Plant Viruses: Soil‐borne

A number of plant viruses in different genera are harboured in and transmitted through the soil. They are found throughout the world and several of them cause significant economic losses to major crops. Vectors for spread of these viruses are fungal-like organisms or plant-parasitic nematodes, but some viruses can also be exuded from infected roots and transmitted abiotically, without the aid of a vector. The long-term persistence of these viruses in soil, often for decades, and a lack of efficient control strategies mean that diseases caused by these viruses are very difficult to control. Eradication of inoculum from infected soil is almost impossible, particularly in those parts of the world where highly toxic soil fumigant chemicals have been banned in recent years. Soil-borne viruses are difficult to study and their biology is relatively poorly understood at present. Key Concepts: A number of plant viruses are transmitted through the soil or by soil-borne vectors to plant roots. Soil-borne viruses infect several economically important crops including wheat, barley, potato, sugar beet, groundnut and fruit crops. Soil-borne viruses belong to fifteen named and two currently unnamed genera that fall within the Secoviridae, Potyviridae, Ophioviridae, Tombusviridae or Virgaviridae families, or have not yet been assigned to a virus family. The main vectors for soil-borne viruses are trichodorid or longidorid nematodes, chytrid fungi in the genus Olpidium and plasmodiophorids in the genera Polymyxa or Spongospora. Soil-borne viruses are relatively poorly understood due to a combination of factors including the complexity of the plant–vector–virus interaction and the fact that soil impedes observations of their biology. Soil-borne viruses are very persistent; they can exist for decades in the soil in long-lived resting spores of their vectors making eradication of disease very difficult once soil is contaminated. There are few effective means available to control soil-borne viruses; few resistant cultivars are available and chemical methods of control are generally expensive and highly toxic. Keywords: soil-borne virus; Secoviridae; Potyviridae; Ophioviridae; Tombusviridae; Virgaviridae; fungal vectors; plasmodiophorid vectors; nematode vectors