Nov 11, 2022

Soybean Rust Spores Detected in Northwestern Rio Grande do Sul

Author: Michael Cordonnier/Soybean & Corn Advisor, Inc.

Farmers in the state of Rio Grande do Sul in far southern Brazil have started to plant their 2022/23 soybeans and Emater has already issued an alert to farmers in the state after they detected asian soybean rust spores in the northwestern part of the state. Asian soybean rust (Phakopsora pachyrhizi) was first found in Brazil during the 2000/01 growing season and over the intervening years, it has become the most important soybean disease in Brazil.

Detecting the presence of rust spores before the disease is confirmed in commercial soybean fields helps farmers prepare their control measures to combat the disease. A rust spore collection system has been in place in the state of Parana for several years and it has proven effective in helping farmers to only apply needed fungicides when there is an imminent threat of the disease.

Emater has established a Rust Monitoring System which has 50 collectors distributed throughout the state of Rio Grande do Sul. The system went operational on October 10th and rust spores were detected a few days after the system was operational. Technicians from Emater indicated that this is early in the growing season to detect the presence of soybean rust spores.

The monitoring system in Rio Grande do Sul was established as a pilot program in during the 2019/20 growing season. During the 2021/22 growing season, the program helped farmers plan their control measures based on soybean development, the amount of spores present, and meteorological conditions.

Soybean rust usually does not infect soybean plants until the soybeans start to flower. If not adequately controlled, it can cause significant yield losses. Brazilian scientists have developed soybean varieties with improved tolerance to the disease, but resistant varieties have not yet been developed.

The presence of soybean rust is what prompted the Brazilian government to establish a "soybean free" period between growing seasons during which no live soybean plants are permitted in fields, along roadways, or around storage and transportation facilities. The absence of live soybean plants between growing seasons has helped to delay the entry of the disease in newly planted soybeans.

Before the disease was present in Brazil, farmers routinely planted two crops of soybeans in the same field during a single growing season. That practice was prohibited as a way to slow the spread of the disease. It also spurred farmers to start planting a second crop of corn instead of a second crop of soybeans. That second crop of corn known as the safrinha is now responsible for more than three quarters of Brazil's total corn production.