Interaction of Rhizoctonia solani and Rhizopus stolonifer Causing Root Rot of Sugar Beet

Authors: Hanson, Linda

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/7/2010
Publication Date: 5/1/2010
Citation: Hanson, L.E. 2010. Interaction of Rhizoctonia solani and Rhizopus stolonifer Causing Root Rot of Sugar Beet. Plant Disease. 94(5):504-509.

Interpretive Summary: Increasing problems with sugar beet root rot in Michigan were associated with the presence of two fungal pathogens in beet, Rhizoctonia solani and Rhizopus stolonifer. Both pathogens can cause root rot alone. When the two pathogens were inoculated together into beets, more severe disease was produced than when either fungus was used alone. This indicates a synergism can occur between these fungi and that a disease complex may be associated with problems in the growing area. Host resistance to Rhizoctonia solani reduced the disease caused by the two pathogens, but was not sufficient to control the disease. This raises concerns about managing root rot in this area.

Technical Abstract: In recent years, growers in Michigan and other sugar beet production areas of the United States have reported increasing incidence of root rot with little or no crown or foliar symptoms in sugar beet with Rhizoctonia crown and root rot. In addition, Rhizoctonia-resistant beets have been reported with higher levels of disease than expected. In examining beets with Rhizoctonia root rot in Michigan, a large number of sampled roots had a second potential root rot pathogen, Rhizopus stolonifer. Growing conditions generally were not conducive to disease production by this pathogen alone, so we investigated the potential for interaction between these two pathogens. In greenhouse tests, four of five sugar beet varieties had more severe root rot symptoms when inoculated with both pathogens than when inoculated with either pathogen alone. This synergism occurred under conditions that were not conducive to disease production by R. stolonifer. Host resistance to Rhizoctonia crown and root rot reduced diseases severity, but was insufficient to control the disease when both pathogens were present. This raises concerns about correct disease diagnosis and management practices and indicates a root rot complex may be important on sugar beet in Michigan.