We are just finishing the bloom period and prune growers are anxiously waiting until the crop load can be accurately assessed.  We all know there is no money in small prunes.    

Matching the crop load with the tree’s ability to size the fruit and achieve desired size is the goal.  Fruit size at reference date, when the endosperm is visible in 80 to 90% of the fruit (Figure 1), can be used to estimate fruit dry fruit size at harvest (Table 1.).  Reference date in the Sacramento Valley usually occurs in early May about one week after the pit tip begins to harden but may be later this year because bloom was delayed.  At reference date, a random sample of sound (non-yellow) fruit should be collected and the number of fruit per pound determined.   Sample 20 fruit from 20 trees.  Use orchard history to determine the sizing potential of the block being considered.  Unfortunately, with large crops this procedure may overestimate fruit size.  Having a good estimate of the number of fruit per tree will help avoid this.  Estimate the number of fruit per tree by removing as much of the fruit as possible with a shaker (prune or walnut) from a representative tree or two.  Place a tarp under the entire tree before shaking.  The remaining fruit should be removed by hand or estimated.  Weigh all the fallen fruit.  A subsample of at least 100 sound fruit is taken from the removed fruit.  The number of sound fruit per pound is determined.   Multiply the weight of the total fruit removed from the tree by the subsample count per pound to determine the number of fruit per tree.   Adjust this number to allow for fruit drop from reference date until harvest to estimate the fruit per tree at harvest.  Work done in the Sutter-Yuba area in the 1970's suggested that approximately 40% of the fruit would drop between reference date and harvest.  More recent work in Glenn and Tehama Counties has suggested that fruit drop may be closer to 20%. 

By dividing the estimated fruit number at harvest by the estimated or desired dry count per pound and then multiplying by the number of trees per acre, you can estimate the dry pounds per acre.  This number will allow you to judge if your estimated fruit size at harvest (from Table 1) is realistic, based on comparisons with crop history – size and yield – from that orchard.  You can then determine how many fruit of the desired dry size are necessary to give the expected dry yield and adjust the number upward by 20% to allow for drop.   Now compare the two sets of numbers.  If the number of fruit per tree measured in your orchard matches the number of fruit per tree at harvest needed to produce a certain size and tonnage of fruit (plus added 20% to account for drop), then you don’t need to thin.  If the number of fruit measured in your orchard far exceeds the needed number of fruit at harvest (+20% for drop) then you should thin.  For example, if your orchard trees should carry 5000 fruit to produce a solid crop in your orchard (for example, 3 dry tons of 60 count fruit) and your trees have 10,000 fruit/tree at reference date – regardless of what Table 1 predicts -- you should thin.