Impact of summer flooding on viability of Sclerotinia minor and S. sclerotiorum sclerotia in soil

Lettuce drop, caused by the soilborne fungi Sclerotinia minor and S. sclerotiorum, is an important and destructive disease of lettuce. Research trials in Arizona have shown that some fungicides, such as boscalid and iprodione, can reduce disease incidence up to 50 to 60%. Prior research also demonstrated that viability of S. minor and S. sclerotiorum sclerotia was completely arrested after subjection to a respective 2- and 3-week period of continuous flooding in soil during the summer. Widespread adoption of this cultural disease management tool led to concerns in southwestern Arizona about misuse of a precious resource in an arid environment, damage to drainage canals from excessive ground water, and rising water tables adversely affecting lettuce growth. Overall data from current studies, which compared reduction of sclerotia viability after continuous flooding to less intensive flooding of one 8-h period for 1 to 3 weeks or two 8-h periods for 3 weeks, revealed that continuous flooding was not better than 8-h flooding periods for sclerotia of S. minor but was superior for sclerotia of S. sclerotiorum. Because Arizona lettuce growers have to contend with both species of Sclerotinia, continuous flooding is still the best flooding alternative to achieve virtual elimination of viable sclerotia of both pathogens.