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Dahlia Stem Rot Caused by Sclerotinia spp.

(Text from Bob Romano, article on American Dahlia Society web site )
Two species of Sclerotinia spp. can cause disease on dahlias. Sclerotinia minor only infects stems or leaves in close contact with the soil. Once infection takes place, water-soaked brown necrotic areas develop. Plants eventually wilt and collapse. Upon examination of diseased plants one finds cottony fungal growth and numerous small, black, hard resting bodies called sclerotia (large reproductive structures) formed on the outside and inside of the stems. Sclerotinia sclerotiorum (white mold) can also infect lower leaves and stems, causing similar symptoms as S. minor, but S. sclerotiorum also has an aerial spore that can infect upper leaves and flowers. S. sclerotiorum forms larger sclerotia than those of S. minor. S. sclerotiorum prefers cool and moist conditions and the aerial spores usually only infect injured leaves and flowers. The sclerotia may survive for years in infected soil.

(Text by Kevin Larkin)
Basal Stalk Rot - Sclerotinia minor rarely infects the solid healthy basal stalk of dahlias. It is when the hollow stems come into contact with soil either by laying over or having soil pushed up, that infection occurs. Avoid this contact by firmly staking dahlias to avoid sprawling stems in direct contact with soil and do not push soil against the plants above the height of the solid stalk.
Aerial Stalk Rot - Sclerotinia sclerotiorum infection in upper stems and leaves is nearly impossible to prevent, however spread can be reduced by cutting away infected parts as soon as you see the cottony mold on the plant surface. Avoid overhead irrigation.
Click to enlarge photos.
Top: White cottony mold on decayed dahlia stem surface caused by Sclerotinia minor.
Below: Split stem of infected dahlia exposing sclerotia inside.


Top: Sclerotia formed on inside of decayed dahlia stem infected by Sclerotinia minor. These are the overwintering structures formed by the fungus and may last in the soil for many years.
Below: Sclerotia will occasionally form on surface of infected stems.