Mar 13, 2023
Retractable nets help regulate apple color

Retractable netting can effectively shape apple color and protect against sunburn and hail. Preliminary results from a study conducted at a Washington orchard on how netting affects apples appears promising.

Lee Kalcsits, a Washington State University (WSU) Department of Horticulture physiologist and associate professor, is examining how netting influences apple color. In its second year, the study, funded by the Washington Tree Fruit Research Commission (WTFRC), is being conducted at Chelan, Washington-based AgriMACS Inc.’s Monument Hill Orchards in Quincy, Washington. The horticulture and viticulture management firm’s 650-acre orchard is mostly Honeycrisp apples.

Lee Kalcsits

“We know there is a color penalty in having trees under nets, in terms of color development,” said Kalcsits, who works in WSU’s Tree Fruit Research and Extension Center in Wenatchee. “The idea is to test the timing of retraction, or pulling the nets back prior to harvest, to see what the impact on red color is and what the tradeoff with that to sunburn might be.”

In the test, AgriMACS managers pulled-back sections of the net 14 and 7 days before harvest and compared results to when nets weren’t retracted. Both retraction dates brought an increase in sunburn risk, with an additional 3% of the fruit suffering severe sunburn penalty, requiring the fruit to be downgraded or culled, Kalcsits said. The netting was pulled back at a time when growers would not have expected sunburn risk, he said.

“As expected, fruit colors much faster once the net is removed,” said Tom Gausman, AgriMACS’ vice president. “A surprising finding was how quickly the fruit colors when the net is removed with the right weather conditions. Overhead cooling after the net is retracted is essential for earlier ripening varieties like Honeycrisp and Premier Honeycrisps to prevent sunburn without the net.”

The netting saved several AgriMACS blocks of Honeycrisp apples from being damaged by early June hail. The netted blocks suffered 15% damage while hail destroyed the non-netted blocks, Gausman said.

Moving the net improves color, but increases risk, Kalcsits said. The test retracted netting during the first year when there was little sunburn pressure, but the trees still experienced an increase in sunburn, he said. 

Careful timing is necessary. With temperatures of 95° Fahrenheit or even in the lower 90s, Kalcsits said he would expect to see even more sunburn damage.

The netting AgriMACS uses costs about $200 in labor per-acre to deploy and $200 to retract, plus the cost of lift rental. More modern “retractable” systems are much cheaper and don’t require scissor lifts, but the construction is not as robust, Gausman said.

“Net usage is still relatively new in our industry and we’re still working out the best operational procedures, especially the focus points of the project which was how does the timing net retraction affect color development,” he said.

Because there was no difference between retracting a week or two weeks before harvest, Kalcsits said growers may need to start retracting seven days before harvest or even earlier. He’s checking to see if there’s a penalty for retracting closer to harvest.

Fruitful results

Netting also aided other fruit development. One AgriMACS block of Geneva 890 rootstocks was highly vigorous. In favorable soil, the first-generation trees won the race to get to the top wire compared to an adjacent block on M337.  However, the 337s have out produced the 890s in the first two crop years, Gausman said.

A non-netted block was a total loss, he said.

“It looked like it had a lot of vigor and looked like it was going to grow very well,” he said. “We chose to pocket those dollars and not extend the net, with the idea that even without the net, the trees would hit the top wire.”

While Kalcsits and his grad student Noah Willsea haven’t specifically looked at differences in net colors, they noticed some variances in growers using white versus black netting. Netting color research conducted in other regions of the world have demonstrated consistent results across several experiments, Kalcsits said.

Black netting appears to absorb all the light coming in. A grower using 20% black netting could cut light by 20%. White netting, on the other hand, scatters light, and trees receive light penetration at different levels within the canopies.

“You get better color under the white netting compared to the black netting,” Kalcsits said.

Some growers use gray nets, a medium between the two colors, which deals more with aesthetics, Kalcsits said.

Kalcsits’ research protocols fit well with AgriMACS’ normal operations and it was easy to be a cooperator with him, Gausman said.

“Grower partnerships like the one with AgriMACS allow us to use mature, commercially managed blocks for larger experiments than we would normally be able to do in our research orchard,” he said. “It also allows us to be nimbler in switching cultivars or training systems based on grower input and need.” 

Key traits for research partnerships include willingness to share information, allowimh frequent visits and a willingness to host industry members for field days and tours, Kalcsits said.

“These partnerships are critical for Extension and information transfer of our funded projects and help growers see what research projects might look like if implemented in their orchards,” he said. 




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