If apple growers could make accurate fruit load assessments and use that information to determine whether or not a follow-up thinner is necessary, would they? Dr. Duane Greene, professor and researcher in the Department of Plant, Soil and Insect Sciences at the University of Massachusetts, believes they would.


“It would be nice to have a system to figure out just how well the thinner worked, and have that answer in time – within the thinning window opportunity,” said Greene, describing the fruitlet model he and other researchers have been working on for several years. “Many of us put on thinners, then when the fruit reaches 7 to 9 millimeters, we try to assess the response to the thinners. However, we never had a good way to do that. The goal is to predict thinner response within six or seven days of application so that there’s time to apply another thinner if necessary.”

The fruitlet model is based on two observations. The first is that fruit that is going to persist starts to grow rapidly after fertilization, and growth will continue somewhat regularly throughout the season. The second is that fruit destined to fall off starts to slow down growth long before abscission. Greene noted that even when fruit growth slowed, it sometimes took several weeks before abscission. “When you look at a fruit, you can tell something about the size,” he said, “but you can’t tell anything about the growth rate, and growth rate is the important thing.”

The fruitlet model is based on measured growth rate, and predictions about whether fruit will persist or abscise are based on those measurements. “Our hypothesis is that if fruit is growing at 50 percent or less of the fastest-growing fruit, it will abscise,” said Greene. “Conversely, if a fruit is growing 50 percent or more than the fastest-growing fruit, it will persist to harvest.”

Greene says that the fastest-growing fruits are those that continue to grow. “They’ll persist 99 percent of the time,” he said. “So we measure them and note that they’re growing rapidly; we know with almost complete certainty that they are the ones that will persist.”

An evenly distributed crop load with premium-size fruit is the result of timely thinning.

Detailed instructions for predicting response to chemical thinner application and a spreadsheet are available at http://www.umass.edu/fruitadvisor/2008/predict thinprocedure.pdf. First, select and mark seven trees or two limbs on seven trees that represent the block in which the assessment will be made. At the pink stage of flowering, count all blossom clusters on the tree or limbs and record that number. Average the bloom and determine the final desired crop load. “Assume we have 200 blossom clusters on a tree, and we want to have 100 fruit on that tree,” said Greene. “That means one fruit for every two spurs. It’s reasonable to expect between 350 and 450 developing fruits. If we want 50 percent set, we want, at the end, 11.9 percent of the flowers to set.”

When fruit is at least 6 to 7 millimeters, select and tag 15 spurs on seven trees. With an indelible marker, number each fruit on the spur, then use a caliper (a digital caliper is ideal) to measure the fruit at the largest diameter. “The goal is to predict the set on the whole tree,” said Greene. “Individual fruits on the spurs are marked and measured before thinner is applied, then fruits are measured at two to three-day intervals.” All data should be recorded on the spreadsheet.

About four days after thinner application, growth reduction should be visible. However, in 13 experiments done over the years, Greene found that temperature following thinner application is a key factor in making accurate predictions. “I looked at growing degree days accumulated, and in general, it turned out to be 120 growing degree days (base 50 degrees Fahrenheit). An accurate prediction can be made within seven to eight days after application. In cool weather, wait until trees have been exposed to 130 to 140 base heating degree units.”

Greene noted that variations in weather from year to year can affect results. “There are years when predictions can’t be made within seven or eight days,” he said. “In these years, temperatures following thinner application were generally cool, which slowed the response of the thinner.”

Continue to measure fruitlets at two to three-day intervals, and enter collected data on the spreadsheet. The spreadsheet is designed for easy use – no calculations are necessary. “All you have to do is cut and paste,” said Greene. “The process should take only a few minutes once the information is in the spreadsheet.”

The author is a freelance writer who farms and raises Great Pyrenees in south central Pennsylvania.