Potato Mop-Top Virus
Possibly spread by contaminated soil, this fungal disease can be difficult to stop once it becomes established.
Potato mop-top virus, or PMTV, is a disease that is on the rise in the United States, says retired American plant pathologist Jim Crosslin. Since pathogens don’t respect borders, some potato experts believe there’s a chance PMTV could become an issue in Canada, too.
While many potato viruses are transmitted by aphids or nematodes, PMTV is spread by spores of the powdery scab pathogen Spongospora subterranea. Powdery scab favours moist soil and cooler temperatures ranging from 11 to 15 C.
PMTV is widespread in parts of both North and South America, and can remain alive for up to 18 years in soil within fungal spores or in host plants. It has also been found in parts of Europe, especially in Scandinavia.
Crosslin says that while PMTV is considered a quarantine pathogen in some countries like South Korea, the disease has not been considered a quarantine pathogen in North America since 2002.
The symptoms of potato mop-top virus include spots, rings and arcs, which are only sometimes visible on the surface of infected tubers.
“Once infected Spongospora is present in the field, it remains infectious for many years,” says Crosslin, who worked with research geneticist Chuck Brown at the U.S. Department of Agriculture Agricultural Research Service’s vegetable and forage crops research laboratory in Prosser, Wash. Brown has been doing a lot of work with PMTV and he’s worried that the virus is more widespread than researchers once believed. Since PMTV is vectored by a fungus, the disease is very mobile, Brown says.
Researchers aren’t sure exactly how the pathogen moves, but Brown thinks that it travels by way of contaminated soil on equipment or on the tubers themselves, making it difficult to manage. Brown is concerned there’s a chance PMTV could get out of control in North America, especially in cases where growers aren’t seeing symptoms.
There can be numerous symptoms once Spongospora has infected potato roots with PMTV, he says. These include spots, rings and arcs, although stacked or concentric rings appear to be the most characteristic symptom of the disease. Sometimes these rings are visible on the tuber’s surface, but more often than not, they are not visible, Brown says.
“Sometimes there will be yellow spots, arcs or chevron patterns on the leaves of infected plants,” says Crosslin. “However, these foliar symptoms seem to be rare in potatoes grown in North America.”
Making identification even more difficult is the fact that PMTV symptoms can sometimes resemble those caused by other soil viruses, including tobacco rattle virus. To identify if powdery scab is present, a commercially available “immunostrip” can be used, says Brown. Test results are obtained in mere minutes, and only a small amount of tissue is needed for testing. To identify and distinguish PMTV from TRV, a well-outfitted virological laboratory may have to employ a highly specialized test since symptoms of these two viruses in tubers can be virtually identical. Knowing which one is the cause of disease is important because fumigation will control the nematode that infects potato with TRV.
According to Khalil Al-Mughrabi, potato pathologist with the New Brunswick Department of Agriculture, Aquaculture, and Fisheries, “PMTV transmission through seed tubers is variable, and the virus may be lost after several generations in the absence of powdery scab.”
“There is evidence that the virus can be present without the symptoms, and a lot of it depends on the variety, the growing conditions, the climate — and there’s a number of factors that come into play.” – Alain Boucher
PMTV in Canada
The Canadian Food Inspection Agency and the United States Department of Agriculture Animal and Plant Health Inspection Service conducted an international survey in 2001 and 2002. More than 3,000 tuber samples from different lots of seed and ware potatoes were tested. The samples originated from various sources, both in the Canada and the U.S.
Before testing began, PMTV was considered a pest of quarantine status in both the United States and Canada. According to an article by H. Xu, T.L. DeHaan and S.H. De Boer published in April 2004 in the journal Plant Disease, determining the impact of PMTV on commercial potato production in North America was difficult.
“After it appeared that we detected PMTV in 4.3 per cent of potato consignments during surveillance of imported and domestic potatoes in Canada, our study concentrated on unequivocal identification of PMTV in potatoes from North American sources,” says the paper. “Although our results clearly indicated that the virus was widespread throughout the potato-growing regions of the United States and Canada, the actual incidence and distribution could not be deduced from the data due to the irregular number of samples tested from any one area.”
Al-Mughrabi says officials in the Canadian potato industry were informed of the presence of PMTV in Canada and the United States during a national conference call with the CFIA back in 2002. At that time, he says, government regulators in both Canada and the U.S. agreed to develop a joint management plan. Under the plan, PMTV would no longer be considered a quarantine pest in either nation, but rather a pest that would be controlled through careful management.
According to Al-Mughrabi, the joint potato virus management plan announced by the CFIA and the U.S. Department of Agriculture-APHIS in December 2002 was focused at maintaining high-quality seed potato production. “This plan is aimed at managing the risks associated with several pests that occur in both countries, including potato mop top virus, potato Y virus complex and tobacco rattle virus, through seed certification measures,” he says.
Alain Boucher, national manager of the potato section at CFIA, says they haven’t seen any PMTV-related issues that would be considered “alarming” in recent years. Boucher says it was determined in 2002 that PMTV did not cause significant economic harm, and therefore, did not need to be treated as a quarantine pest.
“There is evidence that the virus can be present without the symptoms, and a lot of it depends on the variety, the growing conditions, the climate — and there’s a number of factors that come into play,” says Boucher. “Presently, PMTV is being regulated as a regulated non-quarantine pest through the seed potato certification program requirements.
Mathuresh Singh, director of Agricultural Certification Services at the potato diagnostic laboratory in Fredericton, N.B., says he’s received calls from potato growers requesting PMTV testing, and some of those tests have come back positive. However, he’s not sure how widespread PMTV may be in Canada. Potato seed frequently moves between Canada and the United States, so if PMTV is on the rise in the U.S., then it’s possible that it could be increasing here in this country too, Singh says.
“We might have [it] already, but we really don’t know the extent because it travels through seed,” he says, adding that once seed is established in the field a disease like PMTV could be difficult to get rid of.
When PMTV was first identified in Canada in 2002, Boucher says it was symptomless, making it extremely hard to detect. Now, though, Singh says internal symptoms have been found in some instances where tubers were tested for PMTV in Canada.
According to both Al-Mughrabi and Brown, tubers infected with PMTV are unmarketable. They also agree that a widespread outbreak of the disease could conceivably have the effect of restricting the trade of seed potatoes and the movement of planting material between Canada and the United States.
According to Al-Mughrabi, PMTV is best managed through exclusion and sanitation practices. “A viable option for management of PMTV is preventing the movement of the virus and its vector from affected to unaffected regions through quarantine and certification of seed tubers,” he says.
Crosslin agrees that the first step in minimizing losses due to PMTV is to keep the virus out in the first place. It’s important for growers to plant clean, powdery scab-free seed, and make sure to clean their equipment between fields to prevent movement of infected soil to new locations, he says.
“Long rotations between potato crops can help reduce infection levels, but may not be practical in all areas,” says Crosslin. “Some weeds, especially nightshades, can serve as hosts of Spongospora, so good weed control may help reduce infection levels.”
Where PMTV is a problem, chemical control has not been an option, but planting less susceptible cultivars could be a viable solution, since there are differences in cultivar susceptibility, says Al-Mughrabi. He suggests avoiding susceptible varieties, like Kennebec, Shepody, Yukon Gold and Red Pontiac in fields where the presence of PMTV may be suspected.
Brown and his team at USDA’s Agricultural Research Service have conducted trials on standard varieties and are particularly optimistic about the PMTV resistance of one variety, POR06V12-3. So far, it has shown low levels of internal infection and zero latent infection during testing, Brown says.
Brown’s team aims to continue these trials this year. They’re hoping to return to fields with particularly high disease pressure, and plan to include all of the standard varieties in those trials.