THE EFFECT OF DATE OF PLANTING AND OF HARVESTING POTATOES ON VIRUS INFECTION AND ON YIELD

Experiments during 1953-6 tested the effects of early and late planting of King Edward potatoes on the incidence of virus diseases in the tubers and the suitability of the tubers for use as seed. Plantings were made each month from April to August and tubers were lifted after 12 weeks' growth or after the haulms had died. Aphids began to colonize the early-planted plots about mid-June each year, but their multiplication and flying were influenced by weather and varied greatly from year to year. The May-planted plots usually carried the most aphids. Winged aphids visited the July and August plantings, but few bred extensively though the plants were young and green. The incidence of virus diseases in the tubers was not related to the maximum aphid populations on the plants. Most of the plants that became infected became so by aphids flying from diseased potato plants in the same field, and the late-planted plots did not escape. Crops planted in mid-Apriland harvested in early July produced the fewest virus-infected tubers. Planting in April and lifting at the end of the season produced most ware-sized tubers. In plots lifted after 12 weeks, May or June planting produced more ware than April planting. Few or no ware-sized tubers were produced from July or August planting, and yields of seed-sized tubers also were poor. Most seed-sized tubers were produced by the late-lifted May or June plantings. The date of lifting had little effect on the total number of tubers, but the date of planting affected it greatly. Most tubers were usually set by the May plantings and the number decreased with successive plantings. Early-lifted immature tubers were easily damaged and were difficult to store. Blighted tubers occurred only in the July or August plantings, which also suffered most from slug and cutworm damage. Infection with leaf-roll virus reduced yield by about three-quarters, whether the seed had been lifted early or late the previous year. Healthy or virus-infected mature seed produced a bigger crop the next year than immature seed obtained by early lifting or late planting. Late-620planted tubers produced more main stems than those planted at the normal time, but growth of axillary shoots was reduced and total leaf area remained small, even when the plants were not attacked by blight.