Diseases of cole crops (
Sclerotinia rot, also called white mold, is caused by the fungus Sclerotinia sclerotiorum. Disease occurs during cool, moist conditions that favor growth of the fungus. This is a soil borne fungus that begins to grow on the old decaying leaves on the soil surface then causes a soft, watery decay of the plant tissue (photo 1). The leaves wilt, shrivel and drop down rapidly. This fungus produces sclerotia, hyphal structures that enable it to persist in soils for long periods of time, especially under dry conditions. S. sclerotiorum 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 (photo 2) form in infected plant tissue on leaf tissue on the soil surface. Under some conditions, reproductive structures called apothecia grow from these sclerotia and produce sexual spores that are easily carried in air currents; however they are not observed in Arizona fields. Sclerotia 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.
White mold may be managed by (1) avoiding excess irrigation, especially
when the leaves cover the soil and can be in contact with germinating sclerotia;
(2) complete turning or deep plowing of the soil to bury sclerotia and promote rotting
of sclerotia; (3) use of the biological control fungus Coniothyrium minitans available in commercial products such as Contans, and (4) chemical control with iprodione (Rovral).
February 20, 2013