By C. M. Ocamb and D. H. Gent
Cause The fungus, Sclerotinia sclerotiorum, causes white mold. It overwinters as small black structures (sclerotia) in soil. Sclerotia may survive 5 to 8 years in the soil. After a conditioning period of several weeks at 40°F in moist soil, sclerotia can germinate in moist soil when temperatures are 59 to 65°F and produce a small, stalked, cup-shaped fruiting structure (apothecium; plural: apothecia), which releases millions of spores into the air. Sporulating apothecia can persist 5 to 10 days. Spores are forcibly ejected and a few may be blown up to a mile but most land nearby. Spores can survive 2 weeks. Under moist conditions, spores may infect senescent tissue such as blossoms and leaves or may germinate and colonize plant debris. After colonizing blossoms or senescing leaves, the fungus can invade any healthy part of the plant it contacts. Moist conditions within the plant canopy favor infection. Rain, dew, and/or irrigation practices that keep foliage wet for long periods favor white mold development.
Most other plants are susceptible, including pea, lettuce, carrot, cabbage, parsnip, potato, sunflower, radish, other crucifers, and cucurbits. Dandelion (Taraxacum officinale) is a very susceptible weed host.
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 that comes 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.
- Reduce humidity and high moisture periods within the plant canopy and field.
- Provide adequate aeration within, and especially between, rows by increasing plant and row spacing. This is not always economically feasible.
- If available and suitable in quality, plant cultivars, which are more erect and upright or that do not produce excessive foliage near the ground level (open-base types).
- Orientating the bean rows in the direction of prevailing winds is useful for white mold control if the placement of irrigation equipment allows row orientation.
- Time irrigations to allow drying of plant canopy before night fall.
- Avoid excessive irrigation after petal fall
- Apply sufficient nitrogen to meet crop demands, but avoid excessive fertilization that can lead to dense, lush plant growth.
- To reduce pathogen population within a field:
- Rotate with non-hosts for 8 years to achieve best control, but for at least 2 years to reduce population of sclerotia; grasses, cereals, and onion are not affected by white mold.
- Deep plowing buries sclerotia but plowing later years may return viable sclerotia to the surface.
- Field flooding during warm temperatures destroys sclerotia.
- A timely harvest with rapid cooling and pod storage at 45 to 50°F will provide an effective postharvest control.
Chemical control In fields with a history of white mold, apply fungicide at 1 to 10% bloom (that is, 1 to 10% of the plants in the field have at least one open bloom). A second application may be necessary with highly susceptible cultivars or heavy disease pressure.
- Botran 75W at 2.25 lb/A for bush variety or 4 lb/A for pole variety on 7-day intervals. Use in the past has shown poor efficacy. 12-hr reentry.
- Cannonball WP 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 at the beginning of flowering and again at full bloom if conditions are favorable for disease or disease is present. Preharvest interval is 7 days. 12-hr reentry.
- Fontelis at 16 to 30 fl oz/A at the beginning of flowering and again at full bloom if conditions are favorable for disease or disease is present. Preharvest interval is 0 days. 12-hr reentry.
- Iprodione products 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.
- Omega 500F 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 14 days of harvest. 48-hr reentry or 72-hr reentry for high exposure activities.
- Switch 62.5WG at 11 to 14 oz/A. Apply when 10 to 20% of plants have at least one open bloom and again 7 days later. Do not apply within 7 days of harvest. 12-hr reentry.
- Thiophanate-methyl formulations 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. Preharvest interval is 14 days. 12-hr reentry.
- Topsin 4.5 FL at 30 to 40 fl oz/A for one (1) application or 20 to 30 fl oz/A for two (2) applications. For two (2) applications, make the first application when 10 to 30% of the plants have at least one bloom open and repeat application 4 to 7 days later. For a single application, apply when 100% of the plants have at least one bloom open. Preharvest interval is 14 days. 12-hr reentry.
- Topsin M 70WP at 1 to 1.5 lb/A with first application when 10 to 30% of the plants have at least one bloom open and a second application 4 to 7 days later; or Topsin M 70 WP at 1.5 to 2 lb/A as a single application when 100% of the plants have at least one bloom open. Do not apply within 14 days of harvest. 12-hr reentry.
- Contans WG at 1 to 6 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.