Lettuce drop, caused by Sclerotinia spp.—specifically S. sclerotiorum and S. minor—is a highly destructive disease that poses significant challenges to lettuce production. Its impact is magnified by its ability to:
- Infect a broad range of hosts, including allium, brassica, solanaceae, cucurbits, lettuce, and sweet potato.
- Survive in soil for over eight years, thanks to its resilient overwintering structures.
Favourable conditions for infection:
The pathogen thrives in cool, moist conditions but can also infect crops in warmer, wet environments. These environmental preferences make Sclerotinia spp. a persistent threat across diverse climatic regions.
Symptoms:
- Initial signs: Symptoms begin on the outer leaves of lettuce plants that come into contact with the soil. The affected leaves show wilting and eventually die off.
- White mycelial growth: Upon inspection, white fungal growth can be observed on the undersides of infected leaves.
- Formation of Sclerotia: As the disease progresses, black, seed-like overwintering structures (sclerotia) develop within the mycelial growth. These sclerotia allow the pathogen to survive in the soil until a suitable host becomes available.
- Plant collapse: Eventually, the entire plant wilts, becomes slimy, and feels slippery to the touch.
Management challenges:
While some chemical treatments can prevent the germination of overwintering spores, relying solely on chemical control is insufficient to manage this disease effectively.Â
Integrated management strategies:
- Crop rotation: Implement a rotation system with non-host crops to disrupt the pathogen’s life cycle.
- Irrigation management: Avoid creating conditions that allow free moisture to persist, as this promotes disease development.
- Field preparation: Ensure deep ploughing during bed preparation to bury sclerotia and reduce their viability.
By combining cultural practices with chemical measures, farmers can reduce the prevalence and severity of lettuce drop, safeguarding their crops against this formidable disease.