Sugar-beet is attacked by many pests which may cause direct injury or may introduce virus diseases to the plant, causing severe economic loss. In most parts of the world virus diseases and nematodes appear to be the principal problem; and, generally speaking, the same pests and diseases occur wherever sugar-beet is grown, although the predominant ones of Europe and neighbouring countries differ from those in America.
The insect pests which cause direct injury to sugar-beet generally do so at the seedling stage. Soil insects like wireworms and pygmy beetles are destructive on crops in America and northern Europe. In the more southern beet-growing regions such as Spain, Italy and Turkey, the beet flea beetle is more important. The beet leaf miner or beet fly is a very common pest of sugar beet in northern Europe, with injuries consisting of blisters in the leaves caused by the feeding of the larvae. The beet cyst nematode is an important pest, present in all established beet-growing countries.
The most important insect pests of sugar beet are aphids, which attack both the root and seed crops. They occur throughout Europe south to Turkey, and can lower the yield of beet and ruin the seed crop entirely.
Apart from this they are also important vectors of disease, notably mosaic and the yellow viruses. Virus yellows is the predominant disease of northern Europe, though it is also present in southern Europe and the USA. The incidence of this disease is extremely variable from year to year, depending on the winter conditions of green peach aphid and virus.
The curly top disease is the predominant disease in North America, where it is a controlling factor in beet cultivation. It is also prevalent in South America, but absent in Europe. The disease is caused by a virus which is transmitted by leaf hoppers. In the USA a bad attack can reduce the normal average yield of 15 tons of beet per acre to 5 tons or less. Severe outbreaks in the west of the USA have forced farmers to give up sugar-beet growing.
Rhizomania is a virus disease transmitted by the fungus Polymyxa betae which can stay alive without a host crop for more than 15 years. The root system of the sugar beet is ruined by this disease, which can lead to complete crop failure. Initially rhizomania occurred mainly in the south of Europe and Japan, but is spreading in northern Europe and America. Until now no sugar beet growing was possible in severely infected fields, but progress in breeding for tolerance and resistance solved this problem to some extent.