By Mike Matheron, UA Extension Plant Pathologist

Lettuce disease management is likely the last thing on the minds of pest control advisers and growers during the this, the hottest part of the year in the Desert Southwest region of Arizona.

However, this is the perfect time to perform pre-plant soil flooding in fields which had high levels of Sclerotinia drop this past season.

One might wonder how a soil flooding treatment can help manage a disease which will not be a problem for several more months.

First, the two fungi that cause lettuce drop, Sclerotinia minor and Sclerotinia sclerotiorum, carry over in fields between crops of lettuce as black structures called sclerotia. These fungal propagules function like seeds and remain dormant until germination in cool moist soil and infect lettuce plants.

Many of these sclerotia decay naturally over time. However, sufficient numbers can remain in a field after one or more years to cause lettuce drop when a planting is established.

If virtually all sclerotia in a field could be destroyed, then a field would no longer be a source of the Sclerotinia lettuce drop pathogens.

This is where summer pre-plant soil flooding comes in. Research conducted at the University of Arizona’s Yuma Agricultural Center showed a three-week period of flooding in the summer destroyed all sclerotia of S. minor and S. sclerotiorum present in soil.

Some growers in the Yuma area have used this soil treatment technique to successfully manage Sclerotinia lettuce drop in fields chronically affected by this disease.

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Contact Matheron: (928) 726-6856 or