High Risk Fungicides Combined to Low Risk Can Be a New Strategy for Management of Asian Soybean Rust at the Beginning of the Epidemic

Asian soybean rust (ASR) caused by Phakopsora pachyrhizi since it was first reported in Brazil in 2001 has been subject of studies regarding the use of fungicides to its control. Low efficiency of triazol (demethylator inhibitors-DMI’s) or stobilurin (quinone outside inhibitor-QoI’s) has been reported on the control of ASR under field conditons. For this reason experiments were conducted to study the effects of these systemic fungicides associate with protectant multi-site fungicides (MSF) on ASR control preventively and at the onset of the epidemic. All combinations of fungicides were effective to control ASR and promoted greater soybean yield gain compared to the control. The analysis of the efficacy of ASR control in the lower, medium and upper part of the plant canopy, showed that application of Epoxiconazole + Pyraclostrobin and Cyproconazole + Azoxystrobin reduced ASR and leaf fall (%), and increased soybean yield grain. The severity of the disease increased from the lower to the upper part of the plant. The application of DMI+ QoI associated with MSF applyed when disease severity reached 2%, promoted effective reduction of ASR, increased yield and reduced leaf fall. Overal the DMI’s + QoI’s associated with MSF was 68% and 70.2% more efficient on ASR control and soybean grain yield, respectively than the control. Mancozeb and Chlorothalonil associated with the lowest dose of Epoxiconazol + Pyraclostrobin (10.2 + 33.2 g.a.i./ha) or Cyproconazole + Azoxystrobin (12.5 + 30.0 g.a.i./ha), respectively increased soybean grain yield at least 45.0%; the same combinations of the fungicides Epoxiconazol + Pyraclostrobin (15.3 + 49.8 g.a.i./ha) and Cyproconazole + Azoxystrobin (25.0 + 60.0 g.a.i./ha) 50% less than the commercial dose, increased 89.5% and 109.0%, respectively. The use of high risk (DMI and QoI) fungicides combined to low risk (MSF) applyied properly, can be a new strategy for the management of ASR at short and long term, even after the epidemics has reached 2.0% severity. Furthermore, due to the residual effect, MSF can promote higher longevity of DMI and QoI molecules.