Incidence of soybean rust declines 92% in Mato Grosso, Brazil

As the soybean harvest wraps up in Mato Grosso, one of the real success stories this growing season has been the 92% reduction in the number of soybean rust cases confirmed in 2010/11. Only 50 cases of rust were confirmed in the state this growing season compared to 623 cases last year. Fourteen municipalities reported the disease this growing season compared to 52 municipalities last year. The number of leaf samples tested also declined to 1,904 compared to 4,003 tested in 2009/10. The municipality with the most cases was Sorriso located in central Mato Grosso with 24 confirmed cases.

Farmers in the state averaged 2.5 applications of fungicides in 2010/11 compared to an average of three applications in 2009/10. According to the Ministry of Agriculture for the state of Mato Grosso, one of the worst years for rust control was during the 2006/07 growing season when all the soybean fields in the state were infected and some fields received up to five applications of fungicides, especially in areas where the soybean planting was later than normal.

Agronomists attribute the reduced number of soybean rust cases to the prolonged dry season in mid-2010 and to the rigorous enforcement of the soybean-free period between June 15th and September 15th. The prolonged dry season (160 days without rain in some cases) eliminated fresh vegetation of developing on most of the potential host plants in the state. The number one host plant is volunteer soybeans of course and by eliminating those volunteer soybeans, the rust spores were denied a way of remaining viable during the dry season.

The ultimate goal in controlling the disease is the development of resistant soybean varieties, but that has been difficult to achieve. The disease has readily mutated into different races which quickly overcomes resistant varieties. Scientists hope to introduce soybean varieties with two genes for resistance which they feel will stand up better to the mutated disease. The disease is also developing resistance to various fungicides requiring more applications and different mixtures of fungicides.

Soybean rust is going to remain a chronic problem for Brazilian soybean producers. Agronomists feel that the best way to control the disease as much as possible is by rigorous enforcement of the soybean-free period across the entire country. They feel that this is the best way to delay the entry of the disease into newly planted soybeans and the longer the disease can be delayed the less damage it could cause. To enforce the soybean-free period, state and federal agencies are beefing up their inspection efforts during the dry season.