Production and Trade Potato Cultivation

Good appearance may have commercial value, but it apparently has little or no influence on the capacity of the seed for crop pro duction. The factors influencing this are two : (i) The history of the crop from which the seed is selected, and (ii) its treatment between the autumn when it was lifted and the spring when it is planted. Scientific research has discovered that "seed" selected from plants affected with "virus" diseases such as leaf-roll, mosaic or crinkle, usually reproduce plants suffering from these diseases. Unfortunately "seed" infected with these "virus" dis eases cannot be distinguished by examining the tubers, and it is now known that many nice looking samples of so-called "good seed" may be highly infected with virus diseases. A sample of seed needs to be true to variety, so that on planting a true stock may be grown. It is almost impossible for experts to recognise varieties by an examination of the tubers alone—and here again the past history is the reliable guide. If the parent crop was a true stock also free from leaf-roll, mosaic or crinkle, the seed selected from it will have a good history and a good potential capacity; if the parent crop was mixed or had much infection the seed from it will be distinctly inferior.

Potato crops grown in the colder and wetter districts are known to be freer of "virus" diseases than those of the warmer and drier regions and in consequence the former districts are the better areas in which to grow potatoes for seed. These "virus" diseases are not peculiar to any country or region ; they exist in England, Scotland and Ireland and are very injurious in France, throughout Southern Europe, Canada and America. To combat this pest and to raise the standard of potato cultivation many countries—including America, Canada, the Netherlands, England, Scotland, Northern Ireland and the Irish Free State— have established systems of seed-potato-certification under which fields of potatoes are examined by inspectors and certificates given for those crops that reach a certain specified standard.

In 1927, the following quantities of potatoes were given certificates for seed :— Acres Acres England and Wales . 8,30o North Ireland . . 5,000 Scotland . . . . 63,00o Irish Free State . . 3,000 In America and Canada similar schemes have made great de velopment. In 1919 there were 475,308 bushels of certified seed grown in the United States; by 1924 the demand for this seed was so great that over 7,500,000 bushels were produced.

Treatment of Seed.

The "seed tuber" contains food sup plies for the development of its buds, which are not all of the same vigour. Under the best treatment, these food supplies would be used for the development of a few of the strongest buds. In practice much wastage occurs, for seed potatoes are often allowed to produce long shoots which get broken off, and in consequence other buds, usually weaker ones, are forced to sprout. The best treatment is one that preserves the first sprouts, and this can be done by causing the buds to make slow growth in bright light, when short and sturdy sprouts are produced. In practice, soon after lifting time, the seed with the bud end upper most should be placed in shallow boxes 2 ft. long, I ft. wide and 7 in. deep, of which only the bottom 3 in. is boarded.

The boxes may be stacked in tiers in a cool place where light is freely admitted. During frosty weather the boxes must be pro tected in the house heated with stoves. If sprouting houses are really well lighted every bud will not grow, but just two or three strong, sturdy, short green sprouts will develop. The boxes at planting time should be taken to the fields and the tubers care fully placed in the drills leaving the sprouts unbroken.

Protection Against Disease.

Wherever grown, the potato crop seems to be subject to attack by blight, Phytophthora in festans. This disease attacks and kills the leaves, and haulm, and in bad cases also attacks the tubers. The loss then is twofold, for when the foliage is killed growth ceases and little or no further development takes place, and where tubers are also at tacked crop wastage occurs. The disease is less troublesome in countries where the atmosphere is dry, but in those where humid or wet summer conditions prevail blight causes much annual loss unless precautionary measures are taken.

Firstly, seed tubers infected with blight should be discarded, for if planted they cause a recurrence of the disease; but, as it is difficult to see tubers only slightly infected some may get planted and in consequence the crop should be sprayed to prevent. infection and to limit the spread of blight when it occurs. The spray must be applied before the blight makes its appearance, it must adhere to the leaves and be preventive against the fungus penetrating into the tissues of the leaves. In practice Bordeaux mixture (copper sulphate combined with lime) or Burgundy mixture (copper sulphate combined with washing soda) are used as sprays; and there is some experimental evidence to show that both are equally efficient.

For small areas the sprays are best applied by means of a knapsack machine, which must be provided with a nozzle that throws a fine misty spray. The Irish growers usually use these small knapsacks. For the first spraying 120 gal. per acre, or three-quarters of a gallon per rod is sufficient, though for second and subsequent spraying i6o gal. per acre or one gallon per rod will be needed to cover the larger amount of foliage. In Eng land, Scotland, Germany, Canada and the United States, it is customary to use horse machines or even motor spraying ma chines, each capable of spraying many drills of potatoes at one time. Formerly, these wet sprays were always used; but in recent years dry spraying or dusting has become popular in England, America and Canada. A machine is used—hand, horse, or motor —for blowing a very fine powder containing copper salts on to the leaves when they are somewhat moist with dew or fine rain. This method is more economical of labour, it can be carried out in districts where water is scarce but is not so effective as wet spraying in preventing attacks of blight. The exact dates at which the spraying should be done vary with each district ; the first must be applied bef ore blight usually appears. The first spraying would need to be made in the south-west of England by June 15, though July 15 would probably be early enough for Lincolnshire and Ireland. Periods for European countries, the United States and Canada vary in similar ways.