The Asian soybean rust, caused by the fungus Phakopsora pachyrhizi, is the disease of greatest virulence and rapid spread affecting soybean culture in Brazil. For the infection to occur in plants, favorable environmental conditions are necessary, especially leaf wetness measured based on the leaf wetness duration (LWD). Changes in the inter-row spacing can modify the LWD. This study aimed to verify the effect of using two inter-row spacings on leaf wetness duration and percentage, as well as the influence on Asian soybean rust initial infection and development. The experiment was conducted in the crop seasons 2011/2012 and 2012/2013 at the farm of "Universidade Estadual de Londrina" (23 º 34 ' S / 51 º 21' W), where SIGA spore traps were installed to identify P. pachyrhizi urediniospores. To quantify LWD and its percentage, Electronic Trees for Wetness containing sensors at three heights (0.9 m, 0.6 m, 0.3 m) were used in 0.45 and 0.8 m inter-row spacings. During the evaluated crop seasons, P. pachyrhizi urediniospores were detected before the first symptoms. About 6 h of LWD over 50% were necessary for the infection to occur and for the manifestation of the first rust symptoms. Additional 2 h of LWD over 50 % were detected in the middle third (0.6 m) of 0.45 m spacing, compared to 0.8 m spacing in both seasons; however, there was no difference in the disease severity between spacings. The largest number of detected urediniospores and the volume of rainfall may explain the greater final severity of soybean rust in the crop season 2012/2013.
Phakopsora pachyrhizi; epidemiology; spore trap; leaf wetness sensor
