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We are now in the hottest part of the year in the desert southwest region of Arizona, with respective average high and low temperatures of 108 and 83°F. Lettuce disease management is probably the last thing on a Pest Control Advisor’s or grower’s mind at this time. However, this is the perfect time to perform preplant soil flooding in fields that had high levels of Sclerotinia drop of lettuce this past season. How can a soil flooding treatment in the summer help manage a disease that will not be a problem for several more months in a yet to be planted lettuce crop? 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 germinating in cool moist soil to infect lettuce plants. Many sclerotia decay naturally over time; however, sufficient numbers can remain in a field after one or more years to cause disease when a lettuce planting is established. If 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 virtually all sclerotia of S. minor and S. sclerotiorum present in soil did not germinate after a 3-week period of flooding in the summer. This soil treatment technique has been used by growers in the Yuma area to successfully manage Sclerotinia lettuce drop in fields chronically affected by this disease. However, some negative effects of the practice have been expressed. One concern is elevation of the water table as a result of this soil flooding procedure, particularly in areas already affected by high ground water. Also, in certain areas of the Yuma Valley, damage to the open drainage system has occurred as a result of erosion of the sides of earthen drains. These concerns led to additional field research to reexamine and possibly refine the duration of soil wetness required to destroy sclerotia of the Sclerotinia drop pathogens. A brief summary of the results from three additional summer flooding research trials follows: 1) 1% and 10% of sclerotia of S. minor and S. sclerotiorum, respectively, survived a 1-week flooding treatment; 2) no sclerotia of S. minor survived a 2- or 3-week continuous flooding period, whereas 1 to 3% of S. sclerotiorum sclerotia did survive this treatment; 3) after flooding soil for one 8-hour period for 1 to 3 weeks or two 8-hour periods for 3 consecutive weeks, from 3 to 7% of S. minor sclerotia and from 28 to 39% of S. sclerotiorum sclerotia could still germinate. This and earlier research demonstrate that the smaller sclerotia of S. minor are more susceptible to summer flooding compared to the larger sclerotia of S. sclerotiorum; therefore, a longer duration of flooding is needed for fields heavily infested with S. sclerotiorum in contrast to S. minor. Short 8-hour flooding periods were inferior to continuous flooding. The choice of duration of continuous flooding will be affected by the species of Sclerotinia in the field, the severity of the infestation, and other factors. Although summer soil flooding may not be appropriate for all ground planted to lettuce, when feasible, this cultural practice can be an effective component of an integrated disease management program for Sclerotinia lettuce drop.
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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