In This Issue

Prune Crop Status

Crop Size: The prune crop in the Southern San Joaquin Valley is "spotty." That is, some orchards have relatively heavy sets while others have mediocre crops. Orchards that are heavy should consider mechanical thinning to ensure good fruit sizes. Remember a 24/32 inch screen will be in use this season.

Lacy Scab: A considerable amount of Lacy Scab is occurring in local prune orchards this season. Certainly those that used Captan/Bravo during bloom this year have less problem, but it's still present. Lacy Scab results from wet conditions during the bloom and immediate post bloom period when the old flower parts don't separate readily from the developing fruit. The full effect of this problem will not be known until harvest.

Brown Rot: Brown rot blossom blight and some green fruit rot are showing up, especially in orchards untreated during bloom. Plan pre-harvest control measures for this disease with your PCA.

Reference Date: At this writing (May 6) tip hardening of the seed (Blossom End) has occurred. Reference date, that point where 90% of the fruits show endosperm development, should occur within a week of tip hardening. Size predictions at harvest can be made at reference date and shaker thinning strategies planned if needed.

Procedure For Mechanically Shake Thinning French Prunes In The San Joaquin Valley

California French prune orchards frequently have moderate to extremely heavy fruit sets. This causes leaf scorch (potassium deficiency), limb and shoot dieback, limb breakage, and a significant amount of small, worthless fruit. Further, the crop is usually lighter the following year because of the alternate bearing tendency of the French prune.

Mechanical thinning, using conventional tree shakers, is an effective method of reducing crop load to improve fruit size and minimize the other disadvantages of overcropping. It is done after the crop set is known and the major frost danger has passed.

Things You Need To Know Before You Thin

1. Amount of fruit your orchard can produce and also size.

   To effectively utilize mechanical thinning, you need to have some idea of the average fruit size your orchard will produce with different yields. A few orchards can produce 4 dry tons per acre averaging 65 fruit per pound, while others can only produce 2 dry tons per acre averaging 65 per pound. The production level you can sustain for a desired fruit size is the thinning goal.

   Table 1 shows the number of fruit per tree needed at harvest for a given tonnage and fruit size. This table is based on a 20' x 20' foot planting distance, or 109 trees per acre. If your orchard has a different tree spacing, adjust the number of fruit per tree as indicated by the footnote formula to Table 1.

2. About natural fruit drop.

   Because natural drop occurs throughout the season, and following thinning, the number of fruit on the tree immediately after thinning must be greater than you will need at harvest. The amount of this drop may vary between seasons and between orchards but has been determined to be 10% - 20% following thinning.

   Table 2 is similar to Table 1 but fruit numbers are adjusted to account for a 20% fruit drop that will occur following thinning. Table 2 reflects the number of fruit that should remain on the tree immediately after thinning.

3. When to thin.
   

   Shaker thinning is best done as the pit within the fruit begins to harden at the tip end, usually within the last week of April and the first two weeks of May, depending on the season. Thinning later than this removes larger fruit and reduces benefit.

4. If there is a need to thin.
   

   At thinning time, if the crop appears too heavy, shake as much fruit as possible from a typically cropped tree on to a tarp. Weigh the leaves, twigs and fruit that are removed. Then weigh several 1 lb subsamples of the material and count the number of sound (not yellow) fruit it contains. That is the number of sound fruit per pound of the material you shook off. Multiply that number times the total weight of material removed to determine number of fruit removed. Estimate and add to that the number of fruit remaining. This is the total fruit per tree. By using Table 1 to determine the crop level needed at harvest for any given dry fruit size, the need to thin or not can be determined.

   Do not use reference date to determine need to thin.

   Reference date should not be used to determine need for thinning. Trees that are obviously overcropped (see #4) will require thinning and should be thinned before reference date for best results. Reference date is useful for planning strategies to maximize fruit size following thinning for the remainder of the season, such as field sizing at harvest.

Mechanical (Shaker) Thinning

Mechanical thinning should not be based on a shaker operator's judgment. It must be done as objectively as possible. Use the following procedure, with the worksheet on page 3, to mechanically thin French prunes.

What you will need:

• This worksheet.
• Mechanized tree shaker configured with a "thinning" weight pattern.
• Two 10' x 20' plastic tarps.
• Several 5-gallon buckets.
• 2-3 small containers about the size of a 2-lb coffee can.
• An accurate scale capable of weighing from 1 lb up to 40 lbs.
• A stop watch.
• Recording tablet.
• Calculator.

1. When the seed begins to harden at the blossom end ("tip hardening"), select a tree that is fairly typical of the orchard and crop load, several trees away from the end of the row.
2. Shake off as much fruit as possible onto a tarp. Note what you have already done to determine need for thinning.
3. Place all fruit, leaves, twigs, removed into the 5-gallon buckets, weigh each bucket (remember to deduct weight of the empty bucket) and add and record the total weights removed. Note what you have already done to determine need for thinning.
4. Determine number of sound fruit per pound of shaken material by counting fruit from several 1-pound samples of the material weighed in step 3. Do not count yellow and shriveled fruit. Note what you have already done to determine need for thinning.
5. Multiply fruit per pound times the total pounds of material removed to determine total number of fruit removed from the tree. Note what you have already done to determine need for thinning.
6. Since it is not possible to remove all the fruit, estimate the number of fruit left on the tree and add that to the total number removed in Step 5 to determine the total fruit per tree. Note what you have already done to determine need for thinning.
7. Determine crop load the orchard should support (Table 1), based on records of previous crops. Use Table 2 to determine how many prunes are needed per tree immediately after thinning. Remember, Table 2 assumes a 20 percent fruit drop between thinning and harvest time.
8. When you know approximately how many prunes are on the tree (step 6) and how many are needed after thinning (step 7), you can determine the number of fruit to remove by simple subtraction.
9. Now use the stopwatch and shake another representative tree for a specified period of time, such as 4 or 5 seconds. By using the weighing procedure described (steps 3, 4 and 5) the number of fruit removed is calculated and compared with the amount that should be removed (Step 8). If the number of fruit removed is either more or less than desired, the length of shake should be adjusted and step 9 repeated.
10. Once the length of shake is established, shake each tree uniformly across the orchard.

Sample Worksheet

Table 1. Number of fruit needed per tree at harvest to give specified fruit sizes and tonnages^1/

^1/Based on a 20- x 20-foot planting109 trees per acre. For other tree spacings, use this formula: