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January 17, 2011

Weather Conditions Ideal for Spread of Rust in Brazilian Soy Fields

Researchers for Embrapa are warning Brazilian soybean producers that the recent wet weather in Brazil is ideal for the rapid spread of soybean rust and that they should consider a preventive fungicide application in an effort to keep the disease out of their fields.

Embrapa scientists have confirmed the lowest number of rust cases in five years, but now is the time when the disease can really start to spread rapidly. Soybeans become more susceptible to rust once the plants start to flower and set pods. The plants are also more susceptible after the canopy closes. With a closed canopy, the relative humidity under the canopy increases and that is an ideal microclimate for rust invasion especially during periods of heavy rainfall. The disease usually invades the lower leaves of the plant first and then it makes its way up the plant.

Scientists are recommending that farmers consider a preventive fungicide application as a way to hopefully keep the disease from entering their fields. A preventive application works best when the soybean plants are smaller because the spray needs to reach the lower leaves and that is easier to do when the plants are smaller. A preventive fungicide application can also help to control other foliar diseases as well, not just soybean rust. Individually these other foliar diseases may not cross the economic threshold justifying the expense of a fungicide application, but collectively, controlling these various foliar diseases is economically justifiable.

During the 2009/10 growing season 2,370 cases of rust were confirmed in Brazil with the first case confirmed in commercial soybean fields on November 17th. According to scientists from Embrapa, the reason why the number of rust cases has been relatively low this growing season is because of the extended dry season and the ultra low humidity during the dry season which restricted the germination of volunteer soybeans and other type of host plants. Without a host plant, soybean rust spores generally do not survive for more than about 60 days.