2018 ASHS Annual Conference

Prunes (Prunus domestica L. Batch) production in California is limited to a single cultivar, ‘Improved French’. The crop is grown for the processed (dried) market with larger fruit, suitable for pitting, delivering higher grower return per pound than small fruit, which is used only for juice or concentrate production. Prune thinning is done by modified harvester more than a month after full bloom, when the fruit reach sufficient mass for mechanical removal. However, shaker thinning is not practiced by all growers and excessive production per acre of mostly small, lower value fruit is not uncommon. Return bloom can be limited in years following heavy crops, producing inconsistent production from year to year, harming grower income and industry marketing strategies. In 2015, a field study was begun in a mature, high yielding prune orchard in Sutter County, California, to test the hypothesis that bloom thinning with a caustic spray material would deliver more consistent production of large ‘Improved French’ prunes than unsprayed trees. For three seasons, 2015-17, potassium thiosulfate (KTS) at 1.0 v/v or 2.0 v/v was applied at 25% and again at 80% full bloom in a volume equivalent to 1871 l/ha to four (2015) or five (2016-17) trees per treatment. The experiment followed a randomized complete block design with blocking by trunk cross-sectional area and using a single tree per treatment per block. At commercial harvest, fresh fruit weight per tree was measured and a 2 kg subsample taken for drying at a commercial dehydrator. Whole tree dry (18% moisture) fruit yield per each treatment tree was determined using the dry to fresh weight ratio of the subsample. Dry fruit count per kg was determined for each subsample and then scaled up to determine large (<36 dry fruit/kg) fruit size yield for each tree. Conditions at bloom in 2016 resulted in crop failure, but large commercial crops were produced in 2015 and 2017. In both of those years, KTS bloom spray treatments did not significantly affect dry fruit yield (kg/tree), but 2% KTS bloom sprays significantly (p=0.05) increased yield of large fruit per tree over that of unthinned trees. Bloom sprays of 1% KTS significantly (p=0.05) increased large fruit production in 2015 compared to control trees, but not in 2017. These results indicate a potential application for caustic spray thinning in ‘Improved French’ prune and encourage further research.