ARS | Publication request: Identification of Rhizopus stolonifer as a Pre-emergence Seedling Disease Pathogen of Beta vulgaris

Submitted to: Annual Beet Sugar Development Foundation Research Report
Publication Type: Research Notes
Publication Acceptance Date: May 15, 2010
Publication Date: June 1, 2010
Citation: Naegele, R.P., Hanson, L.E., Mcgrath, J.M. 2010. Identification of Rhizopus stolonifer as a Pre-emergence Seedling Disease Pathogen of Beta vulgaris [CD-ROM]. 2010 Annual Beet Sugar Development Foundation Research Report. Denver, Colorado: Beet Sugar Development Foundation.

Technical Abstract: Rhizopus stolonifer, a common soil borne fungus in Michigan, is a known root rot pathogen on mature sugar beet. In 2008, Rs was isolated from a sugar beet seed lot showing consistently low germination rates in both the field and lab, and Rs was morphologically identified on malt extract agar. Much of the lower end of the beet germination spectrum is associated with cooler temperatures and excess moisture, conditions often associated with root rot. Pathogenicity was confirmed by repeated lab and greenhouse inoculation and germination studies of six beet varieties. In replicated lab experiments (3 containers per treatment), seeds (15 seeds/container) were grown in both liquid and on solid media inoculated with spore suspensions (1x103 spores/mL, 10 ml for solid, 25 ml for liquid) and incubated at 28 deg C and germinated seeds were counted after 3 days. Seeds from inoculated treatments showed symptoms such as little to no radical emergence, or blackened radicals, and subsequently germination was reduced up to 50% compared to controls. In greenhouse experiments, beet seeds (5 seeds/pot) were planted on a mesh screen placed into 15.2 cm pots filled with autoclaved potting mix and inoculated. Pots were kept saturated with ddH2O and seedlings were grown for three weeks with greenhouse temperatures averaging 25-29 deg C. After three weeks, un-emerged seeds and seedlings were removed from the potting mix and examined for symptoms. Re-isolated Rhizopus stolonife was used for the re-inoculation of germinating seeds in the second replication of greenhouse testing. Under both lab and greenhouse conditions, at least one germplasm of beets showed a significant reduction (P< 0.05) in germination, however other varietal differences were observed. This may indicate the presence of host resistance in certain germplasms. To our knowledge this is the first report of Rhizopus stolonifer as a cause of pre-emergence damping-off in Beta vulgaris.