If your lettuce leaves in the garden are wilting and yellowing with brownish decaying spots, you may have sclerotinia lettuce disease, a fungal infection. This kind of infection can destroy entire heads of lettuce, making it inedible, but cultural practices or fungicides can help you limit the damage.
What is Lettuce Drop?
Lettuce drop is a disease caused by a fungal infection. There are two species of fungus that can cause the disease, one of which only attacks lettuce, peppers, basil, cauliflower, legumes, and radicchio, called Sclerotinia minor. The other species, Sclerotinia sclerotiorum, may infect hundreds of different plants, including many that may be in your garden.
As with most fungal infections, lettuce sclerotinia favors humid, wet environments. A lot of rain, lack of airflow between plants, and leaves touching damp ground can all make lettuce beds more susceptible to the infection.
The symptoms of this disease vary a little depending on the infecting species. Both species cause the lettuce leaves to wilt, beginning with those touching the soil. They also cause brown spots of decay on the leaves. Eventually, usually when the lettuce plant is nearly mature, the entire plant will collapse.
Plants infected by S. sclerotiorum may also develop decay on higher leaves because the fungus produces airborne spores. These lettuce plants may develop soft rot on upper leaves along with white fungal growths. On plants infected by either species, you may also see black growths called scerlotia.
Treating Lettuce Drop
Treating lettuce drop is most often a matter of cultural control, although you can also use fungicides to treat it. Fungicides have to be applied at the base of young plants to stop the spread of the disease. If you do not want to use chemical controls, there are other things you can do to manage lettuce drop.
Management necessitates that you take all reasonable measures to make sure your lettuce plants stay dry. Make sure your bed drains well and water early in the morning so the soil can dry out throughout the day. It’s also important to avoid over-fertilizing with nitrogen, which promotes fungal growth. If you do see infection in your plants, remove the diseased leaves and plants and destroy them. At the end of the season you can plow infected plant matter under, but it needs to be at least ten inches (25.5 cm.) deep.