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Potato yield, storage and exports 
By Hina Kamal

POTATO (Solanum Tuberesum) is a source of vitamin C, even though the vitamin content per unit weight is comparatively low. The amount is highest in new potatoes and falls gradually during the post harvest storage.

As large amounts of potatoes are consumed, these contribute more protein and iron than other vegetables in the average diet and are also useful sources of thiamine, niacin and several other nutrients including fibre.

Factors affecting the stability of potato during the extensive period of pre-harvest growth, storage, transport, packaging and export can be classified into three margins, including infection during pre-harvest growth, physiological and mechanical damages during post-harvest handling.

Pre-harvest factors are largely responsible for significant post-harvest losses occurring either during storage, transport or packaging. These factors include; seed selection, seed storage, field pests, infection by disease organisms, pest infestation, environmental and cultural practices, and genetic factors.

High quality seed should be chosen with low level of disease or those plants should be used which are moderately resistant to late blight with thoroughly clean and disinfect storage facilities and handling equipment prior to receiving seed potatoes.

During the pre-harvest growth phase, the plant is exposed to different diseases initiated by infection, if handling is not appropriate. The infection may start before harvest, through natural pores in the above and below ground parts of the plant, which allows the movement of air, CO2 and water vapour into and out of the plant and since the pathogenic and spoilage micro organisms are widely distributed in the air, soil and on dead and decaying plant material, they result in inoculating infection.

Severe infections can completely wipe out an otherwise good crop. In the field, late blight reduces the ability of the plant to provide nutrients to the tuber, often resulting in a decrease in the number of marketable potatoes.

In addition to the loss or yield reduction of a specific field, neighbouring field can also be at risk due to mobility of the organism. The quality can be severely impaired. Late blight tuber rot is a direct result of the late blight organism and can develop and spread during the growing season, at harvest and in storage (if potatoes going into storage are infected). To avoid late blight disease spores washing onto tubers from infected vines is conducted. The use of an effective combination of contact/systemic fungicide needs to be applied in two phases firstly, when the plants are small i.e., at the beginning and at the end of season. The magnitude of loss due to infection is variable.

Another factor is the presence and activity of field insects which affect the plant in two ways; by boring holes in tubers thus reducing the quantity and quality of the produce and germination capacity, and by causing damage to the epidermis providing entry for moulds and bacteria to penetrate into the tuber.

Pyralid Moth and Tineidae species are known to create losses during the pre-harvest stage of plant growth.

Pre-harvest factors are largely responsible for significant post-harvest losses due to the interrelationship and interaction between different components of production and harvesting, of which respiration is a vital intrinsic force such that: tubers are living organisms and as such, they respire. The respiration process results in the oxidation of the starch (a polymer of glucose) contained within the cells of the potato tuber which converts it into water, CO2 and heat energy. For respiration to occur freely a supply of oxygen is needed and the resulting CO2 and heat have to be removed from the products environment because a limited supply of oxygen and inadequate removal of CO2 may lead to effective asphyxiation and death of product tissue.

Respiration rate of potato varies with the physiological age of the tuber i.e., whether it is in sprouting or in dormant phase, whether or not it has been damaged and is healing its wounds and even by the storage conditions, mainly temperature. The rate of respiration is relatively higher at harvest followed shortly by a decrease, especially during storage, then followed by an increase once sprouting begins.

Dormancy is considered to be the point of physiological maturity of potato. During dormancy, the endogenous metabolic rates are reduced and the tuber depicts no intrinsic or bud growth, although it retains the potential for future growth. Hence dry matter losses are correspondingly reduced. Dormancy is affected by major extrinsic factors such as temperature, moisture content, oxygen and CO2 content of the storage atmosphere.

Apart from it, the extent of wounding and any disease of the tuber may also have an overriding effect. This provides the fact that while tubers remain dormant they can be stored satisfactorily. As soon as dormancy is broken and sprouting begins, the rate of dry matter loss increases since formation of sprouts require energy which is drawn from the tubersí carbohydrate reserves. This also results in an increase in the rate of water loss, pathogen penetration. However, it should also be taken into account that avoidance of sprouting applied by low temperature leads to sweetening of the tuber, which is considered to decrease the value of the stored crop.

The permeability of the skin of the tuber is a function of its maturity and is significant in the rate of respiration for example the periderm of freshly harvested immature tubers is most permeable and thus permits higher levels of respiration than similarly harvested mature tubers.

Damage and wound healing increases influence respiration and the incidence of invasion of pathogens such that the initial infections cause by the pathogens produces a breakdown of the host tissues and once these primary pathogens are established, they are followed by an invasion by a broad spectrum of secondary pathogens.

Mechanical damages are indeed also vital in keeping the stability of potato. There are different degrees of mechanical damages, from small bruises to deep cuts and they may be sustained at any stage, from pre-harvest operations, through harvesting and subsequent handling operations when the product is graded, packed and transported to market. Serious mechanical injury, which may result in the product being rejected during grading, is a direct loss.

Lastly, storage conditions including temperature, humidity, air movement and water loss are influential in masking the stability of potato tuber. In this regard lower storage temperature gradually, maintaining steady levels of humidity and temperature until all of the field heat is removed. Continue to maintain steady levels of temperature and humidity throughout storing potato, but the rate of air movement needs to be kept as low as practical to prevent excessive loss of moisture to avoid the production and spread of moulds, especially Fusarium Tuber Rot and Silver Scruf. If some decay occurs, completely open outside air intakes and increase the ventilation rate until the diseased potatoes dry out.

Pakistani potatoes are losing export markets which have created a crisis for growers. The traditional export markets are Iran, Sri Lanka and Afghanistan. The streamlines discussed earlier can greatly increase the yield, storage conditions and export of potato, for these reasons, the total production market and marketing system needs to be address as a whole.

Courtesy: The DAWN
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