If you suspect rust, here's a recap of how to identify, collect samples and treat, according to USDA pest management centers in cooperation with the National Plant Diagnostic Network, APHIS and ARS.
Identification: Soybean rust symptoms are similar for both fungal species: Phakopsora pachyrhizi and P. meibomiae. Check lower leaves first for small lesions that increase in size and change from gray to tan or reddish brown on the underside of leaves. Lesions are most common on leaves but may occur on petioles, stems and pods.
Once pod set begins on soybeans, infection can spread rapidly to the middle and upper leaves of the plant.
Environmental conditions impact the incidence and severity of rust. Prolonged leaf wetness combined with temperatures between 59°F and 86°F and humidity of 75-80% is required for spore germination and infection. Under these conditions, pustules form within 5-10 days and spores are produced within 10-21 days. High levels of infection result in distinct yellowing and browning of fields and premature senescence in plants.
Using a 20X hand lens along with a pocket picture guide helps in identifying rust spores, says Glenn Hartman, plant pathologist at the Agricultural Research Service.
Collect Samples: Collect samples by placing each plant sample in a self-locking plastic bag and maintain under cool conditions such as refrigeration. Place samples in a sealed paper bag if cool conditions are not available. Then when refrigeration is available, place the paper bag inside a self-locking plastic bag.
Leaves should be kept flat by placing them between paper towels or pieces of paper. Don't add water to the samples.
Record information for each sample: date, host plant, collector's name; phone number; collection location within field; and location of field, including state, county, township and nearest road intersection. Also, record global positioning system location information if available.
Mark sample containers with a permanent marker and print all information.
Submit samples to your state's university diagnostic lab or Department of Ag diagnostic lab for identification. Also, contact your state university extension personnel for additional help. Each state has developed an invasive species response program as part of the USDA National Plant Diagnostic Network.
Treatment: So far, all commercial soybean varieties available are highly susceptible. Current research is screening germplasm for resistance. Soybean breeders say the U.S. is six to eight years away from developing a resistant variety.
For now, early detection is the key for best control. Once confirmed, check with your state university personnel for fungicide treatments available.
Three registered fungicides in the U.S. are available that have shown control on soybean rust: Bravo, Echo and Quadris. In addition, Environmental Protection Agency Section 18 Quarantine Exemption requests have been submitted by several states to expand the number of approved fungicides available. They include Bumper, Eminent, Folicur, Headline, Laredo, Pristine, Propimax, Stratego and Tilt. At press time, 16 of the possible 30 soybean growing states had signed on for section 18s, according to Kent Smith, plant pathologist at USDA's Office of Pest Management Policy.
“Section 18s don't go into effect until soybean rust is found in the continental U.S.,” explains Anne Dorrance, Ohio State University plant pathologist.
Even with all the fungicide possibilities, there's still a decision-making process. “All fungicides are not equal,” says Miles. “Even though the disease is very controllable with fungicides, farmers need to be familiar with what chemistries work and at what disease levels they are effective. It's a lot like selecting herbicides.”
Experts say to always read, understand and follow label information on each fungicide.
For a complete rundown on soybean rust, check out the North Central Integrated Pest Management Center site at: www.ncipm.org/soybeanrust or USDA-APHIS at: www.aphis.usda.gov/ppq/ep/soybean_rust/. In addition, log on to: www.planthealthinfo.com.
You can also find a wealth of information at: www.cornandsoybeandigest.com. Just click on “special reports” on the left side of the homepage, then click on Asian soybean rust.
Many state university and state soybean association Web sites have rust information or call your local extension service or state agriculture departments.