Diseases of lettuce (
Leaf drop
Leaf drop of lettuce is caused by two soil borne pathogenic fungi, Sclerotinia minor and Sclerotinia sclerotiorum. These fungi infect lettuce during cool, moist conditions causing a soft, watery decay of the plant tissue (photo 1). The leaves wilt, shrivel and drop down rapidly (photo 2). Both fungi produce sclerotia, hyphal structures that enable them to persist in soils for long periods of time, especially under dry conditions. S. sclerotiorum in particular has a very wide host range, including many vegetables and bedding plants.
S. sclerotiorum can be easily diagnosed in the field when large, black sclerotia form in infected plant tissue (photo 3) on leaf tissue on the soil surface. Reproductive structures called apothecia (photo 4) grow from these sclerotia and produce sexual spores (photo 5) that are easily carried in air currents; however, they are not observed in Arizona soils. Sclerotia may also germinate directly in the soil and infect plants. Sclerotia are produced on the soil surface, and do not survive well if buried in moist soils where they are attacked by other soil microorganisms.
Leaf drop may be managed by: avoiding excess irrigation, especially when the leaves cover the soil and can be in contact with germinating sclerotia; complete turning or deep plowing of the soil to bury and promote rotting of sclerotia; rotation with resistant crops such as corn and grasses; application of effective registered fungicides such as iprodione and the biological control product Contans.
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February 10, 2003