Sensitivity of Phakopsora pachyrhizi Isolates to Fungicides and Reduction of Fungal Infection Based on Fungicide and Timing of Application.

Soybean rust (SBR), caused by Phakopsora pachyrhizi, is a damaging foliar fungal disease in many soybean-growing areas of the world. Strategies to manage SBR include the use of foliar fungicides. Fungicide types, the rate of product application, and the number and timing of applications are critical components for successful rust management. The objectives of this study were to determine i) the sensitivity of P. pachyrhizi isolates collected in the U.S. to a range of fungicides and ii) the reduction of fungal infection based on fungicide type and timing of applications on soybean. There were differences (P < 0.05) in effective concentration (EC50) values among the fungicides tested. Azoxystrobin had low EC50 values for both urediniospore germination and fungal sporulation on inoculated leaflets. There were differences (P < 0.05) in fungal sporulation for application times, fungicide treatments, and their interaction when the fungus was inoculated on plants. All application times and nearly all fungicide treatments reduced (alpha = 0.05) fungal infection compared with the nonfungicide control. Information on fungicide sensitivity of P. pachyrhizi isolates and the preventive and curative effects of different fungicides are important in the management of SBR.