A greater incidence of fruit twinning (technically termed fruit doubling) was encountered the last several seasons in many fruits including apple, pear, peach, sweet and tart cherry, and plum. Even quadruple fruit were observed in peach and sweet cherry.

This season (2003) fruit doubling has been seen in peach, nectarine, sweet cherry, tart cherry, and apple. The question from growers invariably is, " Why?"

The answer apparently is, "Stress - stress on the tree during the time the flower buds are being developed for the next season." This stress can be in the form of water stress due to drought (reported for peaches by a number of researchers since 1980) or in the form of heat stress (reported for cherries by two researchers in the 1930's). Fruit doubles result from flowers with two fruit initials (or carpels). If stress conditions are encountered during the time the flower buds are forming for the next season, the cells that will ultimately form the fruit initial likely are partially separated from each other and then each cluster of cells proceeds to develop as though it were the only carpel even though they are still attached at some point toward the upper end of the carpels. This results in two (or more) carpels within a single flower.

The next question is what is the time period during which the developing fruit are susceptible to stress influence and injury. In California, peaches appear to be susceptible to fruit doubling from stress from early August to early September. This may also be the time here in Colorado. If one looks back at the 2000 season, the period between July 19 and August 15 saw high temperatures of 95 or higher for each day. Pan evaporation during this period ranged from 0.36 to 0.48 inches (which is high for the period). That was also the season that peaches ripened faster than growers could harvest them. The following season, 2001, a number of strange, doubled fruit were found, including apples and pears. In 2001, the comparable highest stress period was from June 29 to July 7 during which high temperatures were 95 F or above and pan evaporation rates ranged between 0.36 and 0.59 inches. No fruit doubling was found for apple or pear in 2002, but substantial doubling was found on sweet cherry, European plums, peaches, nectarines, and some tart cherries. Based on the above and on our extremely hot, dry weather during July, growers should expect a high incidence of fruit doubling and deep suture in stone fruits (other than apricot) in 2003.

How does one avoid or minimize fruit doubling? The answer, according to researchers in California, is to be sure trees are given adequate water during the period of fruit formation. For Colorado growers, this would appear to be the period during July and August, based on the prior years observations.