Identifying Soybean Rust Can Be Tricky

Soybean growers from the Gulf coast to the northern plains are on high alert for symptoms of soybean rust in their fields. The problem is, few U.S. growers have actually seen this disease. Making identification even more difficult, soybean rust symptoms can easily be confused with those of brown spot, bacterial pustule or bacterial blight.

Growers need to watch for a progression of symptoms:

Reddish-brown spots appear on plant leaves.
These spots first appear at flowering as tiny spots, usually on the bottoms of leaves.
Lesions grow in size along with plants, becoming raised clusters of tan or brown bumps on leaves.
Plants turn yellow, then defoliate prematurely.
Fewer and smaller pods are produced, leading directly to reduced yields.

Be especially alert for these symptoms when environmental conditions favor rapid soybean rust development:

• At least six hours of moisture
  
  
• Temperatures between 59 and 83 degrees Fahrenheit
  
  
• Additional moisture (rain or dew) to promote the spread after infection has taken place
  
  

The worst-case scenario in the United States would be for a soybean rust infection to explode before it is detected. But at the other end of the spectrum, jumping the gun and assuming that a disease problem is soybean rust could lead to possible panic and unwarranted fungicide treatments.

The best approach is to scout carefully and send samples of potentially infected plants to the state Extension service or department of agriculture for confirmation.