Comments on Thinning and Crop Load Management

I was doing some rough calculations the other day trying to figure out how much it would cost to hand thin Ontario's apple acreage and came up with an estimate of $18,000,000.00. This was based on a labour crew of 10,000 workers employed for about one month.

In comparison, a good Spray thinning program over Ontario's commercial acreage probably cost growers around $ 750,000.00.

Thinning of the crop load on a well set apple tree is probably the single most valuable cultural practice carried on in the orchard. Not only does it produce the best potential crop value, it can influence everything from insect and disease control to prospects for next years return bloom and crop.

Ontario's apple crop for the current year could be the first king bloom crop we have seen in a while. The development of fruit buds this spring has been slower than some of the past springs where buds developed quickly and invariably got caught in frost conditions. This frost factor has historically made thinning decisions quite tough to judge. This year, those decisions should be easier to make.

The nice thing about a king bloom set is that the king will normally dominate the cluster of fruitlets and make it easier to separate it from the side bloom fruitlets when suitable thinning materials are applied. The king bloom fruitlet is considered to be the largest and best connected fruitlet on the spur.

I would have to rate the number of days and hours of conditions suitable for good bee activity in the orchard this year as being quite good. However, it's rare to have perfect dry warm conditions throughout bloom as we did in 1999.

Potential fruit size of well thinned crops and ultimately crop volume are directly related to pollen movement in the orchard and this only takes place when weather conditions favour good bee activity.

We have had in most cases an excellent return bloom in Ontario. Some of trees did come out in what I would call a snowball bloom. This type of bloom takes a lot of energy out of the trees and in theory makes them easier to thin.

A snowball bloom will create a sink for nitrogen because of the pollen production by such massive numbers of flowers.

There is always the possibility of over thinning or under thinning. Many growers would prefer to deal with a crop that's slightly overthinned than one that's under thinned. It's a huge commitment in dollars and time to hand thin large blocks of trees. Check out the thinning and growth regulator section of Publication 360. It does have a lot of information regarding different cultivars and how to evaluate the strength of the set in the orchard.

If you are using the growth regulator Apogee for the first time, you may find that it will increase the strength of the set to a point where you will have to adjust rates or materials. This is where good record keeping comes into play to know what was done in the past and what worked and what didn't work so well.

The one thing I would encourage growers to do in early June is to section fruitlets to get some idea of potential seed count. This can be important as it relates to management decisions through the growing season.

Figure 1. Potential for high seed count, embryos are developing rapidly.

Poorly seeded fruit is at a disadvantage in terms of potential growth and development. It also may not have the same ability to influence uptake of a variety of nutrients including calcium. Mutsu or Crispin with low seed count will demonstrate a high % of corking or bitter pit. Low seed counts may also influence the keeping ability of fruit when compared to apples with 7-10 seeds in the core.

One lesson we have learned is that we need to finish relatively few numbers of set fruitlets to actually make a crop of decent volume. Varieties like Gala and Fuji need only 5-10 % of the bloom finished in an average year to make a good crop. This means the elimination of all side bloom fruitlets plus every 2nd or 3rd king depending on tree vigour and the availability of water. These cultivars do best when the fruitlets are singulated.

A cultivar like Honeycrisp which is relatively new to Ontario may produce fruit that are too large if they are reduced to one fruitlet per spur. Based on the growers own experience with Honeycrisp it may be best to leave a high percentage of spurs with two fruitlets so the individual apples don't get too large. Honeycrisp is a cultivar that requires skilled crop load management to ensure fruit quality for the current season and return bloom the following year.