Apr 30, 2015

Spreadsheet tool allows growers to evaluate chemical thinning applications

Chemical thinning is a critical annual apple orchard practice, and the most stressful and difficult practice to implement. Over time, new approaches to cropload management have been developed, and they have been receiving a lot of attention during recent grower gatherings.

One intriguing new strategy involves the use of a spreadsheet tool called Predicting Fruitset. The model allows high-density apple growers to evaluate ongoing fruitset and/or help assess the effectiveness of the their chemical thinning applications. The model is based on the assumption that setting fruitlets grow faster than abscising fruitlets.

“It is quite accurate and provides a decision point for thinning before you can visually determine fruitset,” said Phil Schwallier, specialty crop educator with Michigan State University Extension.

“Growers that have used this method find it very helpful in their thinning process,” said Schwallier, who has made numerous presentations on the topic – among the most recent, a late-March appearance at the West Central Spring Horticultural Meeting in Hart, Michigan.

He said the model improves thinning, lessens the risk of over- or under-thinning, soothes grower stress and uses tree carbohydrate stress to the orchardist’s advantage.

Duane Greene at the University of Massachusetts developed the model, and Schwallier designed an Excel spreadsheet to aid growers.

Previous research reports have provided evidence that measuring fruit growth rate may be a viable method to predict if a fruit will abscise or persist through the June drop period. A series of experiments were carried out over several years to develop a procedure that could be used to predict the response to a chemical thinner application within seven to eight days after application and before thinners exhibit their final effect.

The procedure developed, and outlined in research reports in 2014, involved tagging 105 spurs on seven individual trees distributed appropriately in the orchard. The report indicated a minimum of two measurements must be made, one three to four days after application and again seven to eight days after application. This model requires that fruit measurement should not start before fruit grow to a diameter of 6 millimeters, and individual fruit within a spur should be numbered and identified.

The model is based on the assumption that if fruit growth rate of a particular fruit over the measurement period is less than 50 percent of the growth rate of the fastest-growing fruit on the tree during the same growth period, it will abscise, whereas if fruit growth rate exceeds 50 percent of the growth rate of the fastest- growing fruit, it will persist.

All data can be entered into an Excel spreadsheet and the output in the summary page gives the predicted fruit set expressed as a percentage of the total number of fruit present.

Strategy hones target

The strategy for crop load adjustment with chemical thinners has evolved over the years to a point where most orchardists plan and are prepared to make two or more thinner applications. The dilemma associated with this approach is to determine if additional thinner applications are necessary. Previously, a tool designed specifically to provide this information had not been developed.

The Predicting Fruitset Model can now be downloaded here. Click on the Horticulture link and scroll down to the Thinning section. It includes complete directions for use of the model.

A fruit is predicted to persist if the growth rate over the measurement period is at least 50 percent or greater of the fastest-growing fruit. A fruit is predicted to abscise if the growth rate of the fruit is slowed to 50 percent or less of the growth rate of the fastest-growing fruit.

Abscising fruitlets will slow down growth in three to four days and stop growth within four to six days following a thinning application. This slowing growth is temperature dependent – warmer temperatures will hasten the stopping and cold temperatures will delay the stopping of growth. The growth rate difference between setting fruitlets (fast growing) and dropping fruitlets (slow growing) is all that is needed to predict fruitset.

Two measurements usually will suffice to predict set. The first diameter measurement should be performed three days after the time of a thinning application or no earlier than the 6-millimeter stage. The second diameter measurement should be performed three to four days later. This will maximize the difference in growth rates. The slowing fruitlets will reveal themselves as abscising fruitlets. The model calculates the growth rates and predicts set.

“This model starts you planning your thinning program early,” Schwallier said. “It encourages a more precise approach to cropload management. It also gives you confidence to strategize, evaluate and achieve a successful thinning plan. This model will encourage appropriate actions based on the predictions. The predictions may require additional thinning applications to reduce cropload.”

Growth diameter rate disparity is the earliest indication of abscising and persisting fruitlets during the fruitset period. Fruitlets that are abscising either on their own or due to the chemical thinning stress are the ones growing at less than half the diameter of the fastest-growing fruitlets.

Abscising fruitlets will normally start slowing their growth rate four to five days after a thinning application. Abscising, slow growing fruitlets stop growing altogether at day seven. The abscising fruitlets appear normal until five days later (day nine or 10), when they start turning an off color (dark green or yellow).

“This is the first visible evidence that fruitlets are going to abscise,” Schwallier said. “Dropping fruitlets will have sepals that fold outward and setting fruitlets will have sepals that fold closed inward over the calyx.”

Seeds will sometimes turn an off color, but all these visible symptoms are inconsistent and can be misleading. The Predicting Fruitset Model will predict fruitset based on the diameter growth disparity of fruitlets.

“It is quite accurate and provides a decision point for thinning before you can visually determine fruitset,” Schwallier said. “Growers that have used this method find it very helpful in their thinning process.”

Step-by-step approach

Schwallier suggests 75 representative flower clusters should be marked by Sharpie pen dots or numbers and the diameter measured every three to four days. The basic steps for the model include evaluating the bloom, selecting representative clusters, marking and measuring fruitlets and evaluating the predictions.

“When measuring fruitlets, measure the diameter on the dots or numbers on the fruit,” he said. “Most fruits are asymmetrical and doing this removes quite a bit of variability.”

A Predicting Fruitset summary sheet is available at the website listed above. The sheet summarizes the data of the grower’s input measurements. The target number of fruit and the target percent fruitset are grower-defined in the setup sheet. The program macro sorts out the 20 fastest-growing fruitlets and calculates their diameter growth rate. All other fruitlets are compared to the 50 percent average of the 20-fastest fruitlet diameter growth rate.

“It counts the number of the fastest-growing fruitlets and the slowest (less than 50 percent of the fastest-growing fruitlets) growing fruitlets,” Schwallier said.

“If after the prediction is calculated, there is a significant difference between the predicted set and the target set, an additional chemical thinning application is necessary,” he said. “If only a small difference between the prediction and the target set, chemical thinning is complete.”

Here’s an example:

On May 1, 505 fruitlets were measured. The target fruit number for this particular block’s measured trees was determined to be 35 percent, or 7 percent of the 505 fruitlets. On May 4, only 483 fruitlets were measured and the model calculated that the 20 fastest-growing fruitlets grew 3.54 millimeters in those three days. Fruitlets growing faster than 1.77 millimeters (50 percent of 3.54) were 64 (12.7 percent) of the original 505 fruitlets.

On May 4, the model predicted 483 fruitlets were not setting. It predicted 12.7 percent fruit set. This 12.7 percent is close to the target (7 percent); therefore no more chemical thinning was warranted. On May 7, six days after the first measurement, the predicted set was 9.9 percent, a more accurate prediction.

Schwallier said the Predicting Fruitset Model is beginning to find favor among some growers. It can be most effective when used in conjunction with a carbohydrate model for thinning apples developed from research by Cornell’s Alan Lakso and improved by that school's Terence Robinson. That model predicts how sensitive trees might be to thinners, based on recent weather and the forecast.

– Gary Pullano