Asian soybean rust control in response to rainfall simulation after fungicide application

Abstract

Asian soybean rust (Phakopsora pachyrhizi) is the main disease that affects soybean in Brazil. Fungicide applications are the main control method, but they can be influenced by the occurrence of rain. We aimed to study the control of Asian soybean rust in response to the occurrence of simulated rainfall at different times after fungicide application. The penetrant fungicides trifloxystrobin + prothioconazole (60 + 70 g a.i. ha-1) and azoxystrobin + benzovindiflupyr (60 + 30 g a.i. ha-1) and the nonpenetrant fungicides mancozeb (1,500 g a.i. ha-1), chlorothalonil (1,440 g a.i. ha-1), and copper oxychloride (672 g a.i. ha-1) were tested using two spray volumes: 70 and 150 L ha-1. Rain was simulated from 30 to 240 minutes after fungicide application. Soybean leaflets were collected and inoculated with a spore suspension of P. pachyrhizi (5.0 x 104 mL-1) and incubated in plastic boxes for 20 days. The trials were repeated twice. Nonpenetrant fungicides were more susceptible to rain washing, mainly when the 70 L ha-1 spray volume was used. For the penetrative fungicides, the best control percentages were obtained when the rainfall occurred between 120 and 180 minutes after application, while the protective fungicides had the best control percentages when the rainfall occurred approximately 240 minutes after application. The Asian rust control is affected by the characteristics of the fungicide applied, by the time interval between fungicide application and rain occurrence and by the spray volume.