The first month after bloom is a critical time for ensuring that apples reach their full potential size at harvest, according to Dr. Alan Lakso of Cornell University, New York.
During the first ten days after bloom, the apples grow by cell division and the cells don’t grow very much in size, Lakso explained during the International Fruit Tree Association’s annual conference. During that period, the number of cells in the fruit grows exponentially.
Early fruit thinning is important, as the number of cells in the fruit is reduced by waiting too long to thin. This is because of competition with other fruit. “For the rest of the season, you pay for it,” Lakso warned.
After about ten days, the cells start to expand. As long as the apples are not limited by a heavy crop load or short season, they grow at a consistent rate throughout the season, he explained.
All cells are growing at the same rate, so the maximum size potential depends on how many cells the fruit has. Small differences in cell numbers during the early season are manifest through the rest of the season.
Apples that maintain their growth rate tend to stay on the tree, while those with a slow growth rate fall off. If a 60 percent growth rate of fruit can be maintained, very few apples drop off, but most fruit with a 20 percent growth rate drop.
Low light levels during the early thinning period can affect fruit growth, Lakso’s research shows. In experiments with shaded trees, a lack of light a couple of weeks after bloom did not affect shoot growth, but it shut down fruit growth.
“At that early period, shoots have a very strong competitive advantage over fruit,” he noted.
Differing yields in various blocks or orchards are related to light interception. The light is the energy that drives the whole system, Lakso pointed out. While there are no guarantees that an orchard with high light interception will produce high yields, if light levels are low, it is a guarantee of low yields.
In plantings with traditional vigorous trees, 30 to 80 percent of the light can be intercepted. If the trees are planted close enough and are large enough, light interception can be high, and there’s the potential for good yields, though it might take a long time to achieve them.
Trees on semidwarfing rootstocks and trained to a central leader system might intercept between 30 and 60 percent of the light. In a slender spindle system, light interception might be between 50 and 55 percent, Lakso reported. Trees on the taller French axe system at the same spacing take more advantage of the light, intercepting between 55 and 65 percent. On V- or Y-trellis systems, 60 to 70 percent of the light can be intercepted, though there can be management concerns.
The goal of good orchard design is to arrange the trees to capture as much light as possible for good yield potential while still being manageable, he said.
As young orchards grow, there is a clear relationship between light interception and yields, but in a mature orchard, there can be big variations, and that has to do with light penetration into the canopy.
During the two to three weeks after bloom, leaves on shoots support the actively growing shoots until the shoots can support themselves. After that, some of the basal leaves can support fruit growth. If there is shade, a shoot can have up to 20 leaves before it starts to support the fruit.
Spur leaves (the primary leaves that come out before bloom) and the bourse shoots support fruit growth. Short shoots can support fruit earlier in the season, but longer shoots don’t support fruit until much later.
For this reason, growers should keep the spur canopy open during the first part of the growing season to make sure that the spurs are well exposed to light, Lakso suggested. They should prune lightly, however, and avoid heavy cuts because heading cuts remove a lot of the short and spur shoots and stimulate growth of shoots that will support themselves instead of fruit.
“The important thing to do is get light to the spurs during the three to four weeks after bloom,” Lakso reiterated. “The goal of good tree design is to capture as much light as possible by the spur leaves, while still being manageable. You don’t want to take all the vigor out of the orchard.”
Speaking about carbohydrate supply and demand, Lakso said 15 to 20 percent of total photosynthesis is burned up in respiration. Once cell division slows down, if the apples are growing at a rate of two grams per day, the demand for carbon is fairly constant.
Research has shown that if the crop is heavy, demand for carbon will exceed supply by two weeks after bloom, and there is never a period in the middle of the season where the crop would not be limited by the carbon supply. With a normal crop, the tree can generally support most of the crop through the season, though the supply might be limited towards the end of the season, especially in short-season locations.
In the middle of the season (from about six weeks after bloom), if the demand is satisfied, the roots grow. This period of root growth seems to correlate with the time when the crop is taken care of and the canopy gets bigger, Lakso said.
In order not to limit crop growth, growers should thin early to make sure the crop is at the right level to grow at the proper rate, he stressed.
Later in the season, if the function of the tree is reduced by European red mites, the carbohydrate supply can be reduced below the demand of the crop because the pest damage reduces the photosynthesis of the tree. If the crop is light, mites won’t have much impact because the demand for carbon will be low, but where the crop load is heavy and the demand for carbon is higher, mites will have a greater impact. “If you have heavily cropped trees, you can’t let them get hit hard by additional stress,” Lakso warned.
Not only could such stresses reduce fruit size, but they could also affect the following year’s crop because the number of return flower clusters is related to the carbohydrate supply-and-demand balance the previous year.
“If you have a problem one year, you’re going to pay for it the next, too,” he said.
Lakso urged caution when summer pruning to improve fruit color.
“I’m not saying summer pruning is bad, but as you remove more and more of that leaf area, it does reduce tree function. It’s best not to do excessive summer pruning if you don’t have to. Get into a cropping situation where you don’t have excessive shoot growth, and keep the trees open throughout the season so you don’t have to do excessive pruning.
“If you summer prune trees that have been hit by European red mite, you’re not going to get much for the rest of the season out of those trees,” he added.
Lakso concluded that trees should be maintained in a healthy, active state so they can size the crop and so that flower buds can develop properly for good, sustained cropping.