The causes of year-to-year variation in the average yield of Cox’s Orange Pippin apple in England

SummaryMultiple regression analysis of the year-to-year variation in national average cv Cox’s Orange Pippin yields, from 1949 to 1975, showed a linear upward trend of +0.254 ±0.039 tha-1 per year (probably attributable to advances in technology) and highly significant effects of three weather factors which, between them, accounted for 63% of the variation around the trend. The four factors together accounted for c. 80% of the total variation. Yields were negatively associated with high temperatures in February, March and April. Although high temperatures at this time were associated with early blossoming, this, in the years concerned, had not led to greater frost damage or lower temperatures at the time of pollination than occurred in other seasons. It. seems probable that the adverse effect of high pre-blossom temperature is mediated by adverse effects on flower quality and fruit-setting potential. High temperatures immediately after full bloom, such as to lead to the rapid completion of pollen-tube growth and expressed in terms relating to this, were associated with high yields. Cold weather in June, as indicated by low maximum temperatures, was associated with low yield. This possibly reflects effects on fruit retention and growth. Possible approaches to the control of these sources of yield variation, and hence of one of the major problems of the English fruit industry, are discussed.