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By Philip Schwallier
and Amy Irish-Brown, Michigan State University Extension Educators
During apple thinning time, we need carbohydrate stress to take excess apples off the trees, but human stress is much less desirable.

Fortunately, we can use Precision Cropload Management (PCM) procedures to improve thinning, reduce the risk of over- or under-thinning, and improve our confidence in thinning, thereby reducing human stress and turning tree carbohydrate stress to our advantage.

Last year, several Michigan growers used a tool called Predicting Fruitset. This model allows growers to evaluate ongoing fruitset and the effectiveness of their chemical thinning applications. By applying some extra effort during the thinning process, they were able to gain some peace of mind.

The growers used a novel approach. They engaged their spouses and children, who teamed up to carry out that extra effort.

Start with a target

To use PCM, growers first set a target cropload based on anticipated or desired yield, calculating it down to desired number of fruits per tree. Once a target cropload is set, then growers dormant prune to a target budload of, say, 1.5 times the target cropload. If the target cropload is 100 fruit per tree, this means the target budload would be 1.5x or 150 flower clusters per tree.

After dormant pruning, the grower chooses five representative trees and counts the dormant flower buds on each tree. If that is greater than the target budload, they do additional dormant pruning to get closer to the target.

Next, at pink stage, 75 flower clusters are marked with flagging ribbon. These clusters will be used to mark and measure individual fruitlets. This is where the growers’ spouses and children entered the process. They took the measurements and recorded the data.

The individual fruitlets need to be measured two to four times during the fruitset window. Starting at about the 6-millimeter stage, the first measurement is taken to give a baseline diameter. Four to five days later, at about the 8-mm to 10-mm stage, a second measurement is taken, and this will determine each fruitlet’s growth and give the first prediction of percent fruitset.

Dr. Duane Greene at the University of Massachusetts developed this process after making critical discovery. Fruitlets that will set will grow larger. A fruit will not persist if it is growing at less than half the rate of those fruitlets.

After the second measurement, if the prediction is close to the target cropload, then no further measurement or chemical thinning is performed. If the predicted fruitset is too high, a chemical thinner will be applied and another measurement will be taken in four to five days, or at about the 12-mm to 15-mm stage. If the predicted fruit set is still too high, then follow up thinning can be done again.

This process allows for precision and will usually fine-tune the thinning process to a near perfect thinning job. However, there will always the need to hand thin small, defective, and damaged fruitlets. Hopefully this becomes a minor activity.

Growers using PCM

A few years ago, PCM was introduced to Michigan apple growers with the express purpose of increasing annual thinning success. At the Great Lakes Fruit, Vegetable, and Farm Market Expo in Grand Rapids, Michigan, last December, three apple growers reported their 2014 experiences with the “Michigan Precision Cropload Management Project.”

The growers are Kim Kropf, who farms with her husband, Chris, at Kropf Orchards near Lowell; Jill Blattner, who farms with her husband, Tony, at Twin Bee Orchards near Lowell; and Bridget Engelsma, who farms with her father, Jim, at Engelsma Apple Barn near Walker.

“I do the hiring for hand thinning,” said Jill Blattner. “So I knew that cutting that down would be wonderful.  We have never been able to thin our Gala block down to a manageable cropload—we were always too heavy—but this year, using this PCM approach, we, for the first time, successfully thinned the Galas. We had great Gala size.”

They dormant pruned their Gala block only to discover they were only down to a 2x budload level and had to dormant prune their block for a second time to get down closer to 1.5x budload.

Jill and her daughter Bethany worked as a team—Jill measuring fruitlets and Bethany recording the data. “We used a laptop, and that saved a lot of time. If the same person does all the measuring, it works better,” Jill said.

Kim Kropf and her daughter Emily teamed up, too, with Kim marking and measuring fruitlets while Emily recorded the data for later entry.

The Kropfs used PCM on three blocks of Gala, Jonagold, and Fuji. “It took time to set up and make the first measurements—about three hours per block to set up the clusters and two hours each time to measure,” Kim said. “But the confidence we developed in predicting the cropload was more than worth it.”

“With so much data to enter, I soon retrieved my laptop and in the orchard, directly entered measurements into the spreadsheet model,” she said.

Growers need to hire two people to measure fruitlets to make the job work smoothly. “Growers do not have time to stay with the protocol, they have too many things on their mind, too many jobs to do, and get too many interruptions to get the measurements all done on time,” she said.

The Engelsma PCM objectives are to obtain consistent high quality packouts and reduce the human stress of thinning by having more thinning options. They’ve been using PCM for three years and have fine-tuned some of the recommendations for their situation, Engelsma said.

“We reduced the number of flower clusters marked and measured in each block,” said Bridget Engelsma, who teamed up with her sister Nicole in the work. “This project has trained me to see fruitset even before I measure the fruitlets. It has given me confidence recognizing my fruitset and confidence in taking thinning action.”

The Engelsmas start thinning early at the full bloom stage with MaxCel (6-benzyladenine) plus the surfactant LI700 and then rethin at petal fall with Sevin (carbaryl) plus LI700. This gets them off to a good start reducing cropload but does not over-thin.

“We then make our first measurements of fruitlets and remeasure in three to four days,” Bridget said. “We hope that 30 to 50 percent of the fruitlets will drop and our thinning job will be done.”

The Engelsmas have met their objectives by recognizing an increase in both fruit quality and packout due to following the PCM protocols.

These growers plan to repeat the PCM program this year and increase the number of blocks measured.

Precision Cropload Management is of great assistance to growers and reduces their annual stress related to routine thinning of apples. The process gives growers confidence in their cropload; it gives them good, solid information early when they still have time to rethin.

It gives peace of mind when accomplishing such a difficult practice. PCM trains the grower eye to visually recognize the ongoing thinning response in their blocks. Perhaps best of all, growers successfully achieve good thinning early, which leads directly to good annual cropping every year. •