INTRODUCTION
Potato (Solanum tuberosum L.) is a very important vegetable and ranks fourth in the world with respect to production, in United States of America it is ranked fifth. Pakistan has further lower ranking in this regard. Potato is also staple food in many communities and it has a large spectrum of cultivar based on shape, size and colour of its edible tuber.
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| Warts on Potato Tubers |
Potato wart disease is an important disease. This is an ancient and widely spread disease across the globe. There had been records of its presence in the ancient tribes of Africa and later it appeared in Americas. Other names also include; black scab, black wart, potato canker, potato cancer, warty disease and many other names in other regional languages. The spread of disease has been mainly due to trade of the diseased potato across the continents. In modern times, the quarantine approaches has largely reduced the impact of disease yet this disease continues to exist.
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| Synchytrium endioticum L. |
Synchytrium endobioticum (Schilbersky) Percival
Potato (Solanum tuberosum L.) is the only cultivated hosts of the Synchytrium endobioticum, where as some wild Solanum spp. can also be affected with the pathogen. A number of solanaceous plants like tomato (Solanum lycopersicum L.) can be inoculated artificially. Potato stands as the major host of the pathogen.
DISSEMINATION AND SURVIVAL OF PATHOGEN
The pathogen, S. endobioticum has a very little natural dissemination capacity; that is why; it is very easy to control the disease. The main source of spread is the infected seed potato. The movement of the soil which has a previous record of being affected by the disease is also a major source of the disease. The disease also spreads with the implements and machinery used for harvest and post harvest of the infected potato. In international trade, its spread has been controlled through quarantine measures.
SYMPTOMS
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| Cross Section of infected tuber |
The evident symptoms are the galls which are produced on several parts of the plants. Galls are mainly produced on the tubers in the form of rounded swelling of the parenchymatous tissues. This is both due to hypertrophy (increase of the size of cell) and hyperplasia (increase in the number of cells). The galls produced are usually spherical outgrowths, 1 - 8 cm in diameter. The galls can also be visible in some cases on the base of the stem near the ground level, or leaf stolons buds or tuber eyes. The above ground gall are green to brown and prone to decay where as below ground galls are yellow to brown in color and turns black as the galls decay on maturing tuber.
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| Symptoms on stem |
Galls during the harvest may not be visible because they may be desiccated and barely noticeable or the galls may decay. The decayed warts are not evident at the time of harvest but retain the pathogen. During longer storage the pathogen becomes active and starts forming the galls againthus destroying the diseased as well as the healthy potato tubers during storage. The disease does not kill the plant but it can damage the meristematic tissue of the seed buds which do not germinate.
The infected plants develop general symptoms of reduced vigor, decreased plant height; yellow greenish warty growths may also be visible at the base of the plants. In contrast, true roots of the plant are not affected by the diseases.
DISEASE MANAGEMENT
i. Resistant Varieties: Resistant varieties are the best solution. The resistant varieties may get the symptoms but the infection is much reduced and the pathogen and its spores on the tuber are killed by the hypersensitive response of the host tissues. The galls remain superficial.
ii. Regulatory Measures: Once the potato wart disease has been identified from a particular site, there should not be cultivation of the further potato or other solanceous crop in the same soil for a considerable long period of time till the pathogen spores present in the soil are not destroyed by the appropriate measures.
There should be prohibition of the movement of soil from such site by any means which would allow the spread of the disease to new sites.
Long term monitoring of the infected soil as well as the record keeping should be done of the treatments given to the soil to ensure complete removal of the pathogen.
iii. Chemical Control: In the past chemical control was carried out but it is out of use in the modern times because of high residual affects and toxicity to the soil. These treatments (copper, mercury, sulphur, chlorine-based chemical) had potential to turn the soil barren. So these methods are no longer in use.
iv. Biological Control: Several non-chemical approaches have been under development and research like use of urea, crab shell and chitin which are hypothesized to be safe and natural.
SIGNIFICANCE
This disease is very persistence in the soil due to formation of resistant, thick walled winter sporangia which can survive for decades. Then under the favorable conditions they germinate and infect the potato tuber. After plant emergence the tuber eyes and stolon may be affected, as a result the disease may produce unrecognizable tuber which are unfit for human consumption. The infected soil with winter sporangia are also unfit for future cultivation of the potato tubers and the treatment of such soils is very expensive and highly unfit for the soil health. Chemical controls are expensive and not recommended because they can not control the resting sporangia.
The potato wart pathogen is readily distributed via movement of infested soil and by infected seed tubers. Because of the great potential for disease loss, any degree of infestation or indication of potato wart world-wide results in strict quarantine and other regulatory measures designed to confine the known infestation and to preclude pathogen re-distribution.
Although direct losses from potato wart may be insignificant when first detected, indirect economic losses resulting from zero-tolerance regulations for potato wart can be devastating to the affected parties.
Indirect economic losses become especially evident in situations where seed tuber production areas become subject to quarantine measures, as well as when the movement of commercial potatoes is restricted. For example, a well-documented detection of potato wart on Prince Edward Island (Canada) late in the 2000 growing season and the ramifications of subsequent regulatory actions resulted in an estimated $30 million loss to the Island’s economy in that first year alone.
