Potato mop-top disease is caused by potato mop-top virus (PMTV), which is transmitted to potatoes by a soil-borne plasmodiophorid vector, Spongospora subterranea. PMTV is a highly destructive pathogen of potato because it causes significant tuber quality damage by inducing brown lines and arcs within and on the surface of tuberof susceptible cultivars. It can be carried symptomlessly in other cultivars.

While PMTV has only been known since 1966, it is transmitted by Spongospora subterranea which causes powdery scab, a disease with a longer, worldwide history. Powdery scab was first discovered in Germany in 1841, but found throughout Europe by 1855, and is believed to have spread from South America. Powdery scab in the United States was traced to potatoes imported from Ireland in 1912. Since the fungal vector of the virus is present in soils worldwide, the potential spread of the virus is also global. PMTV may also be artificially transmitted to some hosts by grafting and manual inoculation, but no other natural vectors are known.

Soil can be infested with S. subterranea without powdery scab occurring if the environmental conditions required for disease development are not met, and powdery scab can occur without PMTV being transmitted. However, the distribution of infested soil or infected tubers can serve to disseminate both powdery scab and PMTV. The virus is retained in the spore balls of the S. subterranea vector, and these infected, long-lived resting spores can persist for up to 18 years in the soil, creating an insidious infection reservoir that is resilient to chemical control methods.

Powdery scab and PMTV are generally associated with cool and wet conditions. Damp soils are particularly favorable to disease development during early tuber initiation, when potatoes are most susceptible to powdery scab. Precipitation at this stage is a good predictor of PMTV incidence, and the disease seldom occurs in areas with less than 30 inches of rain per year, increasing thereafter to a high likelihood of occurrence in areas above 45 inches of rain per year. With the likelihood of global warming bringing wetter weather to the UK, and especially to Scotland, PMTV is a disease that may become more prevalent in future.

Resistance and control

Varietal susceptibility of potatoes to powdery scab varies considerably but no varieties are immune to either powdery scab or PMTV. Resistance to powdery scab and to PMTV are genetically independent traits. A number of plant families are susceptible to infection by powdery scab, but although the fungal pathogen can infect a limited number of other plant species, spore balls (the long-term reservoir of PMTV infection) are only associated with potatoes and black nightshade (Solanum nigrum).

There is no effective control for PMTV at present but research at the James Hutton Institute is aimed at understanding the pathogen and its interactions in order to devise a sustainable control method. Other than varietal selection and certification programmes designed to detect or flush out viruses, control of PMTV is generally directed at its vector, Spongospora subterranea. Using clean seed and minimising the spread of soil and other forms of contamination restricts spread of the pathogen. Infection may be reduced by environmental control such as improved drainage, reduced irrigation during early tuber development, delayed planting until soils are warmer or drier and crop rotation with brassicas and other crops. Seed and soil treatments may also help reduce the spread of the pathogen from infected seed and reduce numbers of viable spores.

The infection process

Zoospores of Spongospora subterranea introduce the virus into the potato plant when they infect the roots, stolons and/or young tubers. Systemic movement of the virus within the plant is generally slow and erratic. The critical period for infection and development of powdery scab on tubers is early in the growth cycle, at stolon formation and tuber set, a period that lasts about 3-4 weeks. Tubers which have matured beyond this period are resistant to infection by zoospores. Little or no spread occurs in areas where soil temperatures are above 20°C, or where moisture is lacking. When PMTV-infected tubers are planted as seed, the virus is passed on as a secondary infection to only limited numbers of progeny tubers (30 – 50%). Therefore, spread via the obligate vector, powdery scab is the most important means of transmission.

Solanum tuberosum (potato) is the main host for PMTV worldwide, although weeds in the Chenopodiaceae and Solanaceae are commonly infected in the Andean region of South America. Under experimental conditions, susceptibility to infection by PMTV is found in the Chenopodiaceae, Solanaceae and Tetragoniaceae families, with species including Beta vulgaris, Capsicum annuum, Chenopodium amaranticolor, Chenopodium quinoa, Datura ferox, Datura stramonium, Lycopersicon esculentum, Nicotiana benthamiana, Nicotiana clevelandii, Nicotiana tabacum, Petunia × hybrida, Physalis spp., Solanum nigrum, Solanum tuberosum and Tetragonia tetragonioides.

Symptomatology and diagnosis

Symptoms may occur on the foliage of potato plants produced from infected tubers and on stems, although not all stems produced will show symptoms. Foliage symptoms develop best at cool temperatures (between 5°C and 15°C), particularly during the early stages of plant growth. The most common foliar symptom is the development of aucuba patterns on the stems which consist of bright yellow blotches and ring or line patterns on lower or middle leaves. Less commonly, a second type of symptom may be observed, consisting of pale, V-shaped, chlorotic chevrons, usually on the leaflets of young upper leaves, and ultimately resulting in a distinct mosaic in the upper leaves. A third type of symptom consists of extreme shortening of internodes accompanied by crowding or bunching of foliage, described as a ‘mop-top’. Some of the smaller leaves may have wavy or rolled margins and the overall effect is a dwarfed and bunched growth habit.

Although the flesh of some potato tubers can be entirely free of symptoms, superficial concentric rings of 1-5 cm in diameter may be visible on the surface of infected tubers, radiating out from the point where the fungal vector first introduced the virus. Tubers may also develop brown necrotic lines, arcs and rings in the flesh, centred around the stolon attachment site, that are known as spraing. Spraing symptoms may not be present in tubers at the time of harvest but can develop in storage, particularly if the storage temperature fluctuates. In other instances, depending on prevailing environmental conditions prior to harvest, spraing symptoms may occur at high levels at the time of harvest and increase only slowly during storage.

In the diagnostic host species Nicotiana debneyi, systemic necrotic oak leaf patterns can be seen, while in Chenopodium amaranticolor, concentric brown local lesions are indicative of PMTV infection. However, in practice the best tests for diagnosis are virion shape and size. The virus is serologically related to soil borne wheat mosaic virus, and also distantly to tobacco mosaic virus. However, there is no homology (using cDNA or RNA hybridisation) between PMTV and soil borne wheat mosaic virus. Diagnosis can therefore be carried out using plant symptoms, electron microscopy to study virion morphology, and serological (antibody) tests.