Soil-borne viruses affecting cereals: known for long but still a threat.

A number of furo- and bymoviruses, which are naturally transmitted by the ubiquitous soil-borne protozoon Polymyxa graminis are of great economic importance for autumn-sown wheat, barley, rye, triticale, and oats. The two barley-affecting bymoviruses, Barley yellow mosaic virus and Barley mild mosaic virus, are widespread in Europe and East Asia. Production of wheat, rye, and triticale is threatened by three related furoviruses and two bymoviruses. Soil-borne wheat mosaic virus mainly occurs in North America and Chinese wheat mosaic virus in Asia, whereas Soil-borne cereal mosaic virus predominates in Europe. The bymovirus Wheat yellow mosaic virus so far is restricted to Asia; Wheat spindle streak mosaic virus occurs in North America and Europe. Oat plants can be separately or mixed-infected by a bymo- and a furovirus, named Oat mosaic virus and Oat golden stripe virus, respectively. Because P. graminis cannot be controlled by chemicals and the viruses retain their infectivity in the resting spores for many years cultivation of resistant plants is the only means of preventing severe losses. The genetic variability of the known viruses and the sporadic reports of newly emerging pathogens, such as Aubian wheat mosaic virus, make continued resistance breeding and careful monitoring of field crops necessary.