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November 28, 2017

First Case of Soybean Rust in Commercial Fields Confirmed in Brazil

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

On November 20th, the first confirmed case of soybean rust was identified in the municipality of Itabera in the state of Sao Paulo. The rust was found in a field planted as soon as the soybean-free period ended on September 15th. Since then, there have been two more cases of rust confirmed in western Parana. Previously, Embrapa had identified about two dozen cases of rust in volunteer soybeans over the last several months.

This is actually a little late for the first case of soybean rust to be confirmed in commercial soybean fields in Brazil. The delay is due to the dry weather that delayed start to the soybean planting. Farmers in western Parana are always concerned about rust spores blowing into the state from neighboring Paraguay, which is just across the Parana River. Farmers in Paraguay are allowed to plant their soybeans earlier than in Brazil, so the disease can advance more rapidly in Paraguay resulting in rust spores blowing into western Parana.

The three cases this year in commercial fields compares to 23 cases last year and 27 cases for the five-year average. The most cases for this period were recorded during the 2015/16 growing season when 69 cases had been confirmed. The fewest number of cases were in 2005/06 and 2007/08 when no cases of rust had been confirmed by the end of November.

Scientists in Brazil are warning farmers to pay close attention on how well their fungicides control the rust because the disease has developed resistance to many of the standard fungicides. Embrapa is recommending that farmers rotate their fungicides and that they reapply fungicide every two weeks after the disease has been confirmed in their area. The previous recommendation was to reapply a fungicide every three weeks. They shortened the time interval between applications because many of the fungicides are losing their effectiveness against the disease.

Soybean Rust Identified in kudzu in northern Argentina - In a bulletin released by university researchers in Argentina, they indicated that soybean rust has been confirmed in kudzu (Pueraria iobata) in the provinces of Misiones and Buenos Aires. It has been long known that kudzu is a host plant for soybean rust, but this is a new occurrence in Argentina.

Generally, soybean rust in Argentina originates as spores are blown into Argentina from neighboring Paraguay and Brazil. The fact that rust has been identified in kudzu plants in Argentina is an indication that the disease may be capable of overwintering within the country.

If soybean rust is left unchecked, it can infect up to 90% of the soybeans in a region within 21 days resulting in significant yield loses. Therefore, it is recommended in Brazil that farmers apply a fungicide as soon as the disease has been identified in their region.

For those of you not familiar with kudzu, it is an extremely fast growing vine that has carpeted areas in the southeastern U.S. Individual vines have an astounding daily growth rate and are capable of completely covering buildings, roads, trees, etc. It produces an underground tuber and the plant is extremely hard to control. It was introduced into the southeastern U.S. in the early 1900's to control soil erosion and it has been spreading ever since.