Weather conditions contributing to crop failure in ‘Improved French’ prune grown in California

The prune (Prunus domestica) production region of California is characterized by a Mediterranean climate with generally cool, wet winters and hot, dry summers along with significant intra- and inter-annual weather variability including temperature. This natural variability of Mediterranean climate combined with greater temperature anomalies of climate change presents a significant challenge to economic viability of prune production in California, where a single cultivar, ‘Improved French’, is used almost exclusively. A field study was undertaken to assess the effect of natural temperature variation on ‘Improved French’ prune fruit set. Bloom timing was tracked and percent fruit set determined in commercial orchards in multiple growing regions of California from 2005 to 2020. Sensors in each study orchard logged hourly average temperatures (HAT). During the entire study, full bloom (FB; 80% open flowers) ranged from Julian Day (JD) 68 to 95, with first open flower to 80% bloom lasting 3-14 days. Maximum HAT (MHAT) for each of 14 days during bloom (7 days before FB to 6 days after FB) ranged from 25 to 29°C with no frost events. Widespread prune crop failures occurred with early full bloom timing (JD ≤ 75) and either extended, cool (daily MHAT 13-15°C), hot (daily MHAT = 27-29°C) from a few days before through a few days after full bloom, or highly variable temperature values with cool weather bracketing a short period of warm weather (>27°C) at or immediately following FB. Temperatures throughout all 14 days of bloom, collated in discrete means of three to four day periods, provided the best correlation with fruit set in eagerly bloom years vulnerable to damage, while individual daily temperature data explained no more than half of the variability in fruit set.