Soybean rust continues to spread in central Brazil

Soybean producers in central Brazil are increasingly concerned about the spread of rust in their soybean fields. While southern Brazil has been suffering from a prolonged drought, central Brazil has been inundated by heavy rains for nearly the last two months. As a result, Mato Grosso with 47 confirmed cases of rust and Goias with 35 cases now represent over 75% of the total number of rust cases being reported in Brazil. Soybean rust, if left untreated, can be devastating for soybean production, but Brazilian scientists and farmers have learned how to adequately control the disease and to keep the losses to a minimum if the weather cooperates.

The problem this growing season is the fact that the weather is not cooperating. The constant overcast skies, frequent rains and high humidity that have been common in central Brazil since mid-December are ideal conditions for the spread of the disease. The disease starts at the bottom of the plant and works its way upward colonizing new leaves as they appear at the top of the plant. That is why it is imperative that the fungicides used to control the disease reach deep into the canopy of the crop.

The heavy rains and wet conditions are complicating control measures. Since the fungicides used to control the disease are contact fungicides, they must be reapplied at regular intervals to protect the new emerging leaves at the top of the plant. In a normal year the fungicides are usually reapplied at intervals of 25 to 30 days. During times of intense rainfall, much of the fungicide is washed off the leaves and the time interval between applications can be a short as 15 days. If the wet conditions keep ground sprayers out of the field and the heavy cloud cover prohibits aerial application, the disease can advance unimpeded and result in yield losses.

That appears to be what is happening in northern Mato Grosso where some producers are concerned that they may have already lost 10 to 15% of their anticipated soybean yield in the most severely affected fields.

In recent years, farmers in central Brazil have been applying slightly less than two fungicide treatments for the control of rust. During this growing season, farmers have already indicated that they may end up applying fungicides three or four times to control the disease.

The different maturity soybeans grown in Mato Grosso can also help to spread the disease. The early maturing soybean varieties may be able to get through their entire life cycle with minimal damage from rust, but a problem arises when these fields are harvested. During the harvest process, the light-weight rust spores can easily become airborne and spread to nearby fields where the soybeans are still green and developing. As a result, agronomists are advising farmers to inspect their fields on a daily basis especially if nearby early maturing soybeans are being harvested.

Embrapa, in conjunction with the Association of Soybean Producers in Mato Grosso (Aprosoja) have set up a series of 11 mini laboratories positioned around the state where trained technicians can identify the presence of soybean rust on plant samples brought into the lab. Agronomist and farmers in Brazil have become very familiar with the symptoms of the disease, but they still need to have the presence of the disease confirmed by trained specialists. The information also goes into a database so that Embrapa can track the spread of the disease throughout Brazil and alert farmers when it has been found in their area.