Rhizomania

Rhizomania is characterized by root stunting and a proliferation of lateral rootlets on the main taproot that give the root a bearded appearance. The storage root is often constricted (turnip-shaped) below the soil level and rotted. The vascular tissue of the taproot becomes discolored and appears as darkened rings when the taproot is cross-sectioned. These symptoms can resemble those caused by curly top disease, but the two are unrelated. Leaves of infected plants often exhibit a pale to bright yellowing of the leaves that can mimic a nitrogen deficiency. The necrotic yellow vein symptom associated with the virus name is rarely observed in the field. Leaves on the plant wilt, especially in periods of high water demand or following irrigation when the fungal vector of the disease is most active. At the beginning of summer, some leaves on plants may crinkle and bleach along veins, but these symptoms can disappear after a few weeks.

Rhizomania is one of the most destructive diseases of sugarbeet. The causal agent, Beet necrotic yellow vein virus, is transmitted by the soilborne fungus Polymyxa betae. Disease development is influenced by the fungus, which is enhanced by saturated soil conditions from rain, irrigation, or poor soil drainage and the warming of soil temperatures in spring. In infested fields, most sugarbeets are affected: roots are usually small, sugar yields are poor, and losses can be as high as 100%. Recent studies suggest that additional losses in fields with infected beets may be the result of secondary invasion by other root pathogens, such as Phytophthora or Pythium.

It is assumed that all commercial sugarbeet fields in California now have rhizomania. Only plant rhizomania-resistant varieties. Current resistant varieties are very high yielding and have provided protection over the last decade; however, a resistance-breaking pathotype was observed in the Imperial Valley in 2003 and is slowly spreading. It may appear in the San Joaquin Valley in the future. Sources of resistance to the new pathotype have been identified and in time may be required in some areas. Consult with seed sales representatives for varietal recommendations. Avoid planting sugarbeets 2 years in a row in the same field, and avoid fields known to contain the new strain until effective new resistant varieties are available.