Rhizomania (RZM) is one of the most widespread sugarbeet diseases. Greek for “crazy root”, Rhizomania is characterized by a proliferation of lateral rootlets along the main tap root. The disease is caused by the Beet Necrotic Yellow Vein Virus (BNYVV) and is transmitted to the sugarbeet by an intermediary root parasite called Polymyxa betae, a protist living in the soil. The disease typically attacks the plant in June during the crop canopy phase. The affected sugarbeet will lose sugar content and increase tare in the field when harvested.
Rhizomania was first reported in the United States in the state of California in 1983. However, the widespread accounts of its devastation that same year suggest that the disease had come to the United States much earlier. Since the first report, Rhizomania has been identified in most domestic sugarbeet production areas.
Over the last few decades, plant breeders have worked tirelessly to improve the availability and quality of varieties resistant to Rhizomania.
Rhizomania is found in all regions of the world where sugarbeets are grown. Domestically, most counties are affected by the disease. Generally speaking, it has become fairly difficult to find estimates of the distribution of Rhizomania or the percentage of the area contaminated in these counties for two reasons. First, the disease is considered to have been present in nearly all sugarbeet producing regions and second, the use of resistant varieties has become essentially a standard procedure.
Many specialists believe climate change will cause a more rapid propagation of Rhizomania. In recent years, unseasonably wet conditions and a reluctance by farmers to discontinue the use of susceptible varieties has resulted in a seven fold increase in inoculum build-up of Rhizomania compared to regions where more disease-resistant varieties were used.
Transmission of the virus occurs through the root parasite, Polymyxa betae, a protist that dwells in the soil. It survives in the form of sporosores – masses of highly resistant, dormant spores. In harsh conditions, the Polymyxa betae/BNYVV complex can remain dormant and maintain its infectious potential in the soil for decades.
When the temperature of the soil becomes favorable, 59 degrees to 77 degrees Fahrenheit, and has a high moisture content, the dormant spores germinate and produce primary zoospores. Attracted by secretions from the rootlets of the host plant, they swim through water in the soil, propelled by their flagellum. When the zoospores reach the outer surface of the rootlets, they hook onto it and discharge their cellular contents inside the plant’s cells. This union results in the formation of plurinucleated cytoplasmic mass more commonly referred to as plasmodium. This is important because when the zoospores release their own cellular contents into the root, they also release the virus.
After the incubation period in the plant cells, the Polymyxa betae plasmodium begins to evolve in one of two ways depending on the climactic conditions.
- If conditions are unfavorable, cold and dry, the parasite will form survival units (the cystospore).
- If conditions are favorable, warm and wet, the parasite will multiply and colonize new host cells.
Regardless of condition, each new spore will contain the Rhizomania virus, Beet Necropic Yellow Vein Virus (BNYVV).
Dispersal & Growth Factors
Polymyxa betae spores can be dispersed by water (rain, run-off, irrigation, etc) or by soil (wind, farming equipment, sugarbeet transportation, etc). The transplant of soil from machinery is the most likely cause of dispersal because it relocates both active and dormant spores.
The environmental factors that will contribute to the development of the disease are:
- the presence of a host plant
- a warm, wet spring (high temperatures, abundant rainfall)
- a neutral to alkaline soil pH. The increase in fertilizer prices has resulted in an increase in the use of composted green waste, which often has very high pH levels. High application rates on low fertility soils may exacerbate the infection.
The Polymyxa betae/BNYVV complex generally feeds on plants belonging to the chenopoiacea family (sugarbeet, chenopodium, spinach) and amaranth family.