Most commercial potato growers in Canada know that if potato wart is detected in a field where potatoes are grown, the economic consequences can be quite serious. Some experts regard potato wart as the most important worldwide quarantine pathogen of potato.
Potato wart also goes by a number of other names, such as black wart of potato, cauliflower disease, potato tumor, potato cancer, potato canker and warty disease. It’s believed the disease originated in the Andean region of South America, although the pathogen’s distribution is now anywhere potatoes are cultivated. Potato wart has been detected in most European countries, and it has more limited distribution in Asia, Africa, North and South America, and even New Zealand.
Potato wart is caused by the soil-borne fungus Synchytrium endobioticum. Since the discovery of the pathogen in Europe in 1896, research efforts have focused on understanding and classifying the causative agent and its mode of infection as well as the origin and geographical distribution of potato wart.
Research into detection and chemical control measures have led to the development of screening methods for host resistance and genetic analyses, which have in turn enabled the development of resistant cultivars. These early successes are currently challenged by new S. endobioticum pathotypes evolving and the increased risk of dissemination by the regional and cross-country trade of infected tubers.
Although potato wart poses no risk to humans or food safety, it causes tubers to become disfigured. According to a scientific research paper by the late American plant pathologist Gary Franc, S. endobiotocum is a primitive fungus characterized by the lack of hyphae, the formation of zoospores and the development of long-lived resting sporangia, which can contain up to 300 zoospores.
The fungus invades certain meristematic tissues of the potato plant, inducing cell divisions in the host that proliferate into tumor-like tissues. These tumors progressively increase in size at the expense of tubers, resulting in yield losses in the range of 50 to 100 per cent. Infected potatoes are rendered unrecognizable and unmarketable.
The disease has a limited ability to disperse naturally, so the main way the pathogen spreads into new areas is primarily through the regional or international trade of infected seed potatoes or tubers with adhering infested soil particles. Sporangia can also be spread through infected soil, contaminated equipment, and in the droppings of animals that have fed on infected plant material, as well as through irrigation water runoff and windblown dust from wart-infested fields.
The symptoms of the disease are usually only apparent below the ground. Early infection of young developing tubers results in distortions and sponginess. Older tubers will exhibit warty, cauliflower-like outgrowths from potato tuber eyes, stolon buds and the base of the stems.
Warts are initially white or green, and darken and decay as they age. They vary in shape and size (from one to eight centimetres in diameter) and large masses may cover the entire tuber. At maturity, the wart tissue turns black and the entire tuber will decay.
According to research scientists at the Max Planck Institute for Plant Breeding Research in Cologne, Germany, the major agricultural problem with potato wart is the contamination of soil with persistent resting spores.
In a recently published scientific paper entitled Managing potato wart: a review of present research status and future perspective, the researchers note that S. endobiotocum spores can potentially remain infectious for decades. A factsheet published by Michigan State University concurs, noting that resting spores of the potato wart fungus may remain viable in the soil for 40 years.
According to scientists at the Max Planck Institute, chemical control is not a practical and sustainable approach to managing the potato wart pathogen. However, they point out that during the first half of the last century, conventional potato breeding schemes were successful in developing varieties resistant to potato wart. The cultivation of resistant varieties, coupled with strict quarantine measures, effectively curtailed the spreading of the disease during the first part of the 1900s.
Until 1941, only one pathotype of S. endobioticum was known and most potato varieties were resistant to it. Since then, however, more than 30 new pathotypes have been identified, making efficient control of the disease more troublesome since resistance to the initial strain of S. endobioticum is not effective against these new pathotypes.
The authors of a scientific paper, entitled History of potato wart disease in Europe — a proposal for harmonisation in defining pathotypes, stress the importance of breeding potato cultivars that are highly resistant to all pathotypes of S. endobioticum.
“We propose a new, standardised coding system for designation of pathotypes, and plead for the establishment of an internationally harmonized set of differential potato cultivars to identify current and new pathotypes,” the paper states. “The lack of an internationally accepted set of differential cultivars currently hampers good comparison of described pathotypes.”
According to Solke De Boer, a retired plant pathologist in Prince Edward Island, earth infected with the fungus could be chemically treated in the past to eradicate surviving soil inoculum. However, this approach is no longer considered acceptable today due to the deleterious effects of this chemical treatment on the environment.
For this reason, De Boer says, “the disease must be controlled by preventing its spread and managing infected fields until the natural demise of all infectious wart spores. To prevent the spread of spores, movement of soil from infected fields must be avoided. Restrictions need to be placed on infected fields to prevent further build-up of inoculum and its movement beyond the infected area.”
Such restrictions, De Boer adds, can include the prohibition of growing potatoes in infected fields and compulsory disinfection of all equipment. “The detection of potato wart in a field of potatoes usually prompts quarantine and containment actions. Such regulatory measures may last for many years because of the pathogen’s potential to survive in the soil for decades,” he says.
According to De Boer, safety measures can include the creation of buffer zones around infected fields, in which potato wart-resistant potatoes or other crops could be grown.
Despite widespread fears about the impact of potato wart on commercial potato production, De Boer says very little is known about the susceptibility of North American varieties to specific pathotypes.
“Potato wart is a frightening disease because it is so devastating and so persistent,” he says. “Yet the economic impact of a potato wart occurrence is largely the result of long-term quarantines and restrictions placed on infected and buffer zone land, rather than direct disease losses.”
Potato Wart Detected in Prince Edward Island
Since 2000, there’s been a protocol in place between Canada and the United States to deal with the detection of potato wart and its ramifications on tuber shipments between the two countries. It is similar to protocols that deal with detection of other quarantine pests such as potato cyst nematode, Columbia root knot nematode and golden nematode.
In August of 2014, the Canadian Food Inspection Service confirmed that potato wart had been detected in a farmer’s field in Prince Edward Island. Officials with the Animal and Plant Health Inspection Service (APHIS) of the United States Department of Agriculture conducted an on-site review in response.
Fresh potato movement continued but shipments of P.E.I. seed potatoes to the U.S. were suspended. In early December, the P.E.I. Potato Board learned that seed potatoes shipments to the United States would be allowed as long as the field in which the potatoes were planted had a soil sample that tested negative for potato wart. Potato seed shipments from the Island to U.S. markets resumed shortly after the announcement.
According to the P.E.I. Potato Board, the USDA is preparing a federal order outlining the requirements for fresh and seed potato imports that is expected to be published soon.
“We are working through this with the U.S. in a science-based manner, as was the case with other challenges in recent years such as potato cyst nematode in Idaho,” says Greg Donald, the board’s general manager. “These protocols allow us to continue trade while following practices to protect our industry on both sides of the border.”