Consistent production (CP) of quality crops supports economic sustainability of individual farms and maintains market share for a larger industry. For the last 30 years, annual prune production in California has been highly variable, ranging from < 1 to >3 dried tons per acre. ‘Improved French’ is the primary cultivar grown in California. This variety, like most plums, often requires thinning to maintain CP and grower income where fruit size (count per pound) is positively correlated to fruit quality and price to the grower. Mechanical shaking at reference date, ~45 days after full bloom, is now a common thinning practice although questions have been raised regarding the potential benefit to earlier thinning a phantom practice that would have to be chemical thinning. However, in the last two decades, extreme bloom weather can virtually eliminate a prune crop if warm weather precedes bloom. This talk reviews work by University of California researchers towards improving CP through 1) better understanding of the weather related factors contributing to very low fruit set in prunes, 2) testing ‘Improved French’ prune fruit growth development to determine if and when source limited fruit growth occurs, and 3) identifying chemical thinning practices, materials, and rates to remove flowers before fruit set. Early bloom years are those with risk of crop loss due to extended and wet or extremely warm (>27oC) bloom weather conditions. [The risk of freezing damage is relatively low for prunes in CA given usual March bloom timing.] Relative prune fruit growth rate studies revealed resource limited fruit growth occurs ahead of current thinning timings (reference date), so earlier thinning may improve fruit growth and possibly yield compared to the current thinning practice. Finally, potassium thiosulfate (KTS) applied 2x at 1-2% (v/v) shows promise in thinning prunes. Lime sulfur and fish oil can be effective but is inconsistent in bloom thinning. Obstacles to CP of ‘Improved French’ prune include difficulty identifying practices to quantify bloom strength and help grower with the decision to thin or not. In addition, no ‘rescue’ spray has been found to improve fruit set in years with early, risky, bloom. Grower testing of the current research results awaits successful overcoming of the two obstacles listed above.
