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Auhors: Jon Clements and Joseph Costante
Journal: "The Apple Press" published by the University of Vermont vol 18, no 1, pp. 11-13
In 1991, we began a research project designed to study the effect of dormant pruning on Mclntosh canopy light distribution, fruit quality characteristics, and fruit pack-out. Although all our data has yet to be analyzed, there are some interesting, yet somewhat predictable, results that indeed quantify the effect of pruning treatment on canopy sunlight penetration and fruit pack-out.
The study was conducted at Sentinel Pine Orchard, than :s to the cooperation of Whitney Blodgett, in a block of vigorous, fifteen year-old 'Rogers' Mclntosh on interstem rootstock. These trees had been pruned lightly over the years, and bad settled into a fruiting mode; in fact, they could have been characterised as somewhat "spur-bound". They are rather large trees, averaging 8 to 10 feet tall.
The dormant pruning treatments initiated in 1991, and continued during 1992 and 1993 included:
1) trees were left largely unpruned, except for broken and diseased branches we'll call these the untreated control's, 'UTC';
2) trees were pruned moderately, removing problem branches. (i.e. Over-lapping, vigorous uprights, low hanging etc.) using thinning cuts. Heading-back cuts were employed to stiffen and shorten branches where appropriate. For example, this was used in the upper portion of the tree so that the top did not get out of control nor too big. We said, "this is how the 'typical' grower would prune., hence we'll call it grower-pruned. 'Gr Pr';
3) these trees were pruned more heavily than 'Gr Pr', utilizing major thinning and heading cuts to create a tree with a distinct whorl of lower scaffold branches, separated from an upper tier of branches by a distinct gap or ~window., wherein more sunlight could be expected to penetrate and be distributed in the lower canopy. Hence, these trees were heavily structured, resembling our '5+4' tree training system. Hereinafter, we'll refer to this pruning treatment es 'Struc S+4'.
Ten trees were randomly selected for each pruning treatment. The experimental hypothesis to be tested was, "would the 'Struc 5+4' pruning treatment let more light into the middle and lower portions of the tree, resulting in better overall fruit quality and improved pack-out?" In general, our preliminary experimental results tend to confirm this hypothesis.
First, let's look at light distribution in the tree canopy. Typically, when we took light measurements in the lower portion of the tree canopies, there was almost 1/3 more sunlight shining on fruit spurs and fruit at this height (approximately four feet) in the 'Struc 5+4' compared to the 'UTC' and 'Gr Pr' treatments. For example, Graph 1. depicts the results of light measurements made in August 1993.
Light measurements were made at mid-day, using a light meter that measures the average amount of photosynthetically active radiation (90 PAR) that falls along a three foot linear sector in the tree canopy at two heights: approximately four and six feet.
Prior research has shown that spur leaves need at least 30% (40 to 5090 is much better) of available sunlight to produce quality fruit (see previous article, "Thinking about Light."). Clearly, the "window created by the 'Struc 5+4' pruning treatment lets more light into the lower canopy wherein a significant portion of the fruit resides. Also note that considerably more light fell on fruit spurs at six feet compared to the amount of light measured at four feet.
What about pack-out? Again, thanks to the patience of Whitney Blodgett, fruit from each treatment were packed on his line in 1991 and 1993 (unfortunately, the 1992 harvest was lost to fire that consumed Mr. Blodgett's storage and packing warehouse). Results of the 1993 packout are presented below.
Close scrutiny of the results reveals a slight shift to luger packed sizes and fewer fancy bags in the 'Gr Pr' and 'Struc 5+4' treatments when compared to the 'UTC'. Note also, that the total S value of the 'Gr Pr' pack-out was the highest. The total $ value of the other two pack-outs ('UTC' and 'Struc 5+4') is comparable, however, there were fewer bushels harvested from the 'Struc 5+4' treatment; pruning has a negative impact on yield. But look at the average $ per bushel: 'UTC' was the lowest at $9.01, while the average price of a 'Struc 5+4' packed box was over one dollar more, at $10.21! Remember that yield, however, was lower from the 'Struc 5+4' treatment. Therefore, the total $ value of packed fruit from both 'UTC' and 'Struc 5+4' treatments were similar, and the 'Gr Pr' treatment had the highest total $ value of all three.
What does this mean? Well, if total S value is extrapolated to a per acre basis, then the 'Gr Pr' treatment is clearly the winner. Don't forget, however, that there will be concurrent expense (mostly picking) involved in growing the extra fruit, and conversely, it will cost more to prune the 'Struc 5+4' treatment (not to mention there are other compelling reasons for pruning). So the economic picture is harder to decipher, and was really not an objective of this experiment. Our feeling, however, is that in order to stay competitive, growers are going to have to focus on producing more high quality, cell-packed fruit.
We still have a lot more data to look at including specific fruit quality characteristics, such as red skin color, fruit size and soluble solids, and fruit spur vigor. Intuitively, one could probably have predicted the outcome of this experiment, i.e. heavier pruning lets more light into the interior of the tree, resulting in better fruit quality and pack-out. We are, however, quantifying this prediction-resulting in information that will help us better understand and balance the relationship between dormant pruning, fruit quality and pack-out.
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