A STUDY OF THE DEGENERATION OF CERTAIN POTATO STOCKS

Summary. 1 Trials carried out at the University College Farm, near Bangor, N. Wales, have demonstrated the intimate relation of virus disease infection to the fall in yield experienced when potato “seed” is saved more than one year. These trials were laid down with seed tubers of three sizes corresponding to commercial “ware,”“seed” and “chats. ” Two varieties were used, viz. Kerr's Pink and Great Scot, which had been grown for varying periods, up to seven years, without change of seed. 2 Whatever size of seed tuber was used, the yield became progressively less from the youngest to the oldest stocks, except that grown for six years, which compared well with slightly younger stocks. Again, with the exception of the six-year-old stocks, the percentage of plants free from virus symptoms fell consistently from the youngest to the oldest stocks. The continued relative superiority of these six-year-old stocks of both varieties is considered to be due to (a) exceptional freedom from infection when received from Scotland, (b) the spread of disease more easily along the drills than across them, and (c) the relatively late infestation by aphides which occurs at the College Farm and results in a large number of tubers on primarily infected plants escaping infection—the effect of (b) and (c) being to maintain any original difference in infection between stocks. The possibility of a difference in the reaction of different strains of a variety to virus infection cannot be ignored but is regarded as unlikely in the present instance. 3 The relationship between virus infection and loss in yield has been shown to be a linear correlation, and that it is of an extremely high order is evident from the fact that the correlation coefficient in the case of the Kerr's Pink is shown to be 0.83 ± 0.046, and with Great Scot 0.97 ± 0.008. 4 “Misses” occurred to the extent of about 5 per cent, in the youngest stocks but increased progressively in the older ones, irrespective of the position they occupied in the chequerboard plots. There is, therefore, another source of loss, which has not hitherto been generally recognised, to be attributed to prolonged home-saving of seed, and probably—though not certainly—to virus infection. 5 The effect of virus diseases on the size of tubers produced has been studied. There is no clear evidence in either variety of any marked reduction in size attributable to virus infection, but definite evidence that the proportion of large tubers in the crop is affected by the size of set planted. 6 No evidence was found that the rate of degeneration was affected by the size of tuber set planted, except possibly in the oldest stock of Kerr's Pink. In the variety Kerr's Pink there was no significant difference in virus infection in the three tuber classes, however long they had been grown without change. This was almost equally true of Great Scot so far as leaf-roll was concerned, but the chats produced a higher percentage of mosaic and crinkle plants than did the ware. 7 The effect of climatic, seasonal, and varietal factors on the rate of degeneration has been studied: a A detailed comparison has been made of the vigour of the same stocks of two varieties maintained at the College Farm, near Bangor, and at Madryn Castle Farm Institute some thirty miles distant. Owing probably to climatic and topographical factors, aphides occurred only sparsely at the latter centre and practically no reduction in vigour in the six-year-old stocks occurred. b Seasonal factors, by influencing the rate of breeding of insect vectors and the vigour of the potatoes, may considerably accelerate or retard the degeneration of a partially infected stock. It is shown that, owing to the difference in the response of healthy and diseased plants to unsuitable conditions, it may happen that a heavily diseased stock in an exceptionally good growing year will show less loss relative to the crop from a healthy stock than it did in a previous year in which less virus disease was present. c The influence of variety on the rate of degeneration is shown to be the resultant of three separate forms of susceptibility, i.e. liability of haulm to infection, liability of tubers to contract infection from the diseased haulm, and the effect of infection on the yield. The variety Crusader showed all three forms of susceptibility in an extreme manner; Up-to-Date was very susceptible to infection both of haulm and tubers, but the virus infection had relatively little effect on the yield; on the other hand, the variety Great Scot exhibited considerably less susceptibility of haulm and tubers to infection, as compared with Up-to-Date, but appreciably more effect was produced on the yield. The value to be attached to field trials of susceptibility to virus diseases is discussed.