Sunflower Production - NDSU Agriculture - North Dakota State ...

■ Stem Rots Caused by SclerotiniaMinor and Sclerotium RolfsiiDescription: Stem rots caused by Sclerotinia minoror Sclerotium rolfsii are very similar in appearanceand will be covered together. Sclerotinia minor onsunflower is seen in California and the southern GreatPlains, but has not been reported in the northernGreat Plains. Stem rot caused by Sclerotium rolfsii,also called Southern blight, primarily is observed onsunflower in warm climates such as California, Floridaand irrigated fields in the southern Great Plains. Sclerotiumrolfsii has not been observed on sunflower inthe northern Great Plains, but has been noted in nurserystock in Minnesota, suggesting it could becomeestablished in the northern latitudes. Both fungi havea very broad host range encompassing many broadleafcrops.The symptoms of both Sclerotinia minor and Sclerotiumrolfsii are outwardly very similar to the rootinfection and wilt caused by Sclerotinia sclerotiorum.Affected plants have a water-soaked lesion on the stemat the soil line that turns light brown. Under humidconditions, white mycelium also may be found on thelesion. In some instances, the lesion also may have aseries of dark rings due to the diurnal growth patternof the fungus. Plants infected with either fungus wiltand die suddenly, without any distinctive foliar symptoms,within a week of the onset of wilting. The majorfield sign to distinguish S. sclerotiorum from S. minoris the size and shape of the sclerotia. In S. minor, thesclerotia are always round and generally less than 1/12inch (2mm) in diameter (Figure 92), and thus muchsmaller than sclerotia of S. sclerotiorum. Sclerotia ofSclerotium rolfsii are also round and the same size (12 inches), especially ifcomplete inversion of the soil profile is possible, willremove most of the sclerotia from the root zone andplace the sclerotia where they are most vulnerable toattack by soil bacteria. Differences in hybrid susceptibilityhave not been investigated in the U.S. becauseneither disease is present in the major sunflowerproduction areas.Diseases■ Figure 92. Sclerotia of Sclerotiniasclerotiorum (left) and S. minor (right)from sunflower stalks. Ruler at top incentimeters. (T. Gulya)71