Causal agent(s) and transmission
The only known vector of Potato mop-top virus is a fungus of the Plasmodiophoridae family (recently reclassified in the Protozoa kingdom), Spongospora subterranea f.sp. subterranea, the causal agent of powdery scab on potato tubers and roots.
The virus is carried in the resting spores (cystosori) in which it persists for at least two years.
Transmission to potato roots is made by motile zoospores released in humid conditions by virus-carrying cystosori which directly infest roots or tubers.
As a consequence, the disease is frequent in cold and humid climates, e.g. Northern Europe, and in damp, heavy or peaty soils.
Transmission of the virus from the seed to the daughter tubers is limited as affected tubers only transmit the viral disease to less than 20% to 50% of their progenies; however seed tubers can serve as a carrier to transfer the fungal disease from one field to another.
Dissemination of PMTV is through powdery scab-infested seeds but also through all activities resulting in the movement of infected soil (machinery, seed tubers, dust storms, manure or organic waste). Tubers may also become contaminated during storage and grading through dust originating from a contaminated seed lot.
Host-plants of PMTV include Solanaceae, e.g. potato, tomato, petunia, and Chenopodiaceae (beet) as well as some common weeds such as Solanum nigrum and Datura stramonium. Some potato cultivars may be hosts but symptomless.
The economic importance of PMTV is mainly related to the presence of necrotic symptoms in tubers which may reduce the value or even lead to the downgrading or rejection of the affected potato lots. The virus and its vector are found in cool and humid climates.
Symptoms on tubers
In the year of infection by the soil, the tubers of susceptible cultivars develop internal brown necrotic arcs (‘spraing’) seen as brown rings on the tuber surface. The arcs develop at the boundary of the virus-infected tissue in response to a specific change of temperature before or after harvest but do not prevent the virus from spreading through the tuber. Temperature variations (for example in cold storage) can induce more or less concentric rings or lines (photo 4), which often extend into the flesh of the tuber (photo 5) almost continuously, unlike TRV symptoms. The tubers of other cultivars may develop superficial raised rings without internal symptoms or may show little evidence of infection.
In the year following infection by the soil, the tubers of some cultivars may be symptomless or cracked and distorted, or they may develop internal brown arcs.
Risk factors
The disease is favoured by cold and humid climates and in damp, heavy or peaty soils as high soil water content is required for the release of zoospores of Spongospora subterranea and the infection of potato roots.
PMTV is spread from field to field in resting spores of S. subterranea carrying the virus and present on potato tubers or in contaminated soil.
Movement of infected tubers or plants, soil, machinery or manure therefore increases the risk of spreading the disease.