Potato Facts Potato Mop-Top Virus (PMTV) Steven B. Johnson, Extension crops specialist Bulletin #2437 History Biology The disease, potato mop-top, is PMTV is vectored by the pathogen caused by the virus, potato mop-top that causes powdery scab disease in virus (PMTV). PMTV is in the viral potatoes, Spongospora subterranea f. sp. genus Pomovirus and is a Furovirus subterranea, which is present through- (fungus-transmitted, rod-shaped out the world. PMTV is not naturally virus). PMTV was first reported in transmissible by aphids or other Britain in 1966 and subsequently vectors, but it may be artificially discovered in cooler areas of Europe, in transmitted to some hosts by grafting. the Andes of South America, and in PMTV is retained in S. subterranea Asia. PMTV was reported in Canada in spore balls. The degree of transmission 1991-92 and in the U.S. in 2002. through seed tubers is variable, and in While PMTV has only been known the absence of powdery scab, it is since 1966, it is transmitted by thought that PMTV may be “self- powdery scab, a disease with a longer, curing” after three generations. That is, world-wide history. Powdery scab was in the absence of the vector, plants may first discovered in Germany in 1841, become free of the virus after three was found throughout Europe by 1855, generations. Movement of the virus and is believed to have spread from within host plants appears to be local PMTV is transmitted by the pathogen that causes South America, where it was eventually or through the xylem but requires powdery scab disease found in 1891. It was first discovered in in potatoes New Brunswick in 1913, and later that year in Maine. Powdery scab was traced to potatoes imported from Ireland in 1912. During that time, USDA researchers presumed that Maine’s outbreaks were fairly recent and derived from New Brunswick. As a result of the presence of the disease, Maine was quarantined, but the quarantine was lifted when powdery scab was recognized in a number of states. movement factor proteins for cell-to- increases spore germination. Under cell movement. In host plants, PMTV favorable conditions, multiple cycles has been found in leaves and in occur. In tubers and sometimes root cytoplasm. galls, resting spores are produced in Powdery scab disease spore balls, which persist for long periods of time. No maximum survival Powdery scab Powdery scab develops from S. time has been determined for powdery subterranea only if conditions are scab spores. and PMTV are favorable. S. subterranea prefers poorly Powdery scab and PMTV generally drained soil and can be infectious across a pH range from 4.7 to 7.6. Soil can be infested with S. associated with While it flourishes in soil temperatures subterranea without powdery scab less than 68 degrees F, the range for occurring if the environmental cool and wet infection is between 52 and 75 degrees, conditions required for disease soils, regions, with an optimal infection temperature development are not met. Powdery of 60. Powdery scab is most severe scab can occur without PMTV being and seasons. when there is more than 15 percent transmitted. The mop-top virus can moisture. High soil moisture early in survive in this pathogen from 10 to 18 the season with a gradual drying out is years after previous potato crops. The thought to encourage the development distribution of infested soil or infected of powdery scab. tubers can serve to disseminate the In potato, when favorable PMTV vector. environmental conditions occur, S. Powdery scab and PMTV are subterranea resting spores germinate to generally associated with cool and wet become motile zoospores that infect soils, regions, and seasons. Wet soil root hairs, roots, stems, stolons or conditions are particularly favorable to tubers. In root hairs, cells produce disease development during early additional zoospores that may infect tuber initiation, when potatoes are tubers or reinfect roots. Wet—but not most susceptible to powdery scab. oxygen-limiting—soils allow movement Precipitation at this stage is a good of the zoospores to roots and tubers. predictor of PMTV incidence. However, periodic soil drying PMTV, being associated with powdery scab, seldom occurs in areas Tuber symptoms are called “spraing” 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. Powdery scab, and therefore PMTV, may occur in warmer, drier areas that are irrigated. Higher soil organic matter may increase infection if it improves moisture retention, or decrease infection if it improves soil structure and drainage. Soil compaction generally increases infection by impeding drainage. Varietal susceptibility to powdery scab varies considerably. No varieties are immune to powdery scab or PMTV. Initial results indicate that resistance to powdery scab and to PMTV development are independent. 2 POTATO MOP-TOP VIRUS, BULLETIN #2437 PMTV Symptoms Tuber symptoms are called “spraing.” Spraing describes the rust- brown discoloration, in the form of arcs or rings and flecks, that appears with internal rust-colored spots on tubers. There may be single or multiple concentric arcs, which extend to the skin surface in some cases. This type of symptom is also shared with tobacco rattle virus (TRV), which causes corky ringspot of potatoes, a nematode- vectored disease. TRV is frequently grouped with PMTV in European countries for regulatory efforts, as the tuber symptoms appear similar. The infection normally remains concealed in the flesh of the tuber, but sometimes There may be single or multiple concentric arcs, reveals its presence on the surface. which extend to the skin surface in some cases Spraing, which in reality is an attempt weeks, followed by storage at 41 to 55 to repel a virus infection, can on degrees, induces symptoms. Tuber occasion develop to the stage where symptoms tend not to develop with the infected site falls out like a plug. In constant 72 to 77 degree or 41 to 55 some cases, tubers may be deformed degree temperatures. Symptom (constricted and bent). Tubers grown development after a storage regime of from PMTV-infected seed may not be one week at 64 degrees and two weeks infected with PMTV, and tubers at 46 degrees is considered to be a infected with PMTV may not show reasonable indication of the final symptoms. Not all stems produced by amount of spraing. Repeated cycles a PMTV-infected tuber may become increase symptom development and infected. are used as a screening technique. The Emergence from infected seed can degree of tuber infection is not a direct be delayed and uneven. Spraing indication of infection and spore appearance and severity differs with production in the root system. Yield variety, hill position and environmental may or may not be affected. The conditions. Symptoms at harvest are primary source of loss is a decrease in more common in tubers closer to the potato quality because of internal soil surface, presumably because symptoms. temperatures there are more variable. Foliar and stem symptoms only Symptoms are seldom centered on a appear if infected seed is planted. powdery scab lesion. Usually, few Symptoms are variable, differing with symptoms are present at and just after variety, weather and season. For these harvest, but expression increases with reasons, disease may be reduced, but further storage. Cutting tubers at not eliminated, by roguing plants with harvest may stimulate spraing symptoms. If the disease develops development. Storage temperature and from infected seed, one or more of the fluctuation are important in symptom stems may show symptoms of various expression. More symptoms occur at types. Symptoms are favored by cool storage temperatures of 46 to 57 weather, particularly cold snaps, and degrees F than at 37 to 39 degrees F. may fade as the season becomes hotter. Initial storage at 64 degrees for two Some varieties show a bright yellow POTATO MOP-TOP VIRUS, BULLETIN #2437 3 blotch pattern (aucuba pattern) in the Control lower leaves, some varieties show V- shaped yellow patterns (chevron There is no good control for this patterns), some varieties show disease. Apart from varietal selection internode shortening (mop-top), some and certification programs designed to varieties show “thistle leaf” patterns detect or flush out viruses, control of Disease may be around the veins, and some varieties PMTV is generally directed at its show deformed leaves and a pale vector, Spongospora subterranea f. sp. — reduced, but not blotchy area. Most potato varieties subterranea the organism that causes eliminated, by grown in North America have not been powdery scab. Using clean seed and evaluated for foliar symptom minimizing the spread of soil and other roguing plants expression. forms of contamination restricts spread with symptoms. A number of plant families are of the pathogen. Infection may be susceptible to infection by powdery reduced, in some cases, by improving scab, although only potato and black drainage, reducing irrigation during nightshade (Solanum nigrum) are early tuber development, and delaying known to produce long-lived resting planting until soils are warmer or spores. The pathogen can infect a drier. Seed treatments may help reduce limited number of other plant species the spread of the pathogen from but does not form spore balls on these infected seed. Soil treatments may other plant species. reduce numbers of viable spores. No maximum survival time has been determined for powdery scab spores. Some reductions in powdery