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February 17, 2020

Soybean Rust in Brazil Lowest on Record, Scientists Speculate Why

Author: Michael Cordonnier/Soybean & Corn Advisor, Inc.

There have been 120 cases of soybean rust confirmed in Brazil compared to 293 cases last year and 317 cases for the 5-year average. The state of Parana continues to be state with the most cases with 44 followed by Mato Grosso do Sul with 30, Rio Grande do Sul with 16, Mato Grosso with 9, Goias with 8, Minas Gerais with 3, Santa Catarina with 3, Sao Paulo with 3, Bahia with 2, Federal District with 1, and Rondonia with 1.

This growing season continues to be the best year by far for controlling soybean rust and Brazilian scientists think they know why. They attribute the low incidence of rust to the dry weather during September and October that extended the soybean-free period and delayed the planting of the soybeans. The dry weather diminished the amount of inoculum (rust spores) available to infect the newly planted soybeans.

The soybean-free period in Brazil generally runs from May/June to September/October depending on the location. During that period, no live soybean plants are permitted and farmers are required to eliminate any soybean plants found on their property. Soybean rust spores can only survive for about 60 days without a host plant, so the longer there are no live soybean plants, the fewer the number of rust spores that can survive to infect the next soybean crop.

When the summer rains did start, they were lighter than normal and not as frequent as normal. As a result, there were no prolonged periods of wet weather or prolonged periods of wet leaves during the time when the soybeans are most vulnerable to infection, which is when the soybeans are flowering and the canopy is closing. The weather is different every year and the weather this growing season was not conducive to a rapid spread of soybean rust.

The next best year for rust control was the 2011/12 growing season when 204 cases had been confirmed by this date. Soybean rust was first confirmed in Brazil during the 2000/01 growing season.