South America Calling
Alastair Stewart South America Correspondent

Tuesday 11/13/12

Brazil Registers First Cases Of Asian Soybean Rust

With only half the soybean crop in the ground, Brazil has registered its first two cases of Asian rust for the 2012-13 season.

The yield-sapping fungus was identified on stray plants growing on roadsides in the center-west state of Mato Grosso and the southern state of Santa Catarina, according to Embrapa, Brazil's crop research agency.

"We still haven't registered rust in commercial crops this year, but the presence of stray soybeans with rust is an important alert for producers. It's an indication that rust could soon be present in all fields," said Rafael Moreira Soares, crop researcher at Embrapa's soybean unit.

The rust fungus, or Phakopsora pachyrhizi, spreads and intensifies throughout the season, reaching its nadir at around harvest time. Therefore the identification of cases so early in the cycle is a worry.

The fungus causes lesions on the leaves of soybean plants, which impede the formation of the bean and can result in massive yield losses. In more extreme cases, the fungus can kill the plant.

It is virtually impossible to prevent rust's arrival as the fungal spores are carried on the wind. However, it can be controlled by fungicide.

Since it arrived in the first half of the last decade, Brazilian farmers have learned to manage the fungus' threat, spraying on the first reports of cases in their region rather than waiting for the telltale yellow mosaic to appear on their own plants.

However, control comes at a cost. Fungicide applications run at approximately $15 to $20 per acre and multiple applications are necessary when the presence of the disease is particularly strong. For example, last season in Mato Grosso some farmers sprayed five times.

The dry weather in the top-producing center-west region over the last couple of months has undoubtedly limited the spread of the fungus this year, but this may be offset by the widespread presence of stray soybeans that grew on roadsides and untended areas through the winter, which offers the fungus a ready environment to ride out the inter-harvest period.

In an attempt to control rust, producing states proscribe soybean production between July and September.

This policy has proven effective but the hot, wet Brazilian summer presents excellent conditions for the fungus to propagate and virtually all areas register numerous instances of the fungus come February and March. Indeed, rust has contributed to a trend for farmers to plant earlier and use shorter cycle beans.

Posted at 8:13AM CST 11/13/12 by Alastair Stewart
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