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March 13, 2017

Soybean Rust in Brazil Developing Resistance to Newest Fungicides

Brazilian farmers have been battling soybean rust since it was first discovered in Brazil during the 2000/01 growing season. It took a number of years for researchers and farmers to develop successful strategies to limit the extent of the damage from the disease. For the last several years, the number of confirmed soybean rust cases in Brazil has been relatively low, but scientists noticed a disturbing development this growing season in the effort to control the disease.

As recently reported by Noticias Agricolas, scientists noticed that the disease is quickly developing resistance to some of the newest and most popular fungicides on the market. This developing resistance has been observed in the states of Parana and Mato Grosso do Sul.

Scientists are speculating that the resistance is developing faster than normal because of the widespread use of chemicals with the same mode of action. This is resulting in a quicker resistance developing in the disease than if a wide mix of chemicals with different modes of action were utilized.

Researchers indicated that the problem was somewhat hidden this growing season because of the relatively low incidence of the disease. Embrapa is reporting 361 confirmed cases of soybean rust in Brazil thus far this growing season compared to 445 cases last year and 371 cases for the 5-year average. The three states with the most confirmed cases this year are Parana with 87, Rio Grande do Sul with 72, and Mato Grosso do Sul with 63.

As an illustration of how the control measures have improved over the years, while the 5-year average is 371 cases, the 11-year average is 1,154 confirmed cases. Embrapa has been tracking soybean rust in Brazil since the 2005/06 growing season and the worst year was 2006/07 with 2,530 confirmed cases and the best year was 2011/12 with 260 confirmed cases.

Scientists believe that it will probably be five or six years before new chemicals come onto the market to control soybean rust, so in the meantime, they are urging farmers to follow the best practices such as:

Soybean-free period - In the state of Parana farmers are not allowed to have live soybean plants on their property from June 15th to September 15th. Rust spores cannot survive for more than 60 days without a host plant, so the elimination of live soybean plants is one of the best ways of preventing the disease from carrying over from one growing season to the next. The state of Mato Grosso has already extended the soybean-free period in the state and Parana is considering following suit.

No safrinha soybeans - Two crops of soybeans should never be planted back-to-back in the same field during the same growing season. If there is a second crop of soybeans, which is called the safrinha, it extends the disease cycle and makes it much more likely that the disease would infect the next regular crop of soybeans.

A problem for southern Brazil is the fact that farmers in Paraguay plant a significant amount of safrinha soybeans right across the border from Parana and Mato Grosso do Sul. Rust spores are very light and they could easily drift across the border from infected fields in Paraguay. Brazilian scientists are urging the government of Paraguay to adopt more stringent measures in an effort to control the disease.

Limit the planting window - Not only should farmers never plant safrinha soybeans, there should also be restrictions on how late soybeans could be planted. Scientists are urging that no soybeans be planted in Parana after December 31st in order to limit the length of the disease cycle.

Plant early maturity soybeans that offer some resistance - The earlier the soybeans mature, the less time they are exposed to the disease and therefore fewer fungicide treatments are needed to control the disease. Seed companies and scientist are continuing to develop early maturing soybean varieties for Brazilian production. If these early maturing soybeans are planted immediately after the soybean-free period has expired, they could be harvested before the disease becomes widespread. Different soybean varieties offer different levels of resistance to the disease so farmers should plant soybean varieties that have the highest level of resistance.

Rotate chemicals with different modes of action - Scientists are urging farmers to either rotate their fungicides or use combinations of fungicides that have different modes of action in order to slow down the development of resistance. Using the same fungicide year after year encourages more rapid development of resistance.