By Mike Matheron, UA Extension Plant Pathologist

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. The pathogens produce structures called sclerotia which allow the organisms to survive in the soil between the 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 the soil followed by the colonization of the base of the plants.

However, when soil moisture and temperature conditions are favorable, sclerotia ofS. sclerotiorum an inch or less below the soil surface can create fruiting bodies that in turn produce vast number of spores dispersed by wind throughout the field and to other fields. The spores 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 two-week period and soil temperatures ranging from approximately 52 to 60 degrees F. Soil in vegetable production fields is normally very wet and soil temperatures from Nov. 26 until the present have been in the favorable temperature range.

The 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 Nov. 26 and 27 resulting in damaged lettuce leaf tissue, and rainfall on Dec. 21 and 22. The favorable conditions for airborne spore production and infection were present in the area.

In other agricultural regions where airborne infection of crops byS. sclerotiorum is common, foliar application of fungicides including Endura, Rovral, or Switch can provide significant disease protection.

For lettuce, the initial application of fungicides during the rosette stage, 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.

Contact Matheron: (928) 726-6856 or matheron@ag.arizona.edu.