Watch for Sclerotinia at Flowering Time

Sclerotinia is responsible for a number of vegetable diseases attacking a wide range of crops. Common names for Sclerotinia diseases in Florida are white mold (beans), drop (lettuce), stem rot (pepper, potato, and tomato), and nesting (postharvest disease of bean).

A good indicator of Sclerotinia disease is the presence of small black sclerotia (resting structures) of the fungus. Sclerotia can form on the surface of plant parts as well as inside the stems of pepper and tomato. The sclerotia enable the fungus to survive from season to season and are the source of inoculum to infect crops.

Another common indicator is the presence of white, cottony-like mycelium of the fungus when weather conditions are cool and moist. Symptoms vary between crops. White mold in beans usually appears after flowering. The disease often appears in leaf axils and advances into the stem, producing water-soaked spots that increase in size, girdling the stem, and killing it above the point of infection. The disease also can enter the plant through leaves or pods that touch the soil where sclerotia or infected plant parts act as inoculum.

In tomato, potato, and pepper, infection typically starts at flowering. Water-soaked spots are usually the first symptom, which is followed by invasion of the stem, girdling, and death of the upper part of the stem that turns a light gray. The disease also can begin where the plant contacts the soil or infected plant debris. Large portions of the field may become diseased, producing large, circular, areas of dead plants. The black sclerotia formed by the fungus often are found inside infected stems.

Survival And Spread
Under wet conditions, S. sclerotiorum will produce an abundance of mycelium and sclerotia. The fungus can survive in the soil mainly on the previous year’s plant debris. High humidity and dewy conditions support the spread and increases the severity of infections. Once a plant is infected, white mycelium will grow on the surface of the infected tissues. At the end of the season, sclerotia are produced. The sclerotia will then remain on the surface of the ground or in the soil on either living or dead plant parts until the next season.

Management Methods
Four to five weeks of flooding of fields that have a history of Sclerotinia diseases may help reduce the numbers of viable sclerotia. The use of plastic mulch may suppress Sclerotinia diseases, while high plant populations may increase the incidence.

In beans, fungicides including Botran 75 W (dicloran, Gowan Co.), Endura 70 WG (boscalid, BASF), Iprodione 4 L (iprodione, Arysta LifeScience), Quadris F (azoxystrobin, Syngenta), Rovral 4 F (iprodione, Bayer CropScience), and Switch (cyprodinil and fludioxonil, Syngenta) applied at bloom stage have been effective in controlling white mold. Iprodione and Rovral 4 F have been used with good results in lettuce. For potato, Iprodione 4 L, Rovral 4 F, Topsin M WSB and 4.5 L (thiophanate methyl, United Phosphorus Inc.) are recommended for control. While in tomato, Amistar 80 DF (azoxystrobin, Syngenta) has given good results.

By Gene McAvoy, Regional Vegetable Specialist IV, University of Florida/IFAS