Pomovirus

Pomoviruses have fragile rod-shaped particles containing single-stranded positive-sense RNA. They have tripartite genomes encoding seven or eight open reading frames (ORFs). Genome sequencing reveals that the replication-associated proteins are encoded on RNA-1 and contain characteristic motifs for methyltransferase, helicase, and RNA-dependent RNA polymerase. A second RNA encodes a triple block of movement proteins and in two of the four member species a smaller ORF encodes proteins of 6-8 kDa of unknown function. A third RNA encodes the coat protein gene which terminates in a suppressible stop codon read through of which results in a large fusion protein. Deletions in the C-terminal half of the readthrough domain of potato mop-top virus (PMTV; the type species) occur in nature and are associated with loss of vector transmission. Pomoviruses are transmitted by zoosporic plasmodiophorids in the genera Polymyxa and Spongospora. The viruses are persistent problems in arable soils because they are carried within the vector resting spores which remain viruliferous in soil for many years and are inadequately controlled by chemicals. Pomoviruses have narrow host ranges mostly confined to Chenopodiaceae, Solanaceae, and Leguminosae. PMTV causes an economically important disease on potato tubers in potato-growing regions, particularly in cool, wet climates. Sugar beet viruses, beet soil-borne virus (BSBV) and beet virus Q (BVQ), are often found in mixed infections with beet necrotic yellow vein benyvirus. BSBV occurs widely but BVQ is reported only in Europe. Broad bean necrosis virus has been reported only from Japan, where it causes a damaging disease on broad beans and peas.