By Dr. Thomas T. Yamashita
Rhizomania, sometimes called “crazy root,” is a disease affecting sugar beets which is caused by the beet necrotic yellow vein virus (BNYVV). The virus is transmitted by the parasitic water mold fungus Polymyxa betae. While the disease is rarely fatal, it stunts plant growth and directly damages the roots of sugar beet plants, resulting in serious reductions in yield.
The surface symptoms of rhizomania make it easy to diagnose as a nutritional deficiency, until the damage to the plant root is directly observed.
The leaves of sugar beets affected by rhizomania appear stunted, wilted, or yellowed. In addition, leaves will grow more vertically and tightly packed, and the number of leaves will be increased. The leaf veins may take on a yellow color in rare cases, for which the virus causing rhizomania was named. But even in mild cases, affected plants can be readily spotted due to their coloration—the contrast between healthy and infected plants is quite notable.
But the full import of the disease cannot be appreciated until the sugar beet is harvested, as the defining symptoms of rhizomania are focused on the root of the sugar beet plant. The infection causes the growth of a thick, haphazard formation of fine, hairy rootlets, which sprout from the main taproot in such a dense fashion that they often resemble a beard. Many of these rootlets die quite quickly, so the ‘beard’ is often comprised of a mixture of living and decaying root tissue.
The impact on the taproot itself can vary. In more mild cases, the top of the taproot appears normal at the crown, but tapers off after a couple inches to produce a wine-glass shape. More severe infections produce stunted roots with darkened, necrotic areas and an overall fleshy appearance. The deformation of the taproot can be quite varied from one plant to the next—greater or lesser amounts of bearing, a single stunted taproot, the taproot splitting into several fingerlike offshoots, etc.—but the significant deviation from the normal structure is unmistakable. However, formal diagnosis requires the use of an ELISA test for identification.
The malformed roots are not as effective in absorbing and transporting water and nutrients, resulting in an observed higher frequency of necrotic root tissue and wilted and stunted leaves. Ultimately, the disease results in yield losses ranging from 20% to 50%.
Part of the reason that losses are often severe is that the damage caused by rhizomania is not limited to the field. Once harvested and placed into storage, sugar beets affected by rhizomania will experience higher than normal sucrose loss. In cases where Rhizomania-infected sugar beets are placed in long term storage, sucrose losses of over 90% have been reported.
Rhizomania is caused by the BNYVV virus, but relies on the fungus P. betae for transport and survival.
Rhizomania is the consequence of infection with the BNYVV virus. However, the virus depends upon the fungus P. betae as a vector. Infection occurs when P. betae zoospores carrying the virus are transported by water and brought into contact with the surface of plant roots. The zoospores penetrate the roots and release the virus.
The fungus and virus both reproduce within the root tissue of the sugar beet, with the fungus producing cysts which virus particles penetrate and reside within. The cysts leave the root tissue and enter the soil, where they and the viruses they contain can survive for more than 15 years in the absence of a suitable host. Dormant cysts are activated when ideal conditions are present:
- The soil reaches temperatures of 59 to 82 degrees Fahrenheit.
- Moisture levels are high, to the point where the soil is nearly fully saturated.
- Sugar beet root tissue is in the vicinity.
Under these conditions, the cysts release zoospores carrying the viruses, and the cycle starts again.
While there are some effective fungicidal treatments and some cultivars resistant to rhizomania, the most effective treatment is sanitation.
Managing rhizomania is a challenge. The soil fumigant Telone II has been shown to be effective at suppressing the virus’s fungal vector. However, there is some evidence that sugar beets grown in soil treated with Telone II may still be susceptible to infection, and perform poorly in storage.
There has recently been some success in developing sugar beet cultivars resistant to rhizomania. But these cultivars are quite expensive, and there is evidence that the disease may already be overcoming their resistance. Occasionally rotating in rhizomania-resistant cultivars for 4-year periods or longer—or other crops entirely—can help to reduce the buildup of P. batae spores in the soil.
Consequently, implementing growing practices that avoid creating conditions that favor the disease are the best means of control. Rhizomania only occurs in warm, moist, and often shady conditions, which is the key to controlling the development and spread of infection.
Crops should be planted early, when conditions are cool. It’s also advisable to prepare for planting by implementing a preplant program that incorporates soil microbial enhancement. This can help to suppress populations of P. batae, and mitigate the risk of infection. Even when infection does occur, early planting results in less severe infections, while enhanced soil quality improves crop health, both helping to reduce yield losses. Early plantings also help to establish a good leaf canopy early, which shades the soil and helps reduce soil temperatures.
More importantly, proper drainage should be implemented so that soils do not remain saturated. Irrigations should be as infrequent as possible, and preferably should only be performed when temperatures are below 59 degrees.
In addition, growers should be careful about cross-contamination of fields. Growing equipment should be cleaned of soil before being moved from one field to the next.
After the crop has been harvested, the soil should be treated to accelerate microbial activity. This ensures that plant and root tissues in the soil are thoroughly decomposed, helping to slow the reproduction of the fungus and attendant virus.