SELECTION OF SEED POTATOES.

New Zealand Journal of Agriculture, Volume XVIII, Issue 1, 20 January 1919, Page 37

SELECTION OF SEED POTATOES.

By

W. H. TAYLOR,

Horticulturist

The significance of the old. saying “ Like begets like” is• doubtless appreciated by every one, yet. very few apply the principles involved to so important a crop as are potatoes. It is quite common experience for a farmer to grow an admirable crop for one or two years, and then a very meagre crop. Bad seasons or something too mysterious to be understood is usually set down as the cause.

There are doubtless a number of factors that may be at work to cause failures, and some of these may be beyond the powers of a grower to alter. It is well known that environment and soil have effect on tubers for seed purposes, but little or nothing is known on that phase of the question as it affects potato-growing in New Zealand. That environment does have great effect is proved by the transformation that takes place, for . instance, in the well-known variety Up-to-Date. In the South Island, particularly in Otago, it exhibits its true form, a half-round with a somewhat rough skin. Sets obtained from there and planted in the North Island produce fluke-shaped tubers with a smooth skin. This alone is evidence of the need for investigation. In other ways there is no lack of information or knowledge, and many failures can be distinctly traced to bad practice. It is a general custom to select the seed-tubers from the crop after it is harvested. All sizable tubers are sold ; the discards are then sorted, and those large enough are kept for seed. The sets are all small; although most of them are fair seed-size, some are really too small, but would be good enough if they were of a good strain. .

Unfortunately, it is practically impossible to keep a strain strong by such methods of seed-selection. When a crop is lifted some of the hills give nothing but small tubers, while others give a preponderance of small ones with a few large.. In both instances there is evidence of deterioration or possibly disease. Hills that are good have but few small tubers. It is evident that when seed is saved in an indiscriminate manner many of the tubers will be from hills that have shown deterioration, the proportion being dependent on the extent of such deterioration. The longer the process is carried on the weaker the strain becomes. It must be evident that this indiscriminate way of saving seed-tubers is attended by very grave risks. Even if large or medium-sized tubers were selected in place of 'small ones there is an almost undiminished

risk, for many of the hills give very few tubers, itself a sign of weakening in the strain.

BETTER METHODS.

If a strain is to be kept strong, methods other than those just outlined must be adopted. There must be special selection, and in many cases special planting of selected sets for > seed-production. Selection from the growing hills is recognized the world over as the best way to obtain prolific sets. When the tubers have nearly finished growing, but before the haulm begins to wither, the most promising-looking hills are lifted. Those that come up to the desired standard,- both in the number of tubers and their form, are kept for planting purposes, discarding those below a certain standard. Appearances are sometimes deceitful, and promising hills are not always good. Tubers lifted in this way will of course be immature, but it is an accepted fact that such tubers produce heavier crops than do'those that were fully matured. Where this cannot be done great improvement could be effected by instituting a process of selection while the ripened crop is being lifted, laying aside the tubers of a sufficient number of good hills. Of course, either plan involves an extra amount of labour, but this is well spent, and improvement in crops cannot be assured without it. So . far the only sets here mentioned for planting have been small ones. Sets cut from large tubers are frequently planted, but these do not always give the crops hoped for. A process of selection is just as necessary with these as with the small sets. They should be taken from good hills, and not from the general bulk of stored tubers. Reverting to small tubers, it is a question how long these could be used without weakening the strain. If small sets are planted and small ones taken from the crop for the next planting, and so on year after year, it is reasonable to expect the strain to weaken, though the period of safety would be extended by annual selection from the hills. It appears reasonable to think that it would eventually be necessary to get back closer to matured parents. •

In former years there was an excellent system in vogue in one potatogrowing district by which small sets were secured direct from matured tubers. Well-grown specimens were planted about the middle of December, the tubers being whole, and the result was a large crop of seed-size tubers. Several advantages were secured besides the one already mentioned. The sets were a nice size for economical planting, they were not fully matured, or at least not so ripe as the earlier crop, and being later grown they were more easily kept till planting-time. This plan is one that could be generally followed with advantage. I believe it is not carried out in the district referred to at the present time, due no doubt to rush methods and the passing of many of the original growers. However that may be, the fame of the district as a potato-growing centre has faded away.

The method adopted this season at Ruakura of sowing the root crops, including mangolds and carrots, on clean land ploughed out of clover has been well substantiated by results. Weeds are conspicuous by their absence in these young crops as compared with some other fields, and the labour of hand-hoeing has been reduced to a minimum.