Lethality of Simulated UV and Solar Diffuse Radiations to Detached Urediniopores of Phakopsora pachyrhizi

Asian soybean rust (ASR), caused by the fungus Phakopsora pachyrhizi, is the most destructive disease on the crop. This work aimed to generate data to understand the detached uredospores survival during the winter fallow in Mato Grosso state, Brazil. Experiment ‘A’—the detached urediniospores maintained and multiplied on Ativa soybean cultivar, were deposited on soybean estract-agar and kept at 25 oC temperature in closed plastic petri dishes and exposed to UV-C (100-280 nm) radiation, distant 30 cm from the plates surface for ‘ 0’; 60; 120; 180; 240; 300; 360, 420, 480 and 560 minutes. Experiment ‘B’—was similar to previous experiment but spores were exposed to UV-B (280-320 nm) radiation. In Experiment ‘C’—the urediniospores were deposited into empty plastic petri dishes without lids and exposed to direct diffuse sunlight for 0; 60; 120; 180; 240; 300; 360; 420; and 300 minutes. Solar radiation was measured with a pyranometer (Spectrum Technologies, Inc.). After the exposition time, urediniospores were plated on soybean-extract agar, incubated at 25 oC under dark for 8 h and germination assessed under an optical microscope (400‍×). Data were submitted to analyses of variance and regression. All experiments were repeated twice. The detached spores exposed to UV-C were killed with 496 minutes, and under UV-B radiation killed with 962.7 minutes and those submitted to direct diffusion solar radiation killed by 15.6 MJ m-2 dose, or 240 minutes exposition to 1,250 w/m2. It can be inferred that the incident solar diffused radiation in Mato Grosso, during the winter fallow, has the daily potential to kill the detached P. pachyrhizi urediniospores.