The American Soybean Association (ASA) has received confirmation from the U.S. Department of Agriculture’s (USDA) Animal Plant Health Inspection Service (APHIS) that Asian Soybean Rust (Phakopsora pachyrhizi), has been found on soybean leaf samples collected from two research plots near Baton Rouge, Louisiana.

ASA President Neal Bredehoeft, a soybean producer from Alma, Mo., said, "Potential yield loss from soybean rust starts to diminish as plants begin to reach maturity. At this time of the year, when soybeans in many areas of the United States have already reach full maturity and the crop is ripe, soybean rust will no longer reduce this year’s crop yields."

Soybean rust has the potential to cause large crop and economic losses to soybean growers and associated industries. Soybean rust poses no health risk to humans or animals. Soybean rust attacks the foliage of soybean plants causing the leaves to drop early, which inhibits pod setting and reduces yield. Host plants infected with soybean rust first exhibit small lesions that gradually increase in size and turn from gray to tan or brown. Once lesions appear, defoliation is rapid, resulting in fewer pods, fewer seeds per pod, lower seed weight and early plant maturity.

The APHIS Soybean Rust Assessment Team is to report to the site where the samples were collected within 24 hours of confirmation. It will be important for the State to secure the confirmed positive field and allow no one to enter until the APHIS team arrives. However, nearby fields should be examined for signs of soybean rust, and State authorities are encourage to take the lead.

"There are currently no rust-resistant or tolerant soybean varieties," Bredehoeft said. "Research is ongoing, but such varieties still are 5 to 10 years away. Fungicide treatments currently represent the only option for containing soybean rust by lessening the spread of spores. Fungicide use in other countries has been effective in keeping soybean rust below the economic threshold of yield loss."

If soybean rust becomes widespread in U.S. soybean production areas, it could cause large crop and economic losses to soybean growers and associated industries. Growers returning from, or hosting visitors from, rust infected soybean production areas should be extremely careful that the disease is not transmitted to their fields.

Growers in areas near the outbreak should survey their fields to inspect for symptoms of soybean rust disease. Inspection consists of a thorough visual examination of soybean plants in the field and of other host plants in the vicinity of the fields being surveyed. A 20-power hand lens will be required to inspect the underside of the lower leaves in the lower crop canopy for uredinial pustules that are powdery, and buff or pale brown in color.

As soybean plants mature and set pods, infection may progress rapidly under favorable environmental conditions to cause high rates of infection in the middle and upper leaves of the plant. Soybean rust thrives on moisture, high humidity and moderate temperatures. Clouds of spores may be observed within and above canopies of highly infected fields. Fields with high infection rates may begin to look yellow or brown.

"Growers with soybean fields near soybean rust infected fields should assume some level of infection in their own fields due to the fact the disease is easily spread from one field to the other and plants can remain symptom-less after infection for about 10 days, depending on the environment," Bredehoeft said.

If a nearby field is populated with less mature plants because it was planted later, and soybean rust is found, the grower should contact their state extension specialist, crop advisor, or local ag dealer for recommendations on fungicide products and application methods to reduce the spread of rust spores.

In nearby fields that are not rust infected, growers will need to decide whether to take any preventative steps, however, this late in the growing season, there is little economic benefit to be gained for this year’s crop from the application of fungicide products after the pods are set.

Soybean rust can infect a variety of other legumes that could serve as an inoculum reservoir with potential for winter carryover. In addition to soybeans, there are also over 100 other plant species reported to be hosts for soybean in nature. One widespread host in the United States is kudzu. It is highly likely that kudzu could serve as an inoculum reservoir for soybean rust, similar to what has been occurring in Brazil.

"The risk of an outbreak during the next growing season will depend on how early soybean rust emerges in the southern U.S. and how quickly prevailing weather conditions promote the dispersal of soybean rust spores," Bredehoeft said.