CURRENT STATUS, MANAGEMENT AND OUTLOOK
Rhizomania is considered the most serious disease of sugarbeets worldwide. It was first reported in northern Italy in 1952 and has since spread to nearly all areas of the world where sugarbeets are grown. Rhizomania was first detected in Idaho near Rupert in 1992. Although the disease will only move a few inches per year through soil without aid, it is easily spread by infected plant material, within soil adhering to non-host root crops, agricultural equipment carrying contaminated soil, irrigation water, and any other means that can move even small amounts of soil. It is not, however, seed transmitted.
All indications are that rhizomania will continue to spread relentlessly and find its way into all sugarbeet growing areas in the United States. Originally diagnosed in the U.S. in California and Texas, it had been thought that the disease would not become an important problem in the cooler, northern tier of states where sugarbeets are grown, but experience has made it clear that this is not the case. It has been detected and is a problem in Idaho, Colorado, Wyoming, Nebraska and the Red River Valley. Rhizomania was detected in only 27 fields (670 acres) out of 354 fields sampled (14,740 acres) the first year it was diagnosed in Idaho. Since that time the disease has spread considerably. Many growers have experienced severe economic loss, and several have had complete loss. We now estimate that that at least 10%, or more than 23,000 acres, are affected in the Amalgamated Sugar Company growing area of Idaho and eastern Oregon.
Roots are stunted and there is a proliferation of lateral rootlets, giving the root a bearded appearance. The taproot may be constricted and there may be excessive crown growth, giving the root a wineglass shape (Figs. 1, 2, 3, 4). Vascular tissue is discolored and the taproot may be rotted (Figs. 5, 6). With mild infections, there may only be slight lateral rootlet proliferation on the taproot or lateral roots. The vascular discoloration may not be very obvious in mild infections, but in longitudinal section the vascular bundles have a disorganized or “marbling” appearance in the vicinity of rootlet proliferation (Figs. 7, 8). This is in contrast to the parallel vascular bundles in normal, healthy tissue.
Leaf symptoms consist of slight yellowing, erect growth habit, and leaf proliferation. Leaves are usually uniformly chlorotic (Fig. 9). In mild infections, there may be little or no stunting of the foliage, with a slightly perceptible and uniform lime-green leaf color. This contrasts with nitrogen deficiency where older leaves are often chlorotic while younger leaves are green.
Rhizomania is caused by beet necrotic yellow vein virus (BNYVV) and transmitted by the soil fungus Polymyxa betae. Both the virus and fungal vector are obligate parasites in that they require a living host to reproduce. Both are host specific and the virus is dependant on the vector for infection. The natural host range, therefore, is limited by those species that P. betae is capable of infecting, and includes primarily species in the same genus as sugarbeets (Beta sp.), a few species in the genus Chenopodium. Experimentally the virus can be mechanically transmitted to several species in the families Chenopodiaceae, Aizoaceae, and Amaranthaceae.
The vector, Polymyxa betae, indefinitely survives in the soil as cysts or groups of cysts called cystosori (Fig. 10). With free water and soil temperature above 60°F, individual cysts germinate in the vicinity of sugarbeet roots and release a single zoospore that infects primary root tissue, carrying the virus into the plant cell. Severity of infection is directly related to the population of viruliferous Polymyxa in the soil. Additional zoospores form within the infected cell which are then released and produce additional infections. The process continues until the rootlet dies and the plant produces new rootlets. Disease is favored by high soil moisture, optimum temperature of 77°F, short rotation, and neutral to alkaline soils.
Resistant Varieties: A resistant variety should be planted if there has been any field history of rhizomania, no matter how small the affected area (Fig. 11). If rhizomania has been found on another field on the farm or on a nearby field, the likelihood that most or all fields are infested is very high. Planting resistant varieties will make long-term disease management far more effective. At least two or three sugarbeet crops are necessary after a field is first contaminated with rhizomania before noticeable symptoms develop. With such a long incubation period, once we observe and diagnose the disease, the inoculum has already increased to a high level.
Available varieties are not immune to rhizomania, but have sufficient resistance to achieve good yields when managed properly. In 1999 variety tests under severe disease field conditions, the best varieties yielded 22.35 tons/acre with 15.35% sugar. In comparison, the standard check varieties (ACH 211, Beta 8422, HM Owyhee, HM PM21) yielded 9.30 tons/acre and 13.80% sugar. There has been concern in the past that the rhizomania varieties were low in curly top virus resistance, but the newer varieties are now approaching the resistance levels to curly top that are found with the standard varieties.
Under disease-free conditions, these same rhizomania resistant varieties yielded 36.7 tons/acre with 16.0% sugar compared with 35.2 tons/acre and 16.6% sugar for the standard check varieties. Although these average slightly lower in sugar, the root yields are comparable. We can expect the agronomic characteristics to continue to improve as seed companies develop new varieties, but the rhizomania resistance levels are likely to remain about the same for some time to come. Seed companies are moving toward incorporating rhizomania resistance into all varieties developed for Idaho and eastern Oregon. Variety trial data may be obtained by visiting the University of Idaho sugarbeet web site at http://www.uidaho.edu/sugarbeet/.
A replicated strip test comparing the performance of a rhizomania resistant and susceptible variety under rhizomania conditions in a commercial field was conducted in 2000. A grower near Weiser, Idaho, planted 12 rows each of rhizomania susceptible HM Canyon and resistant HM 2980 in a 50 acre sprinkler irrigated field that showed symptoms on only about two acres in the previous sugarbeet crop. The field had been in a 3-year rotation of sugarbeets and two years of grain for at least four cycles. For each strip, six rows, 1180 ft long (0.3 acres) were harvested. Rhizomania symptoms were obvious on the susceptible variety throughout the entire field (Fig.12). Based on $35.00/ton for 16.00% sugar ($21 “nets”), there was $445.00 increased return per acre with the resistant variety. In addition, for the replications in the part of the field where there were no previous rhizomania symptoms, the difference in return averaged $259.00 per acre, while in the most severe portion of the field, the difference averaged $529.00. This test clearly showed that if rhizomania symptoms occur in a field, no matter how small the area, it is highly likely there is disease in other parts of the field where symptoms are not obvious. Once rhizomania is observed, a resistant variety should be planted in the subsequent sugarbeet crop.
Rotation: Planting resistant varieties alone will not allow maximum yields to be achieved. A minimum 4-year rotation is essential for good management. For example, our research has clearly shown that with the best resistant varieties and best management practices except for rotation, we measured 16.1 tons with back to back beets, 18.7 tons with a 2-year rotation, 23.8 tons with a 3-year rotation, and 30.7 tons with a 4-year rotation. In addition, short rotations will result in greater difficulty in managing rhizomania in subsequent years. Short rotations are a poor decision for management of any disease, and can be disastrous with rhizomania because high populations of the pathogen develop and last in the soil for many years. Data from preliminary experiments indicate a benefit from planting oilseed radish and incorporating as a green manure after eight weeks growth.
Soil Moisture: Sugarbeets grow best when soil moisture is kept between –40 and –60 centibars (cb). Obviously it’s not possible to irrigate and continually maintain these levels of soil moisture, but excessive irrigation should be avoided. When soil is wetter than –40 cb (i.e. –30 to 0 cb), infection by Polymyxa increases greatly. After planting and prior to seed germination, when soil temperatures are lower than required for infection, it is best to fill the soil profile then wait as long as possible before applying additional irrigation. The disease will be more severe in low spots, areas with poor soil structure and inadequate drainage, and other places where soil moisture is excessive.
Site Selection: Know the field history before planting. Whenever possible, choose fields without a history of rhizomania. If that’s not possible, choose a site where the minimum 4-year rotation can be maintained.
Early Planting: The earlier that plants become infected the more severe the damage from rhizomania will be. The disease is not active below 60°F, and early planting to establish the crop before infection will reduce losses.
Stand: Good plant populations can help reduce the severity of rhizomania. Closing the rows early with high plant populations tends to maintain cooler soil temperature which can reduce the rate of disease development.
Although the fear of rhizomania becoming responsible for the loss of our industry is no longer considered a major issue, there are examples that complacency and ignoring the disease are resulting in severe losses. It is unfortunate that we need to experience major problems to be reminded that the threat is by no means idle. To manage rhizomania, it is essential to sample suspected fields and correctly diagnose the problem. There is no reason to disregard or overlook rhizomania. We have the tools for good management, and we can take the steps necessary to achieve acceptable yields.
Presented at the Snake River Sugarbeet Conference on January 11-12, 2001.
Asher, M. J. C. 1993. Rhizomania. In: The Sugar Beet Crop. Science Into Practice. D. A. Cooke and R. K. Scott, eds. Chapman and Hall, London.
Compendium of beet diseases and insects. 1986. E. D. Whitney and J. E. Duffus, eds. American Phytopathological Society, St. Paul, MN.
John J. Gallian, Extension Sugarbeet Specialist and Associate Professor of Plant Pathology; Department of Plant, soil & Entomological Sciences, University of Idaho, Twin Falls Research and Extension Center, PO Box 1827, Twin Falls, ID 83303-1827, Phone: (208) 736-3633; http://www.uidaho.edu/sugarbeet/