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Sclerotinia White Mold
Initial infection of the pathogen is on tissues within the plant canopy, often near the base of the stem at the soil line. Bleached areas and watery soft rots form on the stems and leaf axils and then wet, fluffy white mold develops inside and outside the plant tissue. The white cottony mycelium spreads to the stems, leaves, petioles, and flowers. As the disease symptoms become more severe, the plant wilts and infected tissues become further bleached and brittle. In 7 to 10 days, clumps of mycelium form black sclerotia that are white-pinkish inside. The sclerotia are easily spread by equipment, plant debris, workers, and water.
Sclerotinia sclerotiorum can remain dormant in the soil for 5 years or more as hard, black sclerotia while the environment is warm and dry. Under relatively cool and wet temperature conditions, sclerotia germinate and form apothecia, which are cup-shaped fruiting bodies. The pale brownish yellow apothecia form just above the soil line and produce ascospores that spread through movement of water, wind, plant debris, and workers. The spores land on healthy plants and enter healthy tissues through wounds or natural openings. The spores germinate and form white mycelium, which appear on the tomato plants at flowering. Pale or dark brown water-soaked lesions form on flowers and at stem joints where senescent flower petals have fallen. The pathogen can also attack at the base of the stem causing the plants to wilt and die. As the disease progresses, infected fruits turn gray and rot. Sclerotinia white mold is a monocyclic disease, meaning once the disease cycle is complete, spores are not produced again until the next season. Sclerotia generally require a period of cold temperatures in soil close to saturation to produce apothecia.
Favorable Environmental Conditions
The pathogen favors relatively cool temperatures from 59 to 70°F, especially nighttime temperatures of 60°F. Sixteen to 72 hours of continuous wetness and relative humidity (> 90%) is favorable for spore infection.
Scouting is most important when the tomato plants are flowering. This is when the plants are most susceptible to sclerotinia white mold infections. White cottony mycelial growth will form on flowers, stems, and leaves. The infected plants should be removed and destroyed to prevent further infections.
Currently, there is no threshold information available for Sclerotinia white mold in high tunnel tomatoes. However, disease-forecasting systems have been developed for field crops such as canola.
- Prevent Excess Moisture – If soils remain warm without continuous wet periods, the pathogen will be unable to germinate. Keeping plant density low and pruning to increase air movement will aid in preventing excess moisture on the foliage.
- Chemical Treatments – Preventative fungicide applications will help prevent infections.
- Start with Clean Soil – Sclerotia overwinter in warm and dry soil. By sanitizing or sterilizing soil by soil fumigation, etc., the sclerotia will be destroyed.
- Sanitation – Clean tools and machinery used in the high tunnels to prevent spread of sclerotia and spores. Remove fallen flower petals or dying plant material so that the pathogen cannot survive in the plant debris. Carefully dig up diseased plants and remove soil in a 4-6 inch radius around the base of the stem to remove any sclerotia that may have fallen from the plant to the soil.