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Regulating crop load to achieve consistent, maximum yields and high fruit quality is a daunting task for apple growers.

"The crop-load challenge to fruit growers is to take the right amount of crop off at the right time so you can achieve the best possible yields and best quality fruit," said Dr. Jens-Norbert Wünsche, professor at the University of Hohenheim, Germany. "Now, this is challenging because we’re dealing with a lot of interactions with the plant, plant processes, and environmental constraints, but also with crop management practices."

Excessive fruit set is a common trait in many apple cultivars selected for fertility, Wünsche said during an educational session at the Great Lakes Fruit, Vegetable and Farm Market Expo in Grand Rapids, Michigan. Oversetting can result in inadequate fruit size and quality at harvest and encourage biennial cropping patterns, resulting in a roller coaster of yields.

Irregular, biennial bearing is a common occurrence in many fruit production regions in the world, he said. "Interestingly, even with very modern orchard practices, we don’t seem to overcome biennial bearing. It is much reduced, much lower and the yields more uniform, but we still see it."

Wünsche told growers to keep in mind two concepts as they use orchard management techniques to overcome biennial bearing. The first is that the presence of fruit on a spur inhibits the flower bud formation on that spur. Secondly, the earlier that spurs can be defruited, the higher the likelihood that they will return a flower in the following year.

He cited research conducted on apple varieties sensitive to biennial bearing—Boskoop, Pacific Rose, and Braeburn—that all showed similar results. "The more you thin, the greater the yield, or the greater the yield in the following year," Wünsche explained. "And, the earlier you do it, the better the spurs seem to come into flowering the following year. And the greater the fruit weight."

While the modes of action of flower thinning and fruit thinning agents differ, ultimately, the agents form a separation layer for flower or fruitlet abscission.

Weather

Environmental factors greatly affect the degree of thinning by chemicals and also make it difficult to separate variability in thinning results from weather factors. Weather before, during, and after the spray application of thinning agents can influence the success of the chemical application.

Research on crop load has revealed a powerful interplay between fruit development and shoot growth and photosynthesis, he stated.

Crop load can affect many other plant processes, from leaf photosynthesis and canopy gas exchange, to water consumption, to tree growth and fruit quality. It can also affect physiological disorders. Generalized effects of cropping vary and are dependent on the time and severity of flower/fruitlet removal, environmental conditions, and source-sink interactions.

"Light-cropping trees, regardless of fruit size, tend to have lower calcium and higher potassium concentrations and a greater susceptibility to bitter pit," he said.

"Fruit from light-cropping trees not only have more bitter pit but may also show a higher degree of radial watercore, moldy core, core rots, internal browning, ­vascular browning, and cracking."