Lettuce drop (white mold)
Sclerotinia spp.
Also Known As - White mold (lettuce drop)What is Lettuce drop (Sclerotinia spp.)?
Sclerotinia lettuce drop disease is caused by two species, Sclerotinia minor and Sclerotinia sclerotiorum, affecting lettuce in California. S. minor primarily infects stems and leaves in contact with the soil, leading to brown, soft decay, wilting, and plant collapse at maturity. White mycelia and small black resting bodies (sclerotia) form on the decayed crown. S. sclerotiorum infects lower leaves and stems, along with aerial spores causing watery, soft rot and larger sclerotia.
How does Lettuce drop (Sclerotinia spp.) occur?
These fungi reproduce and spread through the production of small, hard, resting structures called sclerotia. These sclerotia can survive in the soil for extended periods, up to 2-3 years, in the absence of suitable hosts. When favorable conditions of cool and moist weather occur, the fungi develop fruiting structures (apothecia) that produce airborne spores. These spores can infect lettuce plants, especially damaged or senescent tissue, leading to disease development and symptom expression.
Symptoms
1 - Effects on Plants
• Infected plants experience rapid wilting due to damage to their vascular systems caused by the fungus. • White mold can cause significant yield loss in affected crops. Infected plants may show reduced quality, including decay and rotting of affected tissues.
2 - Effects on Soil
• Sclerotia, the survival structures of the white mold fungus, can persist in the soil for several years and can lead to disease recurrence in subsequent planting seasons. • White mold's wide host range can make crop rotation challenging as many different crops can be affected.
Solutions
1 - Cultural Practices
• Canopy Management: Proper row spacing and removal of obstructions for airflow. • Irrigation Management: Avoid long periods of leaf wetness and use drip irrigation if possible. • Crop Rotation: Rotate to non-host crops to reduce sclerotia buildup. • Weed Management: Control weeds are known to be hosts of white mould. • Tillage Practices: Consider ploughing to bury sclerotia but be cautious of bringing buried sclerotia back to the surface.
2 - Biological Controls
• Coniothyrium minitans: Use products containing C. minitans to parasitize sclerotia. Sufficient time is needed for C. minitans to colonize and provide control.
3 - Chemical Controls
• Apply fungicides as protectants against aerial spores and manage sclerotia. • Consider using products like Endura, Fontelis, and Miravis Prime to reduce plant death caused by white mold.
