In recent years, many growers were faced with huge losses due to a detrimental disease on beetroot, called Rhizomania (Beet Necrotic, Yellow Vain Virus).
AGRI- PULSE - NORTH WEST - Rhizomania refers to an abnormal development in the taproots of certain plants, particularly sugar beets and beetroot. It is characterised by the growth of fine, hairy secondary roots. This condition is caused by the infection of the plant with a virus transmitted by a fungus. When beets are infected with rhizomania, they exhibit excessive growth of secondary roots along the taproot, giving them a “bearded” appearance.
Additionally, the disease leads to large necks with petioles emerging across the top and even down the side of the beet. The internal tissue of the affected beets may also appear whiter than normal. The virus responsible for rhizomania is transmitted by a soilborne fungus called Polymyxa betae, which forms long-lived survival structures, making it impossible to eradicate it from infected soil.
Symptoms
Early infection may result in stunted plants and plant death. Massive development of fine, hairy secondary roots can give the taproot a rough and beardy appearance. Later infections may cause roots to become slender with characteristic high crowns. Water and nutrient uptake of infected roots will function less than optimal. Consequently, the foliage shows symptoms similar to water stress or nitrogen deficiency. Yellowing of veins and necrosis of leaf tissue is found less frequently. Foliage and roots of plants infected late in the growing season may look healthy. Diseased plants usually occur in patches in the field. During transmission the wet conditions that the fungus thrives on will also promote secondary root decay by other root rotting soil borne pathogens.
Development and infection
BNYVV and some other viruses are transmitted by the soil borne fungus Polymyxa betae. Red beet is a host to both the vector fungus and the virus. Polymyxa betae is a widespread soil inhabitant which by itself does not cause significant damage. Once infected, it will spread the virus by means of stationary resting spores and motile zoospores. The resting spores released into the soil by decaying tissue permit the virus and its vector to survive for more than ten years. These spores germinate and release numerous zoospores in the presence of a host, free moisture and soil temperatures ranging from 15 to 28°C. Infected roots produce and release new zoospores attracted to surrounding root tissue. This repetitive infection cycle rapidly increases fungus and virus in soil. Neutral to slightly alkaline soils increase disease development.
Three strains have been identified: A, wB and P. Type A is widespread throughout the world. Type B and P are more restricted.
Prevention and control
Resistance is the most important and effective means of control. Factors that affect spread and development of Polymyxa betea will directly impact the incidence and severity of Rhizomania. Early planting, when soil temperatures are cooler, rapid establishment of the plant canopy and soil drainage will reduce the risk of loss. Avoid spread of infested soil and tare dirt into uninfested fields. Once a field becomes infested, crop rotation will not significantly reduce the risk of infection due to the persistence of the resting spores.
Bejo is at the forefront of breeding resistance against the virus and have several commercial varieties available.
Contact your regional sales representative for more details or visit our website for more information. You can also use the Bejo Cropalyser app, available on the Playstore and Apple store to identify major pests, diseases, and disorders in vegetable crops.
