The sugar beet is vulnerable to several diseases, including some fungal diseases such as rust, beet cercosporiosis and powdery mildew. Although rust rarely poses a major threat, beet cercosporiosis (drying of the rosette of leaves) and powdery mildew (presence of a white film on the leaves) must be treated because they are harmful to the plant and seed bearers.
Two viral infections can also damage crops. The beet yellows virus, spread by certain aphids, causes foliage to change colour and become brittle. It can cause a 20 to 30% reduction in yields.
Rhizomania, caused by a virus spread by a soil fungus (Polymyxa betae) is highly contagious and very harmful, leading to a massive drop in yields (over half). Major efforts have been undertaken to detect and limit the spread of this virus and the fungus that carries and transmits it. Plant breeders have also developed varieties that are resistant to this disease.