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December 27, 2016

Yield Losses can occur if Soybean Rust is less than 80% Controlled

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

Scientists from Embrapa are informing farmers in southern Brazil to be alert for the presence of soybean rust in their soybean fields and to apply a preventative application of fungicide as soon as the disease is reported in their region. They feel the weather is favorable for the spread of the disease in the southern Brazilian states of Mato Grosso do Sul, Parana, and Rio Grande do Sul. The region has had frequent rains with temperatures in the range that allows the disease to spread rapidly.

In the state of Mato Grosso, there have only been a few confirmed cases of soybean rust in commercial fields, but scientists have found the rust spores all throughout the state and they are also advising farmers to apply a preventative application of fungicide.

Scientists have determined that the most cost effective control measure is to achieve an 83% control of the disease. If the control is below 80%, there is the possibility of yield losses occurring in the field. Control can slip below 80% if the wrong chemicals are used or if farmers cut back on the rate of application or wet weather prevents timely applications of the fungicides.

Rust is most likely to invade a soybean plant after the plant starts to flower and when there are prolonged periods of wetness with temperatures in the range of 22°C (72°F). Leaves must be continually wet for at least eight hours for the spores to penetrate the leaf surface. These conditions are most likely to occur after the canopy has closed, which allows for higher moisture levels at the bottom of the plant.

The ideal temperatures for the disease is 22°C (72°F). If the temperature falls to 6°C (43°F), it does not kill the disease, but it stops the growth of the disease. The same thing happens if the temperatures are above 32°C (90°F), the disease just stops growing. So the most likely time for the disease to enter the plant is at night when the leaves may be wet for a prolonged period of time.

When the disease first appeared in Brazil during the 2000/01 growing season, there was only one type of soybean rust, but the disease has now mutated into different races, which has made some of the original fungicides ineffective in controlling the disease. That is why scientists recommend that farmers use a mixture of chemicals each with a different mode of action to slow the development of further resistance. By the 2012/13 growing season, some of the "old standby" chemicals used earlier no longer controlled the disease.

The fungicides used to control the disease are contact fungicides, which means that they must be regularly reapplied as new foliage emerges. If wet weather prevents spraying for an extended period of time, the disease can get ahead of the control measures making it very difficult to maintain at least an 80% control.

Generally, the fungicides are applied every 20-30 days after the disease has been confirmed in the field. In cases of severe in infestation, the interval between applications may be reduced to every 15 days or so. Farmers in Mato Grosso are currently concerned because recent wet weather has prevented them from making timely fungicide applications. There have only been two confirmed cases of soybean rust in commercial fields in Mato Grosso, but scientists have found rust spores throughout the state and they expect the number of confirmed cases to increase significantly.