Progress of Asian soybean rust and airborne urediniospores of Phakopsora pachyrhizi in southern Brazil Progresso de ferrugem asiática da soja e quantifica o de urediósporos de Phakopsora pachyrhizi no ar, na regi o sul do Brasil

Asian soybean rust, caused by the fungus Phakopsora pachyrhizi, was reported at epidemic levels in 2003/2004 and is the main soybean disease in Brazil. The aim of this study was to investigate the spread of Asian soybean rust and to quantify airborne urediniospores in the region of Campo Mour o, Paraná State, Brazil. Three experiments were conducted under field conditions during the 2007/08 and 2008/09 crop seasons. Using the disease gradient method, provided by the application of increasing levels of the fungicide tebuconazole, four Asian soybean rust epidemics at different intensities were obtained in each experiment. To quantify the urediniospores, weathercock-type spore collectors were installed during and between the two crop seasons. Disease progress curves were plotted for each epidemic, and maximum severity was estimated. The curves were fit to the logistic model, which provided higher coefficients of determination and more randomly distributed residuals plotted over time. Analyses of the area under the disease progress curve showed that the largest epidemics occurred in the 2007/2008 crop season and that the progress rates were higher for severity, even among plants protected with the fungicide. The number of urediniospores collected in the air was related to the presence of soybean plants in the cultivated crops. The quantity of urediniospores was also positively correlated to the disease severity and incidence, as well as to cumulative rainfall and favorable days for P. Pachyrhizi infection. A ferrugem asiática, causada por Phakopsora pachyrhizi, relatada em nível epidêmico em 2003/04, é a principal doen a na cultura as soja no Brasil. Este trabalho teve como objetivos estudar o progresso da ferrugem-asiática da soja e quantificar os urediniósporos no ar na regi o de Campo Mour o no Estado do Paraná, regi o Sul do Brasil. Três experimentos foram instalados em condi es de campo, nas safras 2006/07 e 2007/08. Utilizando-se o método do gradiente de doen as, proporcionado pela aplica o de doses crescentes do fungicida tebuconazole, foram obtidas quatro epidemias em cada experimento com diferentes intensidades de ferrugem asiática. Para quantificar os uredinióporos, foram instalados coletores de esporos tipo catavento durante as duas safras e entressafras. As curvas de progresso da doen a das epidemias foram plotadas e estimaram-se a severidade máxima. As curvas foram ajustadas ao modelo logístico por apresentarem maiores coeficientes de determina o e melhor aleatoriedade dos resíduos plotados no tempo. As análises da área abaixo da curva de pr