Dr. Luca Corelli is studying a rainbow of hail net colors to  see if fruit growth and development can be influenced.

Dr. Luca Corelli is studying a rainbow of hail net colors to see if fruit growth and development can be influenced.

Photo courtesy of University of Bologna

The worldwide transformation of tree fruit orchards from low to high densities and from wide to narrow canopies marked a major revolution in orchard systems. But future improvements to orchard systems likely will be made through fine-tuning techniques instead of radical changes, says Italy’s Dr. Luca Corelli.

Previous scientific studies have helped growers understand the importance that light plays in fruit production, said Corelli, plant physiologist in the horticultural department of the University of Bologna. Growers responded in the last few decades by adopting vertical and V-type training systems that better intercept light.

With the major tree-training changes already implemented by growers, Corelli believes that improving size and quality in the future will be through small steps that result in only incremental changes.

“The orchard today is a highly technological enterprise,” he said, likening the orchardist to a racecar technician who must expertly keep the car at peak performance. “If you miss the fine-tuning by one-quarter of a turn of a screw, you miss your chance of winning.”

Managing competition

The process that turns light into fruit, is complex and fraught with chances of mistakes. If all goes well, Corelli explains, light is used for photosynthesis and carbohydrates are partitioned throughout the plant in the fruit, leaves, trunk and shoots, and roots. Dry matter partitioning is a highly competitive phenomenon, and growers must learn to manage the partitioning if they want the tree to produce large fruit, which goes against the tree’s natural instinct of producing lots of small fruit.

“If we manage this competition properly, you can shift some percentage points in the dry matter partitioning to fruit growth,” he said, adding that growers must understand that on the road of turning light into fruit, the tree can’t make up lost time.

For example, he noted that Fuji apples in the Bologna area grow about two grams per day. “If you make a mistake with irrigation and it’s off for ten days in a row, you can’t make up that twenty grams unless you remove plant material.

“Be aware that a day lost is a day lost. We cannot ­accelerate above the influential rate of light from the sun.”

Corelli, who spent time at Cornell University and has collaborated with Cornell’s Dr. Alan Lakso on light interception and fruit development, is studying how different aspects of light interception (shading, row orientation, and hail net color) affect fruit production. He shared the details of his research projects with an international group of tree fruit growers who visited the University of Bologna last fall.

One experiment involves using shading as a way to thin the crop load, “shocking” the fruit to make it fall off without chemical thinning. European Union growers can no longer use carbaryl for chemical thinning, and Corelli is worried about the long-term future in Europe for other chemical thinners. Most Italian growers already use hail nets, so the structures to install shade cloths are already in place.

He is using Lakso’s apple thinning model that was developed to help growers factor in weather data, the carbohydrate supply, and the demand of apple trees when deciding how aggressive their chemical thinning efforts should be. Corelli said the model should help them avoid excessive fruit drop, as well as to know when to remove the nets.

“In our shading trials, we know we can cause fruit to drop, but we don’t want excessive thinning. Already, he’s observed that peaches are more sensitive to shading than apples. “We wiped out a peach crop from one week of shading,” he said, noting that three days of shading in peaches can cause fruit drop. However, it took five to seven days of shading to show effects in Pink Lady and Golden Delicious apples. Corelli also thinks shading could have thinning applications for self-fertile varieties of sweet cherries, though he hasn’t studied cherries.

Rainbow of nets

Italian scientists are also looking at the effects of different hail net colors on other aspects of fruit production. Some of the interest in netting color comes from attempts by local governments in the South Tyrol region to mandate specific net colors to minimize the landscape blight that officials believe has a negative impact on tourism.

Corelli said that some believe that the extensive use of dark gray hail nets in the tourist region is ruining the view. “The environmentalists want hail nets to be green, but that is the one color to avoid because it depresses ­photosynthesis.”

Initial research shows that blue hail nets result in more buds breaking than some of the other colors, he said. “If you go to Zurich, the color of choice for hail nets on tree fruit and grapes is blue. It’s a nice color because it fits in well with the lake region, but is it the best choice in terms of fruit production? I have strong reservations that it is not.”

As scientists learn more about how the plant uses different light frequencies, including ultraviolet and infrared, he envisions orchards in the future that would use three or four different colors of netting material, at different times. The colored nets could be layered in the appropriate order needed.

“You’d see one color used for bud break, another color if you want to make shoot growth be shorter, and the top layer may reflect the sky to make the net disappear for environmental impact.”

Moreover, Corelli sees a future when growers might plant rows at specific angles, such as a northeast to southwest orientation or northwest to southeast, with trees angled to the east or west, to maximize light interception and photosynthesis while minimizing damage from too much light.

He has a trial with trees planted in different directions and angles, and is measuring photosynthesis by putting plastic bags over whole trees. In addition, he is studying the interaction of plant stress and the timing of irrigation to learn if fruit size can be influenced by irrigating in the afternoon rather than the morning.