Gauging apple crop load

Apple growers use a variety of untested hand-thinning heuristics (rules of thumb) to adjust final crop load after insufficient chemical thinning. Depending on the cultivar and market, there are often financial incentives for growers to produce large fruit. However, excessive hand thinning can cause a reduction in yield that ­diminishes the value of the crop.

Pennsylvania fruit grower Dan Boyer of Ridgetop Orchards, returned from a recent fruit growers’ tour with an Equilifruit disk and asked Pennsylvania State University researchers to evaluate it as a hand-thinning gauge. The Equilifruit disk was developed by the National Institute for Agricultural Research (INRA) in France. The ­plastic disk was designed to perform two functions:

1) To provide crop load recommendations for hand thinning, based on the diameter of a limb; and

2) To serve as a pruning gauge in the Solaxe training system.

We tested the Equilifruit disk for its potential as a hand-thinning gauge on tall-spindle apple trees. Using the Equilifruit for crop load management is a simple process. The worker selects a notch in the disk that fits snugly around a selected limb, and counts the number of fruit on the limb. If the number of fruit is greater than the recommended fruit number (F-value) on the disk that corresponds to the selected notch, then the worker reduces the number of fruit appropriately.

When adjusting crop load based on the specifications of the disk, the worker should remove fruit selectively. Fruit should be removed in the following order: 1) damaged fruit, 2) small fruit, and 3) clustered fruit. Using the Equilifruit places less emphasis on the spacing of fruit within the canopy, because it uses limb size to estimate bearing surface. Therefore, any major alterations of the bearing surface (i.e., bench cuts) will result in an inaccurate estimate of the number of fruit that should remain. This thinning gauge is best suited for trees trained to  spindle-type training system with renewal style pruning. The Equilifruit disk was tested for two years as a ­thinning gauge at Penn State’s Fruit Research and ­Extension Center in Biglerville.

Use of the Equilifruit disk was evaluated on Golden Delicious, Buckeye Gala, and Honeycrisp. The Golden Delicious and Buckeye Gala trees used in the study had excessive crop loads, due to insufficient chemical thinning. The Honeycrisp had a crop load marginally higher than desired, with clusters of fruit occurring on some limbs and not on others. All treatments were applied after June drop.

The treatments were as follows:

• Control: No hand thinning treatment
• Equilifruit disk: Using the standard values present on the disk (F-value)
• Hand-thinning heuristic: Thinning fruit to 7-8 inches apart

In Golden Delicious, trees thinned with the Equilifruit had a 24 percent increase in fruit weight while maintaining yields comparable to the control trees. Hand thinning with the heuristic increased fruit size by 34 percent, but total yield was reduced by 30 percent.

Buckeye Gala trees had a high crop load before hand thinning (10 fruit per square centimeter of trunk cross-­sectional area). Both the Equilifruit and the heuristic were successful in reducing the crop load of Buckeye Gala (to around 6 and 5 fruit per square centimeter, respectively). Yields were not reduced by either hand thinning treatment. On trees in the Equilifruit treatment, 15 percent of the fruit were larger than 3 inches, compared with 12 percent in the heuristic treatment and 5 percent for the control.

Neither of the hand thinning treatments affected ­Honeycrisp yields. However, 44 percent of the fruit from trees subjected to the heuristic thinning were greater than 3.5 inches in diameter. These large fruit are more susceptible to poor storage life and are prone to postharvest disorders. Only 12 percent of fruit from trees treated with the Equilifruit were larger than 3.5 inches in diameter. The Equilifruit had 73 percent of its fruit in the size range of 3.0 to 3.5 inches, compared with 61 percent of the control and 55 percent of the heuristic.

In all cultivars, the Equilifruit did not reduce yield when compared to the control, and promoted either larger fruit or a higher proportion of fruit in desirable size categories.

The Equilifruit system provides some flexibility in the intensity of hand thinning, because each F-value has an accompanying delta value shown on the disk. Subtracting or adding the delta value from the F-value can intensify or reduce the thinning treatment. This provides an opportunity to alter hand-thinning practices based on cultivar, desired crop load, and tree age, etc. We have tested the use of the delta value to modify the intensity of thinning, and it is capable of consistently adjusting crop load to various levels. In our trials on mature trees, the use of the F-value provided the best results.

We suggest that the Equilifruit is best used as a training tool for workers in the orchard. After strictly following the specifications of the disk for a few hours, the worker should be comfortable in visually relating limb size to the number of fruit that should remain. After this level of training is achieved, the workers can increase efficiency by assessing the need for fruit removal visually, using the disk only occasionally to recalibrate. The gauge can also be used by supervisors to have a consistent method of evaluating the crew’s performance.