The lettuce growing season is starting in the San Joaquin Valley, and it is a good time for growers and pest control advisors to review their strategies for control of Sclerotinia drop disease. Without appropriate management, Sclerotinia drop of lettuce can cause severe losses. The disease occurs in all lettuce producing areas of California.

Sclerotinia drop of lettuce is typified by a progressive drooping and collapse that results when the fungus colonizes the root or lower portion of the plants. Two species of Sclerotinia can cause the drop, S. minor and S. sclerotiorum. S. sclerotiorum is the main type found in the San Joaquin Valley, but S. minor can also cause lettuce losses here.

These fungi make sclerotia, compacted spherical masses of fungal cells that turn black at maturity. The sclerotia are resistant to breakdown and persist in soil or debris from previous crops. Sclerotia of S. minor are smaller (1-1.5 mm diameter) and more spherical in shape than those of S. sclerotiorum (usually 12 mm or more in diameter) and more oblong to irregular in shape.

Although both species produce sclerotia, they each cause infections differently.   The sclerotia of S. minor germinate and infect thecrown of lettuce plant directly. An individual sclerotium of this fungus must be within a few inches of a lettuce plant to infect it. Plants can be infected as young as the 4 to 5 leaf stage and continue up until harvest.

Infection of lettuce plants by S. sclerotiorum is more complicated. Under cool, moist conditions the sclerotia of this fungus produce small cup-like structures which is visible to a careful observer.  These small cup-like structures  are actually tiny mushrooms each of which can shoot thousands of spores into the air. These spores can survive for up to 2 weeks once they land on lettuce plants. When free moisture occurs on the leaves, the spores can germinate and infect the plant. S. sclerotiorum infections usually begin in senescent lower leaves, but the fungus then moves into the crown, girdling it and causing the characteristic drop symptoms, the same as S. minor.

The different life cycles of S. minor and S. sclerotiorum requires slightly different control strategies for each species. With S. minor, the first goal is to reduce the number of sclerotia in the field. Before prepping the field for planting, the sclerotia should be buried by plowing to at least 10 inches beneath the soil surface. This places sclerotia at a depth where they will not germinate and where listing operations will not bring them up again. The buried sclerotia are then susceptible to break down by soil microbes. Metham sodium applied through sprinklers well before planting has also been used to kill sclerotia.

Once the lettuce seedlings are growing, the next step in a control strategy against S. minor lettuce drop should be to protect against the germinating sclerotia that may remain. Protective fungicides should be applied as soon after thinning as possible. Good coverage of the soil and basal portion of the plants is important.

The above-mentioned control strategy for S. minor is not as effective against S. sclerotiorum. Burying the sclerotia of S. sclerotiorum is not as effective because often the source of infection is from outside the field from the airborne spores.  To control S. sclerotiorum  growers should manage weeds near the field which can serve as host of the fungus. Nearby areas under orchard canopies can also serve as sources of the airborne inoculum.

Fungicide protection against S. sclerotiorum should begin when plants reach the rosette stage, or when some of the lower leaves begin to turn yellow or die. This susceptible stage of the crop is often accompanied by cool wet weather that favors ascospore production.  A number of applications may be needed during the season because spore release can occur several times during the season. 

One strategy that will help for control both pathogens is to keep bed surfaces as dry as possible while meeting the water needs of the crop. A moist soil surface contributes to a microclimate that will favor infective activity of both S. minor and S. sclerotiorum.