June 30, 2015

Chemicals used to Control Soy Rust in Brazil losing Effectiveness

Scientists in Brazil are becoming increasingly concerned that soybean rust is developing resistance to the fungicides used to control it. Currently, the chemicals used in Brazil to control rust have three modes of action and scientists are worried that the diseases could develop multiple resistance to chemicals with different modes of action.

Part of the problem is that the soybean growing season is extended especially when safrinha soybeans are planted. In some areas of Brazil there are live soybeans growing in the field from September to June allowing ample time for the disease to develop resistance due to multiple fungicide applications. Additionally, many Brazilian farmers only use chemicals with one mode of action which can accelerate the development of resistance.

According to the Embrapa scientist, Claudia Godoy, the diseases is becoming less sensitive to the various fungicides. Some chemicals that originally controlled 90% of the disease are now down to 20% to 40% effectiveness. In the worst-case scenarios, some farmers have had to spray up to ten times during the growing season to adequately control the disease, which indicates that they are not listening to the recommendations of scientists to rotate the chemicals use to combat rust. Multiple applications of just one chemical can also accelerate the development of resistance. Scientists fear that some chemicals will be completely ineffective against the disease in as little as two years.

A loss of effectiveness of the fungicides was expected by the scientists, but the speed by which the disease has developed resistance has been a surprise. When the disease first appeared in Brazil during the 2000/01 growing season, the control methods were very ineffective, but since then, scientists and farmers have done a good job in controlling the disease.

But, with the chemicals losing their effectiveness and no new chemicals on the near term horizon, scientists are worried the disease could start to get worse. That is one of the reasons why they have pushed for the elimination of safrinha soybean production by expanding the soybean-free period. Starting with the 2015/16 growing season, safrinha soybean production in Mato Grosso will be eliminated and other states such as Parana are debating doing the same thing.

A lack of crop rotations is also making it harder to control the disease. In the state of Mato Grosso, the first crop planted is virtually a monocrop of soybeans with virtually all the corn grown as the safrinha crop. The state of Parana is also nearly a monocrop of soybeans as well. During the 2015/16 growing season, as much as 90% of the first crop in Parana will be soybeans as farmers plant more of their corn as a second crop. This decline in crop rotations is very worrisome for scientists.

Soybean rust has not been a major concern in Brazil for the last four growing season but as fungicides lose their effectiveness, that may not be the case going forward.