Rhizopus Root Rot

This disease first appears as a temporary wilting of foliage during periods of stress; as the disease advances wilting becomes permanent. After death of the beet, the foliage and root become very brittle and dry. Infected root tissue appears gray brown with darker vascular rings. The disease generally progresses downward with the infected tissue becoming dark and spongy. The taproot can eventually be completely consumed with white mycelium. Black sporangia (spores) are produced on the white mycelium, giving the fungal mass a dark appearance. Often the fungus will decay the internal tissue creating a cavity filled with a clear fluid. The roots may have an odor of acetic acid.

Rhizopus stolonifer and R. arrhizus are common in most agricultural soils throughout the world; in California, R. arrhizus is the more common species. In spite of their wide distribution, both fungi are weak sugarbeet pathogens and tend to only be a problem when the crop is compromised by some other factor such as excess soil moisture, crown injuries, or insect injury (cutworms, armyworms) to roots. While the symptoms caused by these species are identical, the optimum temperatures for disease development are different: Rhizopus stolonifer causes disease at low temperatures of 57° to 61°F, while high temperatures, 86° to 104°F, favor R. arrhizus.

Avoid conditions that cause injury to the taproot. Control insects, see insect section for specific control measures for cutworms and armyworms.