A large, luscious, juicy red apple is truly a tasty treat. However, it takes a combination of many factors to achieve such a beautiful fruit. To maintain regular cropping, there is a delicate balance between the number of leaves and the number of fruits on a tree. Most apple cultivars require 80 to 90 square inches of healthy green leaves to support one, 3 inch-diameter apple. This is equivalent to 10 mature leaves per fruit for a tree grafted onto a M.9 dwarfing rootstock. Dwarfing rootstocks are capable of sending about 70 percent of the carbon the tree fixes to the crop. In contrast, a larger tree on a seedling rootstock puts more than half the carbon into growing wood so a greater number of leaves are required to support a fruit.
There are several ways to achieve the optimum leaf to fruit ratio. Annual applications of nitrogen fertilizer will help maintain a balance of healthy leaves and fruit. Nitrogen fertilizer should be applied in late March and again in late April. Splitting the total amount of fertilizer needed into at least two applications will provide sustained growth. Also, two smaller applications help prevent leaching of the fertilizer if heavy rainfall occurs after a surface application of soluble fertilizer such as ammonium nitrate.
A general rule of thumb used for determining the nitrogen requirement is one-tenth pound of actual nitrogen per year of tree age. This means that a five- year-old tree would need 0.5 lb actual nitrogen. To calculate the correct amount of actual nitrogen, simply divide the amount of actual nitrogen by the percent nitrogen listed on the bag of fertilizer. Thus, a 5 year-old tree would require 4.2 lbs of 12-12-12 fertilizer or 1.5 lbs of ammonium nitrate. To determine if the foliage is actually receiving the proper amount of nitrogen, leaves are collected in late-July through mid-August. The sample can be submitted through your local Extension office to the University of Missouri for foliar analysis. Healthy leaves have a nitrogen content of 2.0 to 2.2 percent. If the nitrogen content falls below this range, increase the amount of fertilizer by 10 percent the following growing season.
The next important step in achieving the correct balance of foliage to fruit is thinning. To produce a 3 inch-diameter apple, other fruitlets should be removed from a cluster when they are no larger than a dime. When there is more than one fruit per cluster, other fruitlets compete for nutrients, resulting in small apples at harvest. Another reason for thinning is to reduce the incidence of pests. When two or more apples are in contact with one another, the humidity is higher within the cluster and drying time after heavy dew or rainfall is prolonged, providing the moisture needed for disease infection. Also, clusters of fruit also offer a protected environment for insects, which can lead to increased populations. It is also harder to achieve adequate spray coverage on the interior surfaces of fruit in a cluster with pesticides than when the fruit is thinned to only one apple per cluster.
Once the proper number of apples has been adjusted to the amount of foliage on the trees, it’s important to maintain them in a healthy condition. Some of the common apple diseases in Missouri include fire blight, powdery mildew, scab, and apple cedar rust. Insect pests include scale, aphids, codling moth, leaf miner, etc. Heavy infections or infestations of these pests can cause defoliation and upsets the leaf to fruit ratio. Protection from these pests is important even in non-crop years such as 2007. Adequate irrigation during periods of drought is also necessary to ensure healthy foliage and to prevent premature defoliation.
Annual pruning will also help maintain the proper balance of foliage and fruit. Shaded leaves do not receive adequate light for photosynthesis and fail to produce the carbon needed for all parts of the tree. Inadequate pruning also leads to poor fruit initiation in buds for next year’s crop and can result in biennial bearing. When pruning, use thinning cuts rather than heading cuts. Thinning cuts, in which entire branches are removed at the main trunk, ensure good light penetration within the tree. Heading cuts made at the ends of branches only produce more vegetative growth and reduce or delay fruiting.
Regular cropping is truly a balancing act. Too much foliage results in little or no fruit. Alternatively, too much fruit results in few leaves on the tree and few apples in the subsequent growing season. Fortunately, we have water, fertilizers, pesticides, and cultural practices to help maintain the proper ratio of leaves to fruit. Unfortunately, we cannot control spring frosts or freezes that upset this delicate balance, but with proper management, the foliage can be protected and the leaf to fruit ratio can be readjusted in the subsequent growing season so trees can bear those large, juicy fruits.
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REVISED: November 19, 2012