The two fungi that cause lettuce drop, Sclerotinia minor and Sclerotinia sclerotiorum, carry over in fields between lettuce crops as structures called sclerotia. These seeds of the fungal pathogens remain dormant in soil until they germinate in cool moist soil and infect lettuce plants. Many of the sclerotia decay naturally over time; however, more than enough can remain in a field after one or more years to cause lettuce drop when the crop is planted. If virtually all sclerotia in a field could be rapidly destroyed, then this field would no longer be a source of the lettuce drop pathogens. This is where summer preplant soil flooding comes in. Past research trials conducted at the University of Arizona Yuma Agricultural Center demonstrated that a 3-week period of flooding in the summer completely destroyed all sclerotia of S. minor and S. sclerotiorum present in soil. Some growers in the Yuma area have used this procedure to successfully control lettuce drop in fields chronically affected by this disease. For more information on this topic, the reader is invited to read the complete research paper published in Plant Disease, Vol. 89, number 1, 2004.

Summer Preplant Soil Flooding

To contact Mike Matheron go to: matheron@ag.arizona.edu.

For questions or comments on any of the topics please contact Marco Pena at the Yuma Agricultural Center.
College of Agriculture, The University of Arizona, Tucson, AZ.