Weather conditions associated with apple production in the Okanagan Valley of British Columbia

An analytical method that generates χ 2 indices of association was used to determine the "critical" levels of daily weather occurrences associated with annual variations in apple production (Malus domestica Borkh.) in the Okanagan Valley of British Columbia over a 72-yr period, 1920–1991. The weather was recorded in the centre of the region (the Pacific Agri-Food Research Centre, Summerland). High temperatures during flower bud initiation in June (≥30 °C) and during flower bud development in August (≥26 °C) were associated with poor production the following year. Low temperature during November, December and February (critical value range, ≤ −7 °C to ≤ −29 °C) was the main climatic factor limiting apple production. These time periods correspond to the occurence of historical winter injury events. Daytime temperatures ≥5 °C at times during January also affects apple production adversely, probably because warm weather leads to tree deacclimation. Daily precipitation in March (≥1.0 mm) was associated with good production years which may indicate a requirement for early irrigation. Production was favoured by mild temperatures during bloom (≥21 °C daytime, ≥11 °C nighttime) and adversely affected by both very low temperatures (≤10 °C nighttime) and unseasonably high temperatures (≥28 °C daytime) coinciding with the temperature requirements of pollination and pollen tube growth. Good production years were also associated with a lack of low nighttime temperature (≤5 °C) in spring, probably associated with frost in low lying areas. Hot, dry weather during August of the harvest year (≥33 °C daytime, ≤2 mm) had a negative impact on apple production possibly because of loss in net photosynthesis, lower fruit size or apple sunburn. Warm weather during harvest (≥17 °C daytime) favoured production, probably because of improved conditions for harvest operations and low fruit losses from autumn frosts. Key words: Malus domestica Borkh., tree fruit, climate, heat stress, spring frost, winter injury