By C. M. Ocamb and D. H. Gent
Bean, Snap (Phaseolus vulgaris) - White Mold (Sclerotinia Rot)
Cause The fungus, Sclerotinia sclerotiorum, is a cosmopolitan fungus that occurs across the globe, infecting a very large number of plant species including radish and Brassica crops. Other susceptible host plants include bean, carrot, pea, lettuce, potato, green pepper, cucurbits, parsnip, radish, sunflower, herbs including mint, fruit trees including apple, nectarine, and sweet cherry, and a large number of ornamentals both woody and herbaceous. Weeds can also be host to S. sclerotiorum; dandelion (Taraxacum officinale) is a very susceptible weed host. Stem infections can cause severe losses in seed crops.
This fungus survives as sclerotia, which may be associated with infected plant residues, contained in seed lots (external or internal to seed), or persist in the soil. Sclerotia can survive three to five years in the soil, depending on environmental conditions and cropping practices. Typically, strains of this fungus outside of tropical zones require a conditioning period in moist soil of several weeks at 40°F or cooler temperatures, after which sclerotia can germinate in soil when the soil water potentials are ≥-100 kPa and temperatures are between 41°F and 68°F. The production of apothecia appears to be prevented when soil matric potentials are below -0.05 MPa. The upper temperature threshold for conditioning and germination of sclerotia is 68°F and 77°F, respectively. Typically, sclerotia germinate and then produce a small, stalked, cup-shaped fruiting structure known as an apothecium, which forcibly ejects millions of ascospores into the air. Sporulating apothecia may persist five to ten days while released ascospores can survive up to two weeks, depending on environmental conditions. Since ascospores require an exogenous nutrient source before infecting a plant, the primary sites for initial infection are senescing tissues such as spent blossoms and declining leaves or plant tissues that have suffered mechanical injuries or wounding through infection by other pathogens. For infection, ascospores require free moisture or a relative humidity close to 100% with a temperature between 50°F and 86°F. Sometimes sclerotia germinate and produce undifferentiated mycelium that directly infects plant portions just below or at the soil line. After infection of the plant has occurred the mycelium spreads by hyphal growth, the fungus can then invade any healthy, vigorous part of a plant with which it comes in contact, and the pathogen may grow from plant to plant if a diseased plant is in direct physical contact with neighboring plants. Moist conditions within the plant canopy favor infection, as do rain, dew, and/or irrigation practices that keep foliage wet for long periods.
Symptoms The first symptom is usually water-soaked lesions on stem and pods. The fungus may invade the stem near the soil line, causing a rapid wilting and death of the entire plant, or it may invade pods or branches and foliage come in contact with colonized tissue. When dry, lesions on infected stems and pods are beige to white due to the production of acetic acid by the fungus. They frequently have sclerotia embedded in them both internally and on external surfaces. In addition, infected stem surfaces may be papery where the epidermis separated from underlying tissue. Newly forming sclerotia are white and change to black after several days.
The beige to white lesion on infected stems and the white mold growth and sclerotia on plant parts help distinguish this disease from Pythium blight, with which white mold may be confused.
Sometimes, the presence of white mold in a field may occur as fine cottony wisps of fungal mycelium growing on plant debris on moist soil. However, many other fungi also can colonize debris, so that is not a useful diagnostic character.
- Plant varieties that do not produce a heavy vine or are more erect and upright. Bush or semivine types allow better air circulation between rows, thus permitting better drying of the soil surface and plant canopy and providing unfavorable environmental conditions for the white mold fungus.
- Do not over irrigate. After pods start to form, allow soil surface to dry out before irrigating again. If white mold is abundant in early August, no additional irrigations are advised. Research in south-central Idaho indicates a final irrigation about four (4) weeks prior to undercutting reduces disease significantly.
- Space plants for good aeration between and within rows and orient to the prevailing wind if the placement of irrigation equipment allows.
- Apply adequate nitrogen for crop needs, but not apply excessive fertilizer that leads to dense, lush plant growth that is favorable for white mold.
- When white mold is present, extend the interval between irrigations to allow plants and the soil to dry.
- Plowing deeply to bury sclerotia may be beneficial once the pathogen is in a field, but plowing the field in later years may return viable sclerotia to the surface where they can produce apothecia.
- Practice crop rotation by growing nonsusceptible crops (grasses and cereals) for 8 to 9 years, before planting susceptible crops (bean, lettuce, potato, crucifers, celery, etc.).
Chemical control Fungicides are recommended if the bean field has a history of white mold and if rows are expected to close by early bloom. One (1) application of fungicide when all plants have one open blossom significantly reduces disease. For southern Idaho growers, a single application of Topsin M applied when 100% of the plants have their first open blossom generally provides effective control through harvest.
Fungicides may be applied by ground sprayers equipped with drop nozzles that apply the fungicide to the lower, interior parts of the plants. Some materials can be applied through sprinkle irrigation. Aerial application for white mold control generally has not given satisfactory results and is not recommended. Applications after the disease is widespread in the field will not satisfactorily control the disease.
- Blocker 4F (Group 14) at 4 pints/A at planting and again on 2- to 3-week intervals when disease is severe. Do not apply after the start of pod formation. Do not feed treated vines to livestock. 12-hr reentry.
- Cannonball WP (Group 12) at 7 oz/A when 10% of the plants have at least one (1) open blossom. Preharvest interval is 7 days. 12-hr reentry.
- Carboxamide (Group 7) formulations are registered for use. Do not make more than two (2) sequential applications before alternating to a labeled fungicide with a different mode of action.
- Endura at 8 to 11 oz/A. Preharvest interval is 21 days. 12-hr reentry.
- Luna Privilege at 6.84 fl oz/A on 14-day intervals. Do not apply within 14 days of harvest. 12-hr reentry.
- Vertisan at 16 to 20 fl oz/A. Preharvest interval is 21 days for seeds; 0 days for hay. 12-hr reentry.
- Cueva (Group M1) is labeled for white mold control but efficacy is unknown. May be applied on the day of harvest. 4-hr reentry. O
- Iprodione products (Group 2) such as Nevado 4F or Rovral 4 Flowable at 1.5 to 2 pints/A when 10% of plants have at least one open bloom and again 5 to 7 days later or up to peak bloom if conditions are favorable for disease. Not recommended in Idaho due to lower level of control. Do not allow foraging for 14 days after last application. Do not feed dry bean hay to livestock until 45 days after last application. 24-hr reentry.
- Jet-Ag at 0.75 to 3.9 fl oz/5 gal water as a weekly preventative treatment. A preliminary study showed good control when tank-mixed with Amplitude or Stargus. 4-hr reentry. O
- LifeGard WG (Group P6) at 1 to 4.5 oz/A on 7- to 14-day intervals for activating plant resistance. Refer to label for appropriate rate per application volume. Preharvest interval is 0 days. 4-hr reentry. O
- Omega 500F (Group 29) at 0.5 to 0.85 pint/A when 10% to 30% of the plants have at least one (1) open blossom and if needed, again 7 to 10 days later. Do not apply within 30 days of harvest. 12-hr reentry or 72-hr reentry for high exposure activities.
- Proline 480 SC (Group 3) at 4.3 to 5.7 fl oz/A on 5- to 14-day intervals. Preharvest interval is 7 days. 12-hr reentry.
- Regalia (Group P5) at 1 to 4 quarts/A plus another fungicide on 7- to 10-day intervals. Does not benefit from the addition of an adjuvant. Preharvest interval is 0 days. 4-hr reentry. O
- Switch 62.5WG (Group 12 + 9) at 11 to 14 oz/A. Apply when 10% to 20% of plants have at least one open bloom and again 7 days later. No more than two (2) sequential applications. Do not apply within 7 days of harvest. 12-hr reentry.
- Thiophanate-methyl formulations (Group 1) are very effective on Sclerotinia.
- T-Methyl 4.5F AG at 30 to 40 fl oz/A. Make first application when 10% to 30% of plants have at least one open bloom and again no earlier than 7 days later. May be applied by ground or air. Preharvest interval is 28 days. 12-hr reentry.
- Topsin M WSB at 1 to 2 lb/A, depending on whether making one (1) or two (2) applications. When making two (2) applications, apply when 10% to 30% of plants have at least one open bloom and again 4 to 7 days later. Do not apply within 28 days of harvest. Do not use if the crop is for forage. May be applied by ground, air, or through sprinkler irrigation lines according to label directions. 3-day reentry.
- Amplitude (Bacillus amyloliquefaciens strain F727) at 2 to 4 quarts/A on 7- to 10-day intervals. Preharvest interval is 0 days. 4-hr reentry. O
- Contans WG at 1 to 4 lb/A, depending on depth of incorporation, as a preplant or postharvest treatment. Incorporate thoroughly in the top 2 inches of soil. 4-hr reentry. Studies in snap bean show that protective fungicides are still required as only a few sclerotia are needed to cause an epidemic in a field. O
- Double Nickel LC at 0.5 to 4.5 pints/A at planting and again at cultivation, can repeat on 10- to 14-day intervals. Can be applied the day of harvest. 4-hr reentry. O
- Serenade Opti at 14 to 20 oz/A on 7- to 10-day intervals. Applications can be made up to and the day of harvest. 4-hr reentry. O
- Stargus (Bacillus amyloliquefaciens strain F727) at 2 to 4 quarts/A plus a nonionic surfactant on 7- to 10-day intervals. Gave good control with white mold on snap bean when used in a 2-spray program in studies conducted by Cornell. Preharvest interval is 0 days. 4-hr reentry. O
References Caesar, A.J., and Pearson, R.C. 1983. Environmental factors affecting survival of ascospores of Sclerotinia sclerotiorum. Phytopathology 73(7):1024-1030.
Hao, J.J., Subbarao, K.V., and Duniway, J.M. 2003. Germination of Sclerotinia minor and S. sclerotiorum sclerotia under various soil moisture and temperature combinations. Phytopathology 93:443-450.
Mila, A.L., and Yang, X.B. 2008. Effects of fluctuating soil temperature and water potential on sclerotia germination and apothecial production of Sclerotinia sclerotiorum. Plant Dis. 92:78-82.