Growing target fruit

Auvil Fruit Company at Vantage, Washington, has developed an optimized orchard system that has a high-volume canopy with the potential to absorb a lot of light and generate high production of target fruit, says WSU horticulturist Dr. Don Elfving. The plan

Light is the key factor in producing target fruit, says Dr. Don Elfving, horticulturist with ­Washington State University in Wenatchee.

"When everything else has been fixed, light management is going to be the determining ­factor—and perhaps the limiting factor—as to whether you can produce a sufficient crop of target fruit," he told growers at a recent Fruit School on Competitive Orchard Systems.

Target fruit is fruit that returns a profit to the grower after all growing, harvesting, and packing costs are paid.

Elfving said fruit production—whether of apples, pears, or cherries—is a function of the amount of light intercepted by the tree canopy. There’s a clear relationship between the amount of light intercepted by the orchard and the amount of fruit the orchard can produce. However, as light interception increases beyond about 80 percent, the orchard tends to be crowded and fruit quality declines. There’s a trade-off between the volume of production and quality of production. Fruit color, size, and possibly internal quality can be affected.

"You can maximize yield at the expense of fruit quality," he said.

Other factors, such as tree structure and crop management, determine whether or not the light that’s intercepted can be converted into useful yield or not, he explained. "If you don’t deploy the canopy to absorb light, you’re not going to get yield."

For good flowering, the wood needs to be exposed to 30 percent or more full sun on a diurnal basis, he said. On large trees, there is commonly less bloom or no bloom on the shaded parts of the tree. It takes a certain amount of energy to drive the process of flower bud formation and initiation, and the light must hit that same part of the tree, not ­elsewhere on the tree, for buds to develop.

Photosynthates

To grow quality fruit, the fruit and its nearby leaves should be exposed to about 50 percent or more full sun so that there is enough light energy to produce photosynthates for the fruit to grow to its maximum size.

"The bigger you want them to grow, the more photosynthates you need to have," ­Elfving explained. "That’s why you see differences in the size of the fruit from different parts of the tree."

The larger fruit tends to be on the most exposed parts of the tree. Elfving noted that very little light passes through a leaf to the leaves below. He estimates that a leaf absorbs at least 90 percent of the light that hits it. Only 10 percent or less shines through to the leaves below, and the quality of that light is changed so that it is not useful for photosynthesis. When leaves are stacked on top of each other, there can be a lot of dark areas in the tree where fruit has poor color and size.

To expose all the bearing surface on a tree, the grower needs to be conscious of what shade the tree casts upon itself and try to overcome that with pruning and training.

"When you’re thinking about your pruning and training program, remember the physiology of the tree, and that light can only travel for short distances in trees before it essentially disappears," Elfving suggested.

Managing light

• First of all, the orchard must have the correct density of trees to deploy enough canopy to absorb light for good production.

• The rootstock must provide vigor control and be precocious and productive. It must be compatible with the spacing and should help create the optimal tree structure.

• Pruning and training are used to create a canopy structure with good light exposure of all productive wood. When light exposure is good, the fruit will be good. When light exposure is poor, the fruit will be poor.

• Uniform vigor from the bottom of the tree to the top is a key to good light distribution. Most trees have too much vigor in the top and not in the bottom. The lower part should have as good light exposure and absorption as the top if that part of the tree is to be productive.

• The tree should have moderate vigor, with enough leaf surface but not too much structural wood, Elfving said. Structural wood is a parasite that uses photosynthates that could otherwise have gone to feed the fruit. One of the reasons for using dwarfing trees is that they have relatively little structural wood in relation to the bearing surface. "You have to have a certain amount of structure, but you don’t want more than you have to have, and you want it to pay its way," he said.