growth temperatures of 24 degrees C and 27 degrees C. Concerning about the strains from MT, two were fit to a linear model, while the other two had the quadratic model at temperatures of 21 degrees C and 24 degrees C. Strains from MS was adjusted to quadratic model at 27 degrees C, while all three strains from BA were fitted to the linear model. The largest number of sclerotia was observed at 15 degrees C and 18 degrees C except for MS strain, which obtained the highest number at 27 degrees C. It was found that 14 strains (six from MG, two from BA, two from GO, three from MT and one from MS) showed a higher severity between 24 degrees C and 27 degrees C, adjusting to the quadratic model, while three isolates (two from MG and one from MT) did not differ significantly for the temperatures evaluated and only one isolate (BA 2 - 101) had set better to the linear model.