By C. M. Ocamb and D. H. Gent
Cause Phakopsora pachyrhizi causes rust on soybean, common bean, lima, cowpea, scarlet runner and other legume species. P. pachyrhizi is considered aggressive on soybean and also infects kudzu (Pueraria lobata). It is considered a minor problem on common bean in Africa, Asia, Australia, and Central and South America. Disease increases on leaf surfaces during prolonged periods of wet weather and temperatures ranging from 59°F to 86°F. Plant debris contained in seed 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 the southeastern part of the United States during 2004. Another soybean rust pathogen (P. meibomia), which causes American soybean rust is present in South America and the Caribbean, but does not pose a risk to common bean.
Symptoms Lesions start as small gray spots on the underside of leaf surfaces, turning tan, reddish brown, or purple. 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.
Chemical control Fungicide applications may not be economical if disease is minor but fungicides are available for use on beans.
- Chlorothalonil formulations (Group M5) are labeled.
- Bravo Weather Stik at 1.375 to 2 pints for dry bean and 1.375 to 3 pints/A for snap bean on 7- to 10-day intervals. Do not apply within 7 days of harvest for snap bean or within 14 days of harvest for dry bean. 12-hr reentry.
- Echo 720 at 1.38 to 2 pints/A for dry bean and 1.375 to 3 pints/A for snap bean. Begin during early bloom and repeat on 7-day intervals for snap bean and 7- to 10-day intervals for dry bean. 12-hr reentry.
- Miravis Neo (Group 7 + 3 + 11) at 13.7 fl oz/A prior to disease development on a 14-day interval. No more than two (2) applications before alternating with a fungicide that has a different mode of action. May be applied up to 14 days before harvest. 12-hr reentry.
- Rally 40WSP (Group 3) at 4 to 5 oz/A on 7- to 10-day intervals is labeled for use on snap bean for bean rust and would aid in controlling soybean rust. Application can be made on day of harvest. 24-hr reentry.
- Strobilurin fungicides (Group 11) are labeled for use. Do not make more than one (1) application of any Group 11 fungicide before alternating to a labeled fungicide with a different mode of action. Do not apply more than two (2) applications of any Group 11 fungicide per season.
- Headline at 6 to 9 fl oz/A prior to disease development on 7- to 14-day intervals. Preharvest interval is 7 days for snap bean, 21 days for dry beans, and 14 days for forage and hay. 12-hr reentry.
- Quadris Flowable at 6 fl oz/A prior to disease development on 7- to 14-day intervals. May be applied the day of harvest for succulent bean; do not apply within 14 days of harvest for dry bean. 4-hr reentry.
- Vertisan (Group 7) at 14 to 20 fl oz/A on 7- to 14-day intervals for dry beans only. 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 21 days for seed; 0 days for hay. 12-hr reentry.
Reference Hall, R. 1991. Compendium of Bean Diseases St. Paul, MN: APS Press.