Performance and Profitability of Rain-Based Thresholds for Timing Fungicide Applications in Soybean Rust Control

Area of Science:

• Plant Pathology
• Agronomy
• Agricultural Economics

Background:

• Soybean rust (SBR), caused by *Phakopsora pachyrhizi*, is a major threat to soybean production in Brazil.
• Current management relies on calendar-based fungicide applications, which may not optimize disease risk assessment.
• Integrating weather-based thresholds could enhance the efficiency and profitability of SBR control strategies.

Purpose of the Study:

• To compare the performance and profitability of rain-based and leaf wetness duration-temperature thresholds for soybean rust management.
• To evaluate the efficacy of different disease severity value (DSV) and daily infection probability value (DVIP) thresholds against a calendar spray program.
• To determine the most economically viable strategy for managing soybean rust in Brazilian soybean fields.

Main Methods:

• Field experiments were conducted across six sites in Brazil over three growing seasons (2012-13, 2014-15, 2015-16).
• Disease severity and yield data were collected from plots treated with different management strategies: DSV50, DSV80, DVIP6, DVIP9, and a calendar program.
• An unsprayed check plot was used to calculate relative control efficacy and economic returns.

Main Results:

• The less conservative rain-based threshold (DSV80) required fewer fungicide applications than other tested thresholds and the calendar program.
• The more conservative rain-based threshold (DSV50) also resulted in fewer sprays compared to the DVIP thresholds.
• While higher spray numbers generally correlated with increased yield, economic analysis revealed no significant differences in the profitability of any tested system.

Conclusions:

• Rain-based action thresholds, particularly DSV80, offer a simpler and potentially more profitable approach to soybean rust management.
• The proposed rain-based models are suitable candidates for integration into SBR management practices in Brazilian soybean production.
• Simplicity and profitability favor the adoption of rain-based thresholds over calendar or leaf wetness duration-temperature models for soybean rust control.