Protective netting in Washington State

Research results demonstrate the many benefits of putting nets over apple orchards. By Anna Mouton.

Sunburn is as much a problem in Washington State as it is in South Africa. Washington growers have historically managed sunburn through overhead evaporative cooling. The problems with evaporative cooling include inefficient water use and loss of control over the amount of water applied. This is partly why growers in Washington State are increasingly adopting nets to reduce sunburn in certain cultivars.

A team led by tree-fruit physiologist Prof. Lee Kalcsits of the Washington State University Tree Fruit Research and Extension Center has been researching nets for the past eight years. The results show that netting not only slashes losses from sunburn and hail but also offers other benefits to growers.

Environmental effects

Sunburn is the single largest contributor to fruit losses in Washington State, according to Kalcsits. Photooxidative sunburn — bleaching — occurs when previously shaded fruit are suddenly exposed to strong sunlight. Sunburn browning is caused by a combination of ultraviolet radiation and fruit surface temperatures of 45–49 °C. Still higher fruit surface temperatures of 52 °C and above will lead to sunburn necrosis.

Nets reduce sunburn by decreasing the amount of radiation that reaches the fruit surface. “If you go into the open sun, the sun on your skin feels hot,” explained Kalcsits. “Underneath the nets, the air temperature isn’t different, but you feel cooler because less radiation reaches your skin. Apples feel exactly the same.”

Counter-intuitively, although nets restrict the amount of incoming light, some may improve the penetration of light into the tree canopy by scattering light. Kalcsits used the example of a shadow to illustrate the difference between direct and scattered light. Stand outside in strong direct sunlight and you will have a strong dark shadow. Go under a net and your shadow is much weaker due to light scattering.

“In Washington State, we get very direct light, so anything that improves light scattering helps canopy penetration,” he said. “Better canopy penetration means better fruit colour, better flower induction, and better tree health.”

Hail is infrequent in Washington State, but mitigating those losses is the second most common reason for erecting protective netting. Trials demonstrated hail damage was reduced from 80% in a non-netted control to 3% in draped-net and 1% in exclusion-net treatments.

Reduction in wind speed is another benefit of nets. “We see about a 40% reduction in wind speed,” said Kalcsits. It is not necessary to have nets around the sides of the orchard to moderate wind. An overhead net is effective on its own.

Fruit quality and tree physiology

There is no question that nets reduce sunburn losses. Kalcsits presented data comparing sunburn under nets to an unprotected control. More than 90% of the netted apples would qualify as Class 1 versus only 70% of the non-netted apples.

He also reported that Honeycrisp apples were about 10% larger under nets. “We’re improving fruit growth by reducing the amount of incoming radiation and decreasing the stress on the tree,” said Kalcsits.

The downside of nets is reduced red colour development. “There are negative consequences, especially if you’re using a grey or a black net,” cautioned Kalcsits, “but even with a white net there is still going to be a colour penalty. And some years will be worse than others.”

However, Kalcsits believes that the loss of colour is offset by the reduction in sunburn and general improvement in fruit quality under nets.

Due to the cost, growers often delay putting up nets until trees come into production. But there may be significant benefits to having nets in place from day 1. “The trees grow better under nets,” said Kalcsits. “We see about a 25% increase, not just in shoot extension, but also in the quantity of branches and shoots underneath nets.”

Nets reduce stress on trees by increasing humidity and moderating wind. “If you’re going to have trouble filling your space it’s better putting that net up earlier because you’re going to fill your space faster,” advised Kalcsits.

Fine-tuning nets

“Sometimes you have fine-tuning and rough-tuning knobs on instruments,” said Kalcsits. “The rough-tuning is the net — the main effect is just getting a net over your block. The fine-tuning is the colour, or the weave, or other little things that you can do.”

The most important attribute of a net is shade percentage. Kalcsits presented data on sunburn in Honeycrisp and Granny Smith under 10%, 17%, and 24% shade nets compared to a non-netted control. For the bicoloured Honeycrisp, there was no significant difference between the different nets, all of which were effective at reducing sunburn. Kalcsits recommends a 10% net, as it would have the least impact on red colour development.

In Granny Smith, however, a 17% net lowered sunburn significantly more than a 10% net. As losing red colour development is not a concern with this cultivar, the 17% net is the better option.

Growers in Washington State routinely use reflective mulches to improve red colour development in both netted and non-netted orchards. Kalcsits reported on trials conducted with white as well as mirror-like reflective mulch. “We saw a strong improvement in red colour underneath nets. The level of red colour underneath nets was about the same as a non-netted tree.”

Reflective mulches can be moved from earlier to later cultivars during the season. Kalcsits also shared that putting out a reflective mulch from six weeks after full bloom for six weeks improves flower induction by improving light penetration into the canopy.

Retracting overhead nets before harvest is another strategy to improve red colour. In comparing retraction at 14 and 7 days before harvest to no retraction, Kalcsits and his team found that in both cases retraction greatly enhanced red colour development. However, about 4% of fruit were downgraded due to sunburn, even though the relatively cool weather was not conducive to sunburn.

Overall, costs are the greatest obstacle to erecting nets. But Kalcsits is convinced that the benefits outweigh the costs. “When you’re getting the hail and sunburn protection, and better fruit quality and size, it doesn’t take long to pay for netting.”

Image: Dr Lee Kalcsits, Washington State University Tree Fruit Research and Extension Center. Supplied by Echo Media.