Dec. 26, 2018
Author: Michael Cordonnier/Soybean & Corn Advisor, Inc.
Embrapa reported that there have been 113 confirmed cases of soybean rust in commercial soybeans fields in Brazil with 44 reported in Parana, 27 in Rio Grande do Sul, 13 in Mato Grosso do Sul, 8 in Santa Catarina, 7 in Sao Paulo, 5 in Minas Gerais, 5 in Goias, 3 in Mato Grosso, and 1 in Rondonia. Last year at this time, there had been 28 confirmed cases and the 5-year average is 78. The worst growing season was 2009/10 when there had been 292 cases by this date and the best growing season was 2011/12 when 16 cases had been confirmed by this date.
The number of rust cases this year is more than what you would expect given the recent dry weather in southern Brazil. Scientists are attributing the high number of rust cases to favorable weather during the off-season that helped the proliferation of volunteer soybeans. These volunteer soybeans acted as host plants for soybean rust which allowed the spores to infect the newly planted soybeans.
Soybean rust spores can only survive for approximately 60 days without a host plant, so that is the reason why the soybean-free period was instituted in 13 Brazilian states. Everyone tries to control volunteer soybeans during the soybean-free period, but it is an impossible task given the fact that soybeans can germinate and grow along the sides of roadways all across Brazil.
Another way they try to control the spread of rust spores is by limiting how late in the growing season farmers are allowed to plant soybeans. Seven Brazilian states have set an end date for soybean planting, which is generally December 31st. These states have also expressly prohibited the planting of a second crop of soybeans in the same field during the same growing season. Prohibiting a second crop of soybeans is a way to reduce the number of fungicide applications during the growing season and thus, holding down the likelihood that the disease will develop resistance to the fungicides.
If soybeans are planted late, there would be a proliferation of spores that could infect the crop shortly after planting. This in turn would require a higher number of fungicide applications increasing the pressure on the disease to develop resistance to the chemicals. This prohibition of safrinha soybean production is necessary because some of the early planted soybeans can actually be harvested before December 31st.
Scientists stress that the primary tools to control rust include: use soybean varieties that are more resistant, respecting the soybean-free period by eliminating all volunteer soybeans, set an end date for soybean planting and not allow safrinha soybean production, and using the most efficient fungicides and rotate fungicides. The cost of controlling soybean rust in Brazil is approximately US$ 2.8 billion due to application costs and lost productivity.