December 7, 2011

First Cases of Rust Identified in Brazilian Soybean Fields

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

Researchers in Brazil have identified the first few cases of soybean rust in the fields of Brazil. The first case of rust was found in a commercial field of soybeans near the town of Cristalina, Goias. The second case was identified in volunteer soybeans near the town of Nova Aurora, Parana. The volunteer soybeans where the rust was found in Parana had earlier germinated unnoticed and was already in the pod filling stage when the disease was discovered. Researchers from the Antirust Consortium are now advising farmers in both states (as well as the other soybean producing regions of Brazil) to keep a close watch on their soybeans because they are starting to bloom and that is when the plant is most susceptible to the disease.

The type of soybeans grown in Brazil are determinates, which means they grow first and then start to flower. Therefore, when flowering starts the plants are generally large and the canopy has filled in, which makes an ideal environment for the introduction of the disease. The disease needs prolonged periods of high humidity to invade the leaves and those conditions are most likely found at the bottom of the plant. That is why the lower leaves are infected first and the disease moves up the plant. If unchecked, the disease can cause losses as high as 80%.

Historically the first cases of rust are generally identified in Brazil at the end of November or early December when the crop starts to flower, so this a normal time for the disease to appear. Fungicides can adequately control the disease, but the farmer must be sure that the chemicals penetrate the canopy to the lower leaves. The fungicides used to combat the disease are contact fungicides, which mean that they must be reapplied as new leaves emerge. During times of prolonged rainy weather, the disease can spread quickly and that is precisely the type of conditions in which farmers may have difficulty in applying the fungicides needed to control the disease.

Since the disease was introduced into Brazil a little more than a decade ago, control methods have improved significantly. Brazilian scientists have recently developed soybean varieties that are more tolerant to the disease. These new soybean varieties still require fungicide applications to control the disease, but the frequency of the applications is reduced. They are currently working on new varieties with two genes of resistance, which should improve the tolerance even more and slow the development of new strains of the disease.

Soybean rust will continue to be a chronic problem for Brazilian soybean farmers for the foreseeable future. Eliminating host plants such as volunteer soybeans during the dry season has helped to slow the spread of the disease, but until scientists develop truly resistant soybean varieties, it is going to be an expensive battle keeping the disease in check.