A widespread outbreak of aerial infections caused by the lettuce drop pathogen Sclerotinia sclerotiorum was reported in several locations in Yuma during the last week of December. A review of the biology of the two lettuce drop pathogens and the environmental conditions required for production of airborne spores may help explain this occurrence. Lettuce drop is caused by two fungal pathogens, Sclerotinia minor and Sclerotinia sclerotiorum. Both pathogens produce structures called sclerotia which allow these organisms to survive in soil between plantings of host crops. In desert plantings, infection of lettuce by S. minor and usually by S. sclerotiorum results from direct germination of sclerotia in soil followed by colonization of the base of plants. However, when soil moisture and temperature conditions are favorable, sclerotia of S. sclerotiorum an inch or less below the soil surface can create fruiting bodies that in turn produce vast number of spores that are dispersed by wind throughout the field and to other fields. These spores will germinate and cause aerial infections when deposited on lettuce leaf tissue. The optimal conditions that stimulate airborne spore production include exposure of sclerotia to nearly saturated soil for at least a 2-week period and soil temperatures ranging from approximately 52 to 60 F. Soil in vegetable production fields is normally very wet and soil temperatures from November 26 until present have been in the favorable temperature range. These airborne spores require free moisture (from rainfall, dew or sprinkler irrigation) on senescent or damaged leaf tissue for optimal infection to occur. The weather record shows freezing temperatures throughout the area on November 26 and 27, resulting in damaged lettuce leaf tissue, and rainfall on December 21 and 22. Thus, the favorable conditions for airborne spore production and infection were present in the area. In other agricultural regions where airborne infection of crops by S. sclerotiorum is common, foliar application of fungicides such as Endura, Rovral, or Switch can provide significant disease protection. For lettuce, initial application of fungicides during the rosette stage, which is about 30 to 40 days before harvest, has been shown to significantly reduce the incidence of lettuce drop caused by airborne infections of Sclerotinia sclerotiorum.

Sclerotinia sclerotiorum

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.