Apple farmers know they need to thin their trees to ensure they produce larger, better-quality fruit. Hand thinning is labor intensive and expensive, so apple growers turn to chemical thinners to simplify the job. 

Jimmy Larson, a postdoctoral researcher in horticulture at the Mountain Horticultural Crops Research and Extension Center in Mills River, N.C., explains that apple growers turn to plant growth regulators that create stress in the tree and due to that stress, the tree will shed fruit in response.  

“That response will be pretty variable, depending on weather conditions, so if you have hot cloudy days in the springtime, those are pretty favorable for fruit abscission. If you spray your chemical thinner you’re pretty likely to get high levels of fruit abscission whereas sunny days that are cooler, you have more photosynthesis going on and more resources. So those trees aren’t going to be as stressed and when you spray that chemical thinner, then you most likely will have more fruit hang on to the tree,” Larson explained at a field day at the station Aug. 10. 

The right timing 

Larson said apple growers often make multiple applications of chemical thinners in the spring in order to produce a crop that will result in better fruit size and quality. If they don’t do that, they resort to the more demanding and costly hand thinning. 

He said there is a three-week period in the spring when they can make thinner applications.  

“The difficulty with making those multiple applications is that it takes two weeks for fruit to actually start falling from that thinner application, so we need tools to give to growers so they can determine how many fruit are going to abscise following a chemical thinner application and then know whether or not they need to make a follow up application while trees are still susceptible to those thinners, or if they were to make another application if that would result in over thinning,” he said.  

Apple growers need to find a way to better time and manage their fruit thinning, so Larson is working on a model that can predict fruit abscission earlier. In his research work, Larson is using a portable visible and near infrared spectrophotometer that shoots a beam of light from a lens to whatever you are measuring and reflects back to the instrument, allowing you to measure the fruit on the tree. 

The spectrophotometer measures light from about 300 nanometers, which is the beginning of the visible portion of the spectrum, all the way up to 1,200 nanometers, which is the near infrared portion of the spectrum. 

“We found there were two big differences in fruit that ends up persisting and those that end up abscising because when you measure them, the first one is right around 650 nanometers, which red light is. And we found that persisting fruitlets absorb more red-light than those that are abscising, right around 1,000 nanometers that’s responsible for water absorption,” Larson said. “We see persisting fruitlets absorbing more of that light because they have higher water content than abscising fruitlets. We used that spectra to build models to be able to predict fruit abscission.” 

Expanded development 

In 2020 and 2021, as part of his Ph.D. work, Larson worked just with the apple variety Honeycrisp and found that the model shows 90% accuracy in predicting abscission as early as three days following a thinner application. Larson said this would give apple growers greater flexibility in being able to make their thinner applications. 

Beginning this year, as part of his postdoctoral work, Larson has expanded the development of the model to multiple cultivars, thinner chemistries, and application timings. This year he looked at the varieties Red Delicious, Gala, Granny Smith, and again, Honeycrisp. 

“We get a wide variety of cultivars that are both easy and difficult to thin and then we tested it in multiple timings in the thinning period. We are collaborating with researchers in New York and Michigan to be able to determine broadly if we need cultivar specific models or models that are specific to different growing regions,” he said. 

“In the future, we will look at how we can use this technology at a larger scale. This same technology could also be applied to drones or passive sensors that can measure whole trees at once,” Larson said. 

The technology is still in the research stages, but Larson is optimistic it will be available to apple growers in the near future to help them better make chemical thinning timing and application decisions.