This is the VOA Special English Agriculture Report.

Alternate bearing is a widespread problem for growers of citrus and other fruit trees. It can affect a large area or just individual trees or even part of a tree. No, it is not a disease. Alternate bearing is when a tree produces a heavy crop one year, called an "on-crop," followed by an "off-crop" the next year.

On-crop trees produce a large number of small fruit with little value. Off-crop trees produce no fruit or a small number of large fruit that often have thick, unappealing skins.

Mandarin trees heavy with fruit in an on-crop year

Two researchers recently did a study to understand how this happens. Johannes Verreynne is now at Stellenbosch University in South Africa. Carol Lovatt is at the University of California, Riverside. They studied "Pixie" mandarin trees in the Ojai Valley of California. Mandarin oranges are also known as tangerines.

The study showed that fruit on the tree reduces the next bloom by stopping buds from appearing. This limits the number and length of summer and fall shoots. As a result, there is a reduction in the number of nodes, or joints, that produce groups of flowers along stems. Fruit on the tree during spring bloom stops the growth of flowering shoots.

During an on-crop year, growers often treat the fruit so it can stay on the tree longer. The purpose is to extend the harvest season. Yet Carol Lovatt says holding fruit on the tree makes alternate bearing worse. The researchers advise growers to investigate the effects of thinning or pruning to reduce the number of fruit in early summer of the on-crop year.

The findings appeared in the Journal of the American Society for Horticultural Science.

Other kinds of trees that can experience alternate bearing include nut trees. Scientists recently studied the effects of mechanical thinning on two kinds of pecan trees in the southeastern United States. The results from the University of Georgia appeared in HortTechnology, also published by the society.

The study compared trees that had been thinned by machine with those that had not been thinned. The findings suggest that thinning during the on-crop year can increase the value of off-year pecan crops.

And that's the VOA Special English Agriculture Report, written by Jerilyn Watson. I'm Bob Doughty.