Sclerotinia minor is one of two Sclerotinia species which cause a basal rot and lesions on sunflower. The other more commonly identified species, Sclerotinia sclerotiorum causes Wilt, Stalk Rot or ‘White Mold’ at the stem base, as well as mid-stem lesions or head infection. Both species may be present in a single field, and both species can cause devastating disease outbreaks and survive on 500+ hosts, both crops and weeds. Although Sclerotinia sclerotiorum is the most dominant species on sunflower in the US, S. minor is present in some areas and yield losses have been recorded on other crops (for example peanuts).
The primary survival structures of S. minor are small, black, angular sclerotia. Size and color of sclerotia can be used to differentiate S. minor (small black) from S. sclerotiorum (large black) and also from a third basal rot pathogen, Athelia (Sclerotium) rolfsii (small brown).
Sclerotia can survive in the soil or on the soil surface in sunflower stubble or other plant residues for at least five years, depending on environmental conditions. Infection usually occurs during the vegetative and budding growth stages, and begins when fungal threads (hyphae) erupting from sclerotia directly penetrate the host tissues. High humidity (greater than 95%) and active root growth initiates infection, particularly lateral root proliferation. Infection is favored by moist soil conditions and temperatures of 18-23˚C.
Symptoms are often first observed as a pale grey to light brown basal lesions which may appear water-soaked (wet). Lesions are first seen near the soil line, and can be more readily observed by scraping soil back from the stem base. Lesions can extend upwards for up to eight inches under favorable conditions and often develop a dark brown edge and diurnal striations (Figure 1).