Submitted to: Compendium on Alfalfa Diseases
Publication Type: Book / Chapter
Publication Acceptance Date: 12/5/2012
Publication Date: 10/17/2014
Citation: Samac, D.A. 2014. Violet root rot. In: Samac, D. A., Rhodes, L. H., and Lamp, W. O., editors. Compendium of Alfalfa Diseases and Pests. 3rd edition. St. Paul, MN: APS Press. p. 58.
Technical Abstract: The fungus causing violet root rot, Helicobasidium brebissonii (anamorph Rhizoctonia crocorum), is widely distributed in Europe and North America but is rarely of much economic importance on alfalfa. The disease has also been reported in Australia, Argentina, and Iran. The disease is characterized by yellowed and dying plants in circular to irregular patches. Infected roots have a compact felt cloak of mycelium that is bright violet to cinnamon colored that often extends 20 cm or more below the soil line and is also abundant in the crown. As the root rots, its bark loosens and the central cylinder becomes soft and shredded. Minute black sclerotia just visible to the naked eye are often present on dying roots. The disease is most common after midsummer. Gramineous plants are highly resistant and can be grown safely where the disease is a problem.