NC State Extension Publications

Pathogen

Lettuce drop is caused by the fungal pathogens Sclerotinia sclerotiorum and Sclerotinia minor.

Host Crops and Plants

Sclerotinia sclerotiorum and S. minor have a wide host range infecting allium (onion, garlic), brassica (cabbage, broccoli), solanaceae (pepper, potato), cucurbits (cucumber, squash), lettuce, and sweetpotato crops. The broad host range of both pathogens significantly limits the efficacy of crop rotation. Cereals (maize, wheat, sorghum) are less susceptible to these pathogens. For more information about reported hosts for these pathogens, visit the USDA Fungus-Host Distributions Database.

Host Parts Affected

Affects the upper and lower leaves as well as the stems of lettuce.

Identification

Lettuce drop initially causes the lower leaves to wilt and drop to the soil surface (Figure 1). When moisture is present, a white mycelium can form (Figure 2) and sclerotia can develop (Figure 3 and Figure 4) where the leaf is in contact with the soil. S. sclerotiorum can also affect the upper leaves of the plant by airborne spores.

Lettuce drop symptoms

Figure 1. Lettuce drop symptoms.

Dr. Lina Quesada, NCSU Vegetable Pathology Lab

Mycelia of Sclerotinia on lettuce

Figure 2. Mycelia of Sclerotinia on lettuce.

Shawn Butler, NCSU PDIC

Sclerotia of Sclerotinia causing drop on lettuce

Figure 3. Sclerotia of Sclerotinia causing drop on lettuce.

Shawn Butler, NCSU PDIC

Sclerotia of Sclerotinia causing drop on lettuce

Figure 4. Sclerotia of Sclerotinia causing drop on lettuce.

Shawn Butler, NCSU PDIC

Favorable Environmental Conditions for the Disease

The disease prefers cool and moist conditions and wet soil.

Disease Transmission

The pathogen survives via sclerotia in the soil and in the case of S. sclerotiorum, by wind with sclerotia that produce ascospores.

General Disease Management

  • Application of fungicides at early plant growth to control S. minor.
  • Applications of fungicides to plants that are more mature to control S. sclerotiorum.
  • Deep plowing to keep sclerotia from the soil surface.
  • Crop rotation with non-hosts.
  • Drip irrigation below the soil surface.

Disease Control for Conventional Growers

Fungicides may be required for control of lettuce drop, especially if disease was present in prior lettuce crops. Before disease appears, apply fungicides on a 14-day interval. For the latest fungicide recommendations for lettuce drop see the Southeastern US Vegetable Crop Handbook. Fungicide labels are legal documents, always read and follow fungicide labels.


Example products for control of lettuce drop:

Active ingredient Example product Pre-harvest Interval (day) Fungicide group
Boscalid Endura 14 7
Fludioxonil Cannonball 7 12
Iprodione Rovral 14 2
Dicloran Botran 14 14


Useful Resources

Acknowledgements

This factsheet was prepared by the NCSU Vegetable Pathology Lab.

Author:

Associate Professor, Vegetable Pathology
Plant Pathology

Publication date: March 21, 2015

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