Backyard Orchard News
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If you look at the current degree days for California red scale in the Lindcove area, we are about 125 degree days behind the 30 year average. In that region, we are accumulating about 10 DD per day and so we are about 2 weeks later than 'normal'. Be sure to follow degree day units either using the web site or your own temperature units to determine the best time to spray (for most insecticides, 550 degree days after the 1st or 2nd male flights).
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.
Prune Reference Size Table
Reference Harvest Size (dry) (count/lb)
Size Green Orchard Sizing Potential
(count/lb) Average Good Excellent
50 32 31 30
55 36 34 32
60 39 37 35
65 42 40 38
70 46 43 41
75 49 45 43
80 53 48 46
85 56 51 48
90 60 54 51
95 63 57 54
100 67 60 56
105 70 63 59
110 74 66 61
115 77 68 63
120 81 71 66
125 84 74 68
130 88 77 70
135 92 79 73
140 95 82 75
Table 1. Prune reference date and average harvest dry size table. Use the reference size fresh count per pound and read across for orchards with average, good or excellent sizing potential.
Figure 1. Extracting endosperm at reference date.
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.
Mechanical thinning with the same machinery as is used for harvest can be used to remove the desired amount of fruit. Shake a tree and, and using the same methodology described above, calculate how much fruit was removed. Adjust the shaker and repeat the procedure until the desired amount of fruit is removed. Set the shaker and thin the block. The earlier thinning can be done, the greater effect it will have on fruit size at harvest.
According to work by Dr. Ted DeJong in the Plant Sciences Department at UC Davis, the first 30 days after bloom can tell a grower quite a bit about 1) the sizing potential for a stone fruit crop and 2) the time to harvest. See back ground info and predictive model at: http://fruitsandnuts.ucdavis.edu/Weather_Services/Harvest_Prediction__About_Growing_Degree_Hours/.
It has been just over a month since full bloom. So, what can we learn from the first 30 days of the 2011 prune crop?
Given a 50% bloom date of March 28, 7000-7600 growing degree hours (GDH) accumulated in much of the Sacramento Valley (Colusa, Nicolaus, Durham) in the 30 days that followed. So, the model (available free on line at: http://harvest.ucanr.org/) predicts harvest between Aug 29 and Sept 1 in 2011. Since the model is always long when applied to dried plums, I’m predicting a harvest date of Aug 18-21 for Sutter/Yuba region. That is when I predict prunes in an orchard with a good crop – say 3 dry ton per acre -- will reach 3-4 pounds pressure. DON”T take that to the bank, but I suspect it will be pretty close. What do you think? Send me a comment (see below), please.
Now for the bad new-- fruit sizing potential for an orchard with a "normal" cropload could be less this year compared to the same sized crop in the same orchard in the last few years. Why? The heat unit accumulation in the first 30 days after bloom also helps give growers an indication of the relative sizing potential of a given crop. A relatively high GDH 30 (accumulated GDH in 30 days after 50% bloom) means a smaller sizing potential. A smallish GDH 30 means a better sizing potential. In the past decade, GDH 30 has ranged from 5000-9000. In 2004 it was almost 9000. In 2006, a great year for sizing, it was around 5000. This year, at 7000-7600 GDH 30, we fall in the warmer side of the range. This suggests that the sizing potential of the crop will be less this year compared to the last few years. What does this mean to a grower? Count fruit/tree in a block as soon as reference date arrives – maybe as early as pit hardening. If you have to thin, thin hard and early.
A post on cropload evaluation post by Bill Krueger follows this one.