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. Dead branches may resemble bleached chicken bones. A good indicator of the disease is the presence of small, black sclerotia (resting structures) of the fungus often found inside infected stems. Another symptom of the disease is the presence of white, cottony-like mycelium of the fungus when weather conditions are cool and moist.
Survival And Spread
Optimal temperatures for growth range from 59°F to 70°F. Under wet conditions, Sclerotinia sclerotiorum will produce an abundance of mycelium and sclerotia. The sclerotia enable the fungus to survive from season to season, and are the source of inoculum to infect future crops.
High humidity and dewy conditions support the spread and increase the severity of infections. When conditions are favorable, the dormant sclerotia germinate to produce fruiting bodies. These produce ascospores, which then germinate on the host and begin to invade the host tissue via mycelium, causing infection.
S. sclerotiorum is capable of invading nearly all tissue types including stems, foliage, flowers, fruits, and roots. 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.
Four to five weeks of flooding of fields during the summer rainy season might help reduce the numbers of viable sclerotia, thereby reducing the amount of disease in succeeding crops. Recycled irrigation tailwater might move sclerotia to fields where sclerotia are not present. The use of plastic mulch may suppress Sclerotinia, while high plant populations can increase the incidence. Timing also is a critical issue of fungicide applications, and growers should try to apply during periods of long cool, wet weather, which also is favorable for other foliar pathogens.
In tomato, choice of fungicides are limited to azoxystrobin (Heritage and Quadris, Syngenta Crop Protection), pyraclostrobin (Cabrio, BASF), and Priaxor (a premix of Cabrio and fluxapryoxad) on tomato and pepper. Unfortunately, use of these products may tend to exacerbate problems with target spot. Thiophanate methyl (Topsin, United Phosphorus Inc.) used to be labeled Special Local Needs on tomato, but is not any more. The other succinate dehydrogenase inhibitor fungicides (Endura, BASF; and Fontellis, DuPont Crop Protection) may suppress Sclerotinia, but are not specifically labeled for Sclerotinia on tomato and pepper.
Biologicals like Contans WG (SipcamAdvan), Regalia (Marrone BioInnovations), Rhapsody, Serenade Max, and Sonata (each AgraQuest respectively) also are labeled and may provide various degrees of control alone or in combination with other fungicides. Contans WG is specifically aimed at limiting the seasonal carryover of sclerotia and must be applied prior to and following the cropping season.