Phakopsora pachyrhizi Airborne Urediniospore Distribution and Contribution to Soybean Rust Disease in Mexico

Phakopsora pachyrhizi, the causal agent of soybean rust, reduces soybean yield by up to 90%. Therefore, the development of preventive disease control measures is needed. We measured the distribution of airborne P. pachyrhizi urediniospores from 18 September 2014 to 21 May 2015, and their contribution to soybean rust in the Altamira municipality of southern Tamaulipas, Mexico. Three experiments, each with a different sowing time (early, late, and very late), were performed using the cultivar 'Huasteca 200', and an additional experiment was conducted with volunteer soybean plants. Urediniospores were trapped at weekly intervals using slide traps placed at two heights and in four different wind directions. Disease incidence and severity were scored beginning 12 July 2014 in each of the three canopy sections, and environmental conditions were registered. Spore distribution was continuous, and only the second of the two spore distribution phases contributed to disease. Disease incidence varied from 30 to 100% in experiments, and severity was the lowest (<0.6%) in experiment 1 (early planting) and highest (74%) in experiment 2 (late planting). In addition, teliospores on leaves were detected for the first time in Mexico. Spore distribution showed significant differences among trapping periods and was significantly correlated with wind direction. Together, these results reinforce cultural control, with early planting as the formal recommendation, because it coincides with the lowest urediniospore distribution and unfavorable temperature (>30 degrees C) during the first phase, as well as low relative humidity (<85%) and zero rainfall during the second phase, thus enabling disease escape during flowering and grain development.