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Introduction
Wart disease of potato is caused by a microscopic soil-bourne fungus. The disease
appeared in Europe around the turn of the century and spread to most regions of the
world where potatoes are cultivated. In Newfoundland the disease is known as potato
canker and is estimated to be present in over 90% of communities. Most infested soils
are in home gardens. Wart disease presents biological, social, and legislative
problems. This pamphlet attempts to put some of these problems in perspective.
Symptoms of Wart Disease
The major symptom of wart disease is a gall at the base of the potato stem, or the
haulm, The gall, which is white when under ground, and black when decaying, may be
as small as a pin or as large as a fist. Its surface is rough and corrugated-warty in
appearance. In a severe infestation the tip of the stolon (where the tuber is normally
formed) becomes infected and develops a gall instead of a tuber. Severe infestations
destroy the potato crop by preventing tuber production. The plant itself does not
appear damaged above ground: the damage is hidden until the plant is harvested.
The Fungus
The organism responsible for wart disease of potato is a tiny, single celled fungus,
Synchytrium endobioticum (Schilberszky) Percival. This fungus, which characteristically
thrives in very wet conditions, belongs to a family of fungi that generally inhabit
freshwater ponds. Although the fungus is tiny-the cell measures approximately less
than 100 micrometers-its life cycle is complicated. When the surface of a gall is
examined with a hand lens, the fungus appears as a myriad of tiny brown dots, which
are actually resting spores. The gall eventually decays and turns black, releasing
these spores into the surrounding soil. Resting spores are known to live for 40 years
of more in undisturbed soil, their longevity contributing to the wart disease problem.
When the resting spore germinates, it releases hundreds of tiny swimming spores into
soil water. The swimming spore, which is about 1/50 the size to the resting spore,
eventually reaches and penetrates susceptible potato tissue, thus perpetuating the
disease cycle. The presence of abundant soil water following heavy rain is a key to
the development of the disease; this and temperature conditions largely determine where the fungus can remain active.
How the Disease Is Spread
The disease is spread when the fungus is transported to new soil. Contaminated
materials include the soil itself, tubers from infested soil, implements used in potato
cultivation, footwear worn when working potato plots, manure from animals that have
been fed warted tubers, hooves of animals tethered on infested soil, and possibly
wind passing over dried infested soil. This is a social disease, spreading only where
people cultivate potato plots. A study of the distribution pattern of the disease in
Newfoundland indicated clearly that it is spread along railway lines, road networks,
and coastal transport routes.
The fungus is believed to have originated in the potato-growing regions of Chile or
Peru. It was probably transported to Europe after the Irish potato famine, when plant
scientists were investigating the breeding of new strains of potato. The connection
between the gall, the reduction in tuber yield, and the tiny soil-borne fungal cell has
apparently not been easy for the public to comprehend. In a recent survey of home
gardeners on the cause of wart disease of potato, not one person considered the
causal agent to be a lowly fungus. This lack of understanding is an important aspect
of the wart disease problem, because it affects the interaction between home
gardeners and the disease.
Controlling the Disease
Disease control in such complicated process is fraught with difficulties. To help solve
the problem, governments have legislated quarantine measures as a first level of
control. Consequently, the disease is now subject to stringent quarantine regulation
around the world, which prevent the entry of contaminated materials. Because the
disease is already established in Newfoundland, quarantine measures are used to
prevent the export of contaminated materials. Car-wash and inspection facilities at the
ferry heads of Port-aux-Basques and Argentia intercept materials that may harbor the
fungus. Soil adhering to vehicles is removed.
Although the focus is on prevention, plants must be protected from the fungus when it
is already present in the soil. Chemical controls that eliminate the fungus from
contaminated soil have not proved to be feasible in Newfoundland; such chemicals as
copper sulfate or formaldehyde are no longer acceptable because they destroy the
crop-producing capacity of soil and interfere with well water and land use. In an
attempt to eradicate wart disease of potato, potato plants are now bred for
resistance to the disease. This breeding program has been successful around the
world. At St. John's Research Station, several resistant cultivars have been produced:
Pink Pearl, Mirton Pearl, Blue Mac, Anson , Brigus, Cupids, and Domino.
The wart disease problem is compounded by the fact that the fungus has developed numerous strains, known as pathotypes. About 20 occur around the world, including four known in Newfoundland. Potatoes that are resistant one pathotype may be susceptible to another.
Although resistant potato plants do yield disease-free crops of tubers, the problem of
contaminated soil remains. Recent investigations into biological control methods at
St. John's Research Station have revealed that the disease can be suppressed when
infested soil is amended with crushed crab leg. The meat is removed from the crab
legs, which are subsequently dried and then crushed. The resulting meal is mixed with
soil in a meal-to-soil ratio of 1:20. After the tubers are planted, the mixture is packed
over the top o f the tuber and covered with additional soil. The plants are then watered
thoroughly. Because weather condition vary at planting time, water should be applied
daily for a week or so. The exact method of application is still under investigation;
however, this system produces fairly consistent results. As a result of this work,
greenhouse experiments have shown that the number of resting spores in soil
diminish over several months after treating soil with crabshell meal. If a substance
can be found that eliminates the fungus from soil to produce, and that is cheap and
plentiful, the quarantine ban on export of Newfoundland produce could be removed in
the near future.
Research on the eraticative properties of crushed crabshell is therefore being
intensively pursued. Crabshell, a waste product form the fishing industry in
Newfoundland, can offer hope for a solution to the wart disease problem.
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