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Lettuce disease management is probably the last thing on a Pest Control Advisor’s
or grower’s mind as we now enter the hottest part of the year in the desert southwest
region of Arizona, where the mean daily high/low temperature is now 108/83°F. However,
this is the perfect time to perform preplant soil flooding in fields that had high
levels of Sclerotinia drop this past season. You might wonder how a soil flooding
treatment in the summer can help manage a disease that will not be a problem for
several more months. The two fungi that cause lettuce drop, Sclerotinia minor and
Sclerotinia sclerotiorum, carry over in fields between crops of lettuce as small
black structures called sclerotia. These fungal propagules function like seeds,
remaining dormant until they germinate in cool moist soil and infect lettuce plants.
Many of these sclerotia will 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 this
field would no longer be a source of the Sclerotinia lettuce drop pathogens. This
is where summer preplant soil flooding comes in. Past research conducted at The
University of Arizona Yuma Agricultural Center demonstrated that a 3-week period
of flooding in the summer destroyed virtually 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.
To contact Mike Matheron go to: matheron@ag.arizona.edu.
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For questions or comments on any of the topics please contact Marco Pena at the Yuma Agricultural Center.
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