How to Manage Pests
UC Pest Management Guidelines
White mold generally appears on tomato plants at flowering. Symptoms include water-soaked areas on flowers and at stem joints where senescent flower petals have fallen. The infection quickly kills stems, which eventually dry and take on a bleached appearance. Water-soaked stem lesions may also appear at the soil line if senescent plant debris is present around the plant. Affected areas generally show white, cottony mycelium that soon produces large, irregularly shaped, black sclerotia. Infected fruits turn gray and rot. Sclerotia on infected fruits are usually produced at the point of attachment with the plant.
Sclerotia survive in the soil. When they are within the top 0.8 to 1.2 inches (2–3 cm) of soil they can germinate to form a saucer-shaped fruiting body called an apothecium. Each apothecium produces millions of ascospores that are disseminated by wind. A single sclerotium may produce up to five to six apothecia, depending on environmental conditions. High soil moisture and temperatures of 49° to 59°F (9° to 15°C) favor apothecial production. Because ascospores germinate on and colonize dead or senescent tissue, senescent flowers are frequently the source of new infections. Occasionally infections may originate before flowering from mycelium in the soil that is in direct contact with injured stem tissue.
Both infection and subsequent spread of the disease are determined by temperature and moisture conditions. White mold is favored by cool (59° to 70°F, 15° to 21°C), moist (16 to 72 hours of continuous wetness) conditions. Disease is most severe in low-lying, waterlogged parts of the field.
White mold is generally a minor disease of tomato and specific control measures are usually not warranted. White mold can be difficult to control because infection is caused by both airborne ascospores and soilborne sclerotia. Because of the very wide host range of the pathogen, routine crop rotations are not effective. Most infections are initiated by airborne ascospores, consequently simple sanitation methods around a tomato field do not provide effective white mold control. In years with severe white mold infections, sclerotia in the soil have the potential to create a long term problem.
Attempts to breed for crop resistance against white mold have largely been unsuccessful and currently no resistant tomato cultivars are available. The usefulness of crop rotations and deep-plowing to reduce soilborne inoculum are also limited. Decreased planting densities may open plant canopies to create a less favorable environment for white mold development. Subsurface drip irrigation prevents alternating soil wetting and drying, which favors germination of sclerotia and keeps the soil surface dry, creating less favorable conditions for infection. Subsurface drip irrigations may, therefore, provide a significant, long term control of white mold.
Organically Acceptable Methods
UC IPM Pest Management Guidelines: Tomato