THE POTATO CROP

Evening Star, Issue 22308, 7 April 1936, Page 7

THE POTATO CROP

FIGHT AGAINST VIRUS DISEASE AGRICULTURE SECTION OF ROYAL SOCIETY The first meeting of the agricultural section of the Royal Society of New Zealand for the 1936 season was hold in the Museum lecture room last evening. The speaker of the evening was Mr .7. M. Smith, fields superintendent of the Department of Agriculture, who delivered an illustrated lantern lecture on ‘ Improvement of the Potato Crop.’ Mr G. A. Holmes presided over a fair attendance of members. In tho North Island commercial potato growers recognise the fact that seed from the South gives better results than local seed, said Mr Smith, and southern seed, as it is commonly termed, is in keen demand. Similarly Canterbury growers appreciate Otago seed, while Otago growers go further south still to procure their stocks. In Great Britain the position is much the same, although being in tho Southern Hemisphere the growers go north for their seed, and Scotland is the reoegnised land of the seed potato. Undoubtedly the further south one travels in New Zealand the more noticeable is the Scottish accent, but it_ is not suggested that the racial position or the gathering of the clans has any bearing ou seed potatoes. Bather does it appear that the colder climes of the south in New Zealand and of the north in Great Britain are responsible. Commercial potato growing in this country has proved that high-class seed, taken to a district where the winter months are temperate, shows rapid degeneration after a season or two, and despite any steps that are taken this degeneration cannot be prevented. Even in the northern districts of the South Island a steady deterioration of certain varieties is very noticeable. Then, again, it was recognised several years ago that commercial stocks of seed potatoes were badly mixed as far as varieties were concerned, and it was getting harder and harder tp procure “ rogue ’’-free lines of potatoes, and indeed in some cases tho “ rogues ” or impurities constituted the greater proportion of the line. These two factors —that is, the general deterioration which greatly impaired cropping power, and seed impurities—were leading potato growing and growers into serious economic trouble, and it was at this stage that the Department of Agriculture introduced its system, of certification. which system was introduced to combat these threatening troubles. It was first necessary to study the factors that were lowering the productiveness of a large proportion of potato crops. This low yielding trouble was proved to be principally the result of a group of diseases collectively known as virus disease. It was found that in most oases this group was wholly responsible, and that they were transmitted from plant to plant in the field, and from season to season, by means of the tubers. Unfortunately, however, virus disease cannot be detected on the tubers—these to all appearanees having a bright, healthy appearance even when badly affected with the disease. When such tubers are used as seed, however, the cropping power is greatly reduced, the infected shaw giving a large number of seed-sized potatoes. Tims the disease becomes progressively worse, as it is impossible to distinguish between diseased and healthy tubers, while in the resultant crop the greater proportion of seed-sized potatoes from diseased shaws are thus virus infected tubers capable of carrying the disease on to tha succeeding season’s crop. Many growers, realising this, used only tablesized tubers as seed, appreciating the fact that virus affected shaws produced few, if any, table-sized potatoes, so that by selecting such seed the chances were greatly in favour of this seed being healthy. This practice of growing crops only from table-sized tubers is carried out by many growers to-day, even though lines of good seed practically virus free can be obtained through certification. While there is not a great deal known about virus disease, it is known to be a sap-borne trouble, many of the forms being apparent in the foliage, where there is a definite breaking down of the tissues. Being a sap-borne trouble, it is transmitted from a diseased to a healthy plant by means of leaf-biting insects, principally the aphis. _ Thus in the colder districts, where aphis are cleaned up each winter, and where, in fact, they never really become very troublesome, potato crops can with a little carp he kept fairly free from virus disease, provided a start is made with healthy stock and the- crop vigorously rogued for any plants showing signs of virus trouble. To illustrate how a line can degenerate from a high standard in a season or two under unfavourable conditions might be cited ap experience at Palmerston North, where three generations of seed of a higli standard yielded 15 tons, 12 tons, and seven tons respectively. This shows how, despite the utmost care, a general lowering of tho cropping power of the seed is experienced. Authorities differ in classifying the many virus diseases; a plant may carry more than one of them, and their ■'Jftual identification thus becomes very difficult. These diseases, which are so very apparent in the tubers after they are dug and while they are in storage, are not brought about by vims or in any way directly affected by virus disease. Possibly the virus troubles will have a weakening effect on a plant union will in turn reduce its vitality and so make it more susceptible to other blights, hut these various late blights, Irish blight and so on must not be contused with virus disease. One form , ii Vlr i" s <lis ? ase lakes is known as leaf 101 l In this the lower leaves are rigid, the leaflets thickened and harsh and rolled upwards and inwards, making the leaf funnel or spoon shaped. Hie rolled leaflets are crisp and dry, ivn rus^e "’hen the plant is disturbed. TV hen a healthy plant becomes infected during the growing season there may be no visible sign of infection, but occasionally a stiff upward rolling of the topmost leaves takes place, extending progressively to the lower ones. _ All stages of infection occur, and in severe cases the plant becomes much dwarfed and incapable ot yielding table sized potatoes, although at times a fair crop of diseased, although to the eye apparently healthy, seed size potatoes results. Another form of virus is known as leaf mosaic. Instead of the leaf being uniformly green, the leaflets are mottled with pale green patches. This condition is recognised most easily if the plant is shaded from tho direct rays of the sun. In severe cases tho foliage becomes paler, the growth more open, and the leaflets very much wrinkled or the margins of the leaflets waved. The whole plant is dwarfed and the yield considerably reduced. The mottling is to a large extent masked in hot sunny weather, and recognition of mild mosaic becomes very difficult. Infection is carried in the same manner as in the case of leaf roll, that is by leaf biting insects.

Crinkle is the name applied to another form of virus, which form resembles severe mosaic. The plants are always very much dwarfed and . the leaves crinkled. This appearance can be detected almost as soon as the plants are through . the ground. The leaves are very much mottled and curled downwards. The lower leaves often turn brown and drop off, and the plant matures at an earlier date than do healthy plants. A form of virus known as stipple 'streak causes the leaflets to develop characteristic dark angular spots which spread down the veins of the leaflets and extend to the midrib of the leaf. In advanced cases the lower leaves fall, leaving only a tuft at the top and giving the plant a palm-like appearance. Spindle sprout is another form that does become apparent when the seed has sprouted. These develop thin weak sprouts, and should bo discarded. The department’s certification scheme was then dealt with at length by Mr Smith, who pointed out that only those crops showing a minimum percentage of virus in the crop were accepted as certified. Purity was also taken into consideration, and over a certain percentage of rogues debarred the crop from certification. Before tags were issued and final certification reached it was also necessary for growers to grade their seed into even grades so as to ensure purchasers getting seed of the same size. At the conclusion of the address a hearty vote of thanks to the speaker was-carried by acclamation. Mr M. Stewart said that the work of the Department of Agriculture in improving the potato stocks of this country was worthy of the highest commendation, but the general public little realised what was being done in this connection. SOUTHERN BUILDING SOCIETY A meeting of the Southern StarrBowkett Building Society was held last night, Mr H. T. Speight presiding over a small attendance of shareholders. As no tenders were received tor the eighty-fourth appropriation of £oW by sale in group 3, this sum was disposed of by ballot, and resulted in cluster No. 21 being drawn, the shares b :ng held by two shareholders. In group 4 the thirty-seventh appropriation of £I,OOO by ballot resulted m cluster No. 3 being drawn, the shares being held by two shareholders. In group 5 the forty-fifth appropriation of £SOO by ballot'resulted m cluster No. 7 being' drawn. In this case the shares were held by three shareholders. SHEEPSKIN MARKET The New Zealand Loan and Mercantile Agency Company Limited have received the following advice from London, dated April 3:— _ Sheepskins.—The position of the market as compared with preceding auctions prices is in the favour by 7i to 10 per cent for Clothing Merino and Clothing Cross-bred. The demand is good. 56-58’s —full-woolled sound pelts 124 d, 3-woolled sound pelts life, J-woolled sound pelts 91(1; 48-50 s fullwoolled sound pelts IOJd, J-woolled sound pelts lOd, I-woo]led sound pe ts 9d. 44-46’s—full-woolled sound pelts 10(1. S-woolled sound pelts BJd, -i-wnolicd sound pelts Bid; 40 s fullwoolled sound pelts B.ld, -i-woo Hod sound pelts. Sid, ■ sound pelts Bdj short and shorn, 6d to 7Jd.