Cause Two fungi, Phakopsora pachyrhizi and P. meibomiae, cause rust on soybean, snap bean, dry bean, cowpea, and numerous other legume species. P. pachyrhizi is considered the more aggressive strain on soybean and also infects kudzu (Pueraria lobata). Disease increases on leaf surfaces during prolonged periods of wet weather and temperatures range from 59°F to 86°F. The soybean rust fungus is not expected to overwinter where temperatures dip below 32°F; however, plant debris contained in seed lots may harbor the rust fungus and enable spread of the pathogen. This disease has not been reported in the Pacific Northwest but P. pachyrhizi was detected on soybean in other regions of the United States
Symptoms Lesions start as small gray spots on the underside of leaf surfaces, turning tan or reddish brown, leaf chlorosis may appear on the upper leaf surface. Lesions are angular due to restriction by leaf veins. Lesions often appear first on older leaves but they also may appear on petioles, stems, and pods. They contain pustules that appear as small, scattered blisters filled with tan urediniospores. Dark brown to black telia develop among uredinia. Severely affected fields have a yellowing or browning coloration and premature senescence.
Cultural control
- Monitor affected fields after confirming the diagnosis with a plant clinic.
- Currently there are no resistant varieties; all soybean varieties grown in the United States are highly susceptible.
Chemical control Fungicide applications may be economical if disease is detected early or if fungicides are applied preventively.
- Echo 720 at 1 to 2.25 pints/A; see label for rates and timing. Do no feed soybean hay or threshings from treated fields to livestock. 12-hr reentry.
- Fontelis (Group 7) at 14 to 30 fl oz/A on 7- to 14-day intervals. Do not make more than two (2) sequential applications before alternating to a labeled fungicide with a different mode of action (non-Group 7). Preharvest interval is 0 days. 12-hr reentry.
- Miravis Neo (Group 7 + 3 + 11) at 13.7 to 20.8 fl oz/A on 14- to 21-day intervals. Do not feed soybean hay, forage and silage. Preharvest interval is 14 days. 12-hr reentry.
- Strobilurin formulations (Group 11) are labeled for use. Do not apply more than one (1) foliar application of a Group 11 fungicide before alternating to a labeled fungicide with a different mode of action.
- Headline at 6 to 12 fl oz/A on 7- to 14-day intervals. Do not apply within 21 days of harvest of bean, 14 days for forage, and 21 days for hay. 12-hr reentry.
- Quadris Flowable at 6 to 15.5 fl oz/A. Do not make more than one (1) application at 15.4 fl oz/A to soybean forage and hay. Do not apply within 14 days of harvest of soybean (bean). May be applied the day of harvest to soybean forage and hay. 4-hr reentry.
- Tebuzol 3.6F (Group 3) at 3 to 4 fl oz/A on 10- to 14-day intervals. Do not apply within 21 days of harvest. 12-hr reentry.
- Trivapro (Group 3 + 7 + 11) at 13.7 to 20.7 fl oz/A on a 14- to 21-day interval. Do not apply more than two (2) applications per year. Preharvest interval for grain is 14 days or R6, whichever is longest. 12-hr reentry.
Reference Hartman, G.L., Sinclair, J.B., and Rupe, J.C. 1999, 4th Edition. Compendium of Soybean Diseases St. Paul, MN: APS Press