Determining Percent Fruit Set in Wild Blueberry Fields

B.2.0

The determination of percent fruit set is a tool which every wild blueberry grower would benefit from using. Percent fruit set refers to the percentage of blossoms which end up forming fruits. In New Brunswick, the average percent fruit set is thought to be in the vicinity of 25-30%. Each producer should aim for an average of a minimum of 30 to 40% fruit set on a constant basis. The yield potential of a wild blueberry field depends on a number of factors, such as: 1) the numbers of blossoms in the field; 2) weather conditions in both the sprout and crop years; 3) insect and disease incidence; 4) soil fertility; 5) soil moisture, as well as 6) the number of flowers which result in a fruit.

When the percent fruit set is known, a wild blueberry grower is in a much better position to know what production practices to improve. For example, a field with low plant cover could have a percent fruit set of 45%, and still have a poor crop. So, in this instance, a major limitation to yield would be the degree of plant cover, which the grower can work to improve. If, on the other hand, a wild blueberry grower has a field of full plant cover with lots of buds and blossoms, and yet the yield is low, he/she should look to improve the percentage of blossoms which set fruit, by controlling the blights, by introducing commercial bees, or by managing the habitat to improve pollination by native bees.

There are a few methods for determining the percent fruit set. Some are simple, but not very accurate; some are accurate, but relatively time consuming. If, during the pollination period, you shake the stems, and this causes the petals (not the full flower) to fall off a lot of the flowers, this is a sign that the degree of pollination is fairly high. If you cut open some berries when they are mature; and you find 10 or so plump seeds, this is also a good sign, since seeds set only when they are successfully pollinated. But, as you might have guessed, a true measure of the actual degree of pollination will take more work than this!

The following method is one that we have found to be a fairly reliable indicator of percent fruit set. It is adapted from some of the excellent work of E.A. Karmo, formerly with the Nova Scotia Department of Agriculture. This test is used to calculate the per cent fruit set.

The important feature of any method is to sample properly. As with any test (soil sampling, harvest tests, scouting for insects), you have to make sure that you do not influence the results. When you sample a field, you should sample ALL the field. Since conditions may vary from field edges to field centres, you will need to sample both types of areas unless you are trying to compare them. If you sample a big area, then you will need to take more samples. Fifty to one hundred samples seem to be adequate for a twenty acre field.

Walking your field during early to mid-bloom, you will take RANDOM samples by crossing the field back and forth in the shape of a "W", as you would for soil sampling. At each one of the sites at which you stop, you should sample in the following fashion:

1. You will be following the progress of one stem at each sample site. At each site, drop a pen or a similar small object and sample the stem which is located closest to where the object falls. This will keep your sampling random. You can identify the stem by tying a piece of coloured synthetic string loosely to the stem, below the flower stalk (known botanically as a raceme). Place a brightly colored flag right next to it, so that you can locate the general location of your sample stem. Identify the sample number with indelible ink on the flag, and record this number and the information described below in a log book.
   
   When sampling at early to mid-bloom, you would be counting and recording the TOTAL NUMBER OF FLOWER STRUCTURES which are found on the sample stem (this includes unopened blossoms, open blossoms, and blossoms which have lost their petals).
   
   
2. Prior to harvest just when most of the fruits have turned blue, return to the same sample and count the TOTAL NUMBER OF BERRIES which will produce a harvestable fruit. Do not include the "pinheads" (those fruits which have larger crowns than berries.
   
   
3. Use the information from (1) and (2) to determine the percent fruit set for the sample site, and then combine the information to learn the average percent fruit set for the field.

Here is an example:

• From section 1. The TOTAL NUMBER OF FLOWERS e.g., 70 unopened buds, open flowers, flowers without petals
• From Section 2. The TOTAL NUMBER OF BERRIES e.g., 35 harvestable berries
• PERCENT FRUIT SET = (35 / 70) x 100 = 50%

To calculate the percent fruit set for the field, add all the sampling sites and divide by the number of sites. If the differences between the sampling sites are very big, you might consider taking more samples next year OR you might try and figure out why. Some growers claim to get up to 70% fruit set; so if your figures are really low, you know that the top priority for managing that field is bees.

You can also use this method to calculate differences between your fields, or differences that you might have from using or not using hives.

ALTERNATIVE METHOD:

The following sampling method can be useful if you do not have access to little flags:

1. Place a stake at each of several locations in a field. Number the stake with a permanent marker.
   
   
2. Using the stake as the starting point, sample 10 to 15 stems along an imaginary line in a consistent direction (for example, north). Stems are chosen at random, as described in the first method. Tie a small brightly-coloured ribbon around the stem, and count and record the number of flowers on the stem, as described in the previous section. The stems should be located at least 50 cm (18 inches) from each other. The number of lines chosen will vary according to the size of the field. The number of stems sampled should be equivalent to that of the first method.
   
   
3. Prior to harvest, return and count the number of fruits that are present on each stem as described in the previous section.
   
   
4. The percentage fruit set is determined in the same way as the first method.

This method is somewhat less precise than the first one, but it requires less material and is a little less work.

Prepared by Gaétan Chiasson, P.Ag. Horticulturlal Specialist and John Argall, P.Ag., Blueberry Specialist, N.B. Department of Agriculture & Rural Development

Winter 1996