The aim of this study was to evaluate changes in the morfology and length of the germ tube of Puccinia kuehnii, under the influence of temperature and incubation time. The development of the germ tube was asssesed for 24 hours. The concentration of uredospores was 2x105 esporos.ml-1 calibrated by a Neubauer chamber and plated in a 0.1 ml aliquot in water – agar (1.5%), placed in BOD and regulated at temperatures of 10, 20 and 30 °C. Readings were performed after 1, 3, 6, 12, 18 and 24 hours, with 5 replications. At the end of each period, germination was stopped by adding 0.1 ml of lactophenol. The length and morphology of 10 germinated spores per plate were evaluated with the help of a Motic Images MCCamera – Plus 2.0 ML software. Data were subjected to analysis of variance in a 3x6 factorial design with an interaction in orthogonal polynomials. The model estimated that a temperature of 20 °C, at 24 hours of incubation, was required to reach the maximum length. Germ tubes morphological changes were observed at incubation temperatures of 10, 20 and 30 °C showing zigzag, rectilinear and branched forms, respectively.

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