Rhizomania is caused by a virus, Beet necrotic yellow vein virus (BNYVV). It is spread with the help of a soil borne "fungus" (vector) Polymyxa betae. The Polymyxa-BNYVV complex can persist in soil for at least 15 years. Warm and humid conditions are optimal for the disease. The cultivated plants that are attacked by BNYVV are sugar beet, fodder beet, chard, spinach and red beetroot. The virus can also propagate in some weeds, especially in different species of Chenopodium.

Rhizomania has been a known sugar beet disease since the 1950's, when it caused yield losses in Italian sugar beet growing. Over the last few decades, rhizomania has spread and been discovered in many beet growing areas around the world. It is now present in most European countries, the USA and Asia. The virus has spread to new growing areas with infected soil attached to plants, such as seed potatoes and onion sets.
It can also be spread via irrigation - if the water is taken from a water system where Polymyxa resting spores, carrying the virus, are present.
Soil "blow" on lighter soils and soil carried on farm machinery and vehicles can also spread the disease from field to field. Small amounts of infected soil can also be spread to new fields by animals or human beings.

It usually takes a number of years from the arrival of rhizomania in a field until the virus has multiplied to the extent that damage is evident. Normally, in the beginning rhizomania appears in small patches in the field. The beet plants can be stunted, the tops yellow in colour and the beet stalks extremely long. The roots get a "bearded" appearance - the result of a lot of small rootlets being created (from which the disease gets its name - rhizomania or root madness).
A cross-section of an infected root would show that the vascular system had become dark-coloured. Only very rarely the virus spreads up from the root to the leaves, giving typical leaf symptoms with large, light areas around the leaf veins. It is these symptoms that have given the name to the virus.
Sometimes a rhizomania infection is not discovered until the beet are delivered to the sugar factory - where a low root yield, extremely low sugar content and increased sodium values are noted.
The symptomatic "bearded" roots can be confused with nematode attack and the yellow coloured tops with nutrient deficiency. In order to be completely sure that rhizomania is present in a field, a laboratory analysis must be done on a soil sample or on the roots of the beet.

For many years, Hilleshög has had an extensive breeding programme to develop rhizomania resistant varieties. High yielding, resistant varieties are sold with success in most of the countries where rhizomania resistance is needed.

• Avoid transfer of rhizomania infected soil from fields e.g. via soil left on farm machines; maintain strict hygiene precautions.
• Only use seed potatoes and onion sets from fields that are guaranteed free from rhizomania.
• Choose rhizomania resistant varieties for contaminated fields. For the rest follow the recommendations of the sugar company.