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November 20, 2012

Three Cases of Soybean Rust Confirmed in Brazil

Researchers in Brazil have indicated that a second case of soybean rust was confirmed in Mato Grosso last week. A third case had previously been found in the southern state of Santa Catarina. All three of the cases were discovered on infected volunteer soybean plants growing along the side of the highway. The infected plants were in the flowering stage or had already started to fill the pods.

With two cases of rust confirmed in the state of Mato Grosso, the Mato Grosso Soybean and Corn Producers Association (Aprosoja) is now sending out technicians across the state to collect leaf samples from commercial soybean fields. These samples will be analyzed for rust at one of the ten laboratories set up in the state for the specific purpose of tracking the spread of the disease.

The Brazilian research agency Embrapa has been tracking the advance of soybean rust in Brazil since the 2005/06 growing season when they set up a series of laboratories across the country where leaf samples could be brought in by agronomists or farmers to be tested for the presence of the disease. The disease was first detected in Brazil in the 2000/01 growing season, but it took several years to get the detection methodology established and to set up the laboratories. Since then, they have been publishing the number of confirmed cases and their location across the country on a daily basis.

It's relatively early to have three cases of rust by mid-November, but it is not unprecedented. Of the seven previous years for which data is available, four of those years had no cases of rust reported by mid-November and three years did have rust reported by mid-November. There were 15 cases reported by mid-November in 2006/07, 18 cases in 2009/10, and 1 case in 2010/11.

How fast the disease spreads is highly dependent on the weather conditions. During periods of excessive rainfall, the numbers of cases can more than double every week. If the weather is dryer than normal, the number of cases may not increase at all from week to week. Soybean producers are advised to start spraying for the disease as soon as it is reported in their area.

It is too early to say how bad the disease may be during the 2012/13 growing season. Even though the cases reported thus far have only been on volunteer soybeans, I suspect they will find the disease in commercial soybean fields within the next few weeks. As soon as soybeans start to flower, they become more susceptible to the disease. How fast the disease progresses will depend on the weather during the remainder of the growing season.