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August 11, 2015

Presence of Rust Spores in Southern Brazil Worries Scientists

Scientists in Brazil are always concerned about eliminating volunteer soybeans in central Brazil during the dry season because they can harbor soybean rust spores capable of infecting the next crop of soybeans. They are generally not too worried about volunteer soybeans in southern Brazil doing the same thing because it is usually too cold for the soybean plants or the rust spores to survive over the winter months (June. July, and August).

This year though, the weather thus far in southern Brazil during their winter period has been mild and wet thanks to the influence of El Nino. In fact, fruit producers in southern Brazil are concerned about how the lack of cold temperatures may impact the flowering of their fruit trees. Over the last two months there have been extended periods of wet weather with temperatures between 18 C and 28 C, which is ideal for the spread of rust spores. As a result, scientists have identified volunteer soybeans in southern Brazil that are showing symptoms of soybean rust. The fear is that these plants will allow the rust spores to move into the newly planted soybeans much earlier than normal.

Generally, rust spores are blown into southern Brazil from states further north such as Mato Grosso and Goias where the soybeans are planted earlier than in Parana or Rio Grande do Sul. That allows the soybean crop in southern Brazil to get established before control measures are initiated. That may not be the case this year if these volunteer soybean plants are not eliminated by freezing temperatures over the next few weeks.

An earlier than normal infection with soybean rust doesn't necessarily mean that the soybean yields will be adversely impacted, but it does mean that farmers will be spraying more to control the disease.

An additional concern is that neighboring Paraguay has also experienced mild temperatures this winter season and rust spores may blow into southern Brazil from the neighboring country. The farmers in Paraguay plant a lot of safrinha soybeans (back-to-back two crops of soybeans during the same growing season) which keeps the disease active until the safrinha soybeans are harvested in May or June. Additionally, farmers in Paraguay are not as concerned about eliminating volunteer soybeans as they are in Brazil because they usually experience periods of freezing temperatures which eliminates the plants.

Scientists have already alerted farmers in southern Brazil to be on the lookout for symptoms of soybean rust earlier than normal and to be prepared to apply fungicides as soon as the disease is confirmed in their area.