The US is the second-largest global producer of apples with 70% of their apples coming from Washington State. The main apple-growing areas in Washington State are semi-arid. Summers are characterised by high solar irradiation and temperatures that frequently exceed 30ºC – conditions that promote sunburn.
Tree-fruit physiologist Prof Lee Kalcsits of the Washington State University Tree Fruit Research and Extension Centre has been helping growers to optimise the application of protective netting to mitigate sunburn. He spoke to Hortgro about his findings.
Q What are the trends in net use in Washington State?
A Primarily, worldwide, netting has been used mainly as hail protection and less for sunburn. We don’t have a lot of hail in Washington, so our growers were more interested in netting from the sunburn side. When I first started at Washington State University in 2014, there had been a couple of growers who had tried it out, but it wasn’t widely adopted by the industry at that time.
Since then, the amount of netting has increased. Granny Smith and Honeycrisp are the two main varieties that are covered. Honeycrisp was bred in the United States, and it ripens just after Gala. It’s bicolour and is actually quite difficult to colour.
Q How do growers manage to achieve good red colour development while preventing sunburn?
A There is a small reduction in red colour from the use of nets. But growers find that overall the finish of the fruit is better and the gains they get from netting outweigh that small loss in colour. But there are mitigation strategies they can use to improve colour.
Reflective ground cover is pretty much standard used across the whole industry for Honeycrisp, whether it’s netted or not. It’s considered an essential. It’s also used in Gala and Fuji – basically growers will use it with any bicolour variety.
They either use a woven white material or a reflective mylar-type silver material. For the white fabric, the growers will generally put it out four weeks before harvest. The silver mylar material will be put out about 10 days before harvest. So it’s just to optimise the light penetrating into the lower parts of the canopy during those final few days of colouring.
They have machines that roll it out and cover up the edges, so the wind doesn’t blow it away. It’s more or less automatic, but it’s still labour intensive.
The reflective ground cover doesn’t affect the soil a lot. For specific materials the orchard grass can get quite hot underneath, and that might create some damage. Sometimes growers will put holes in the material just to allow it to vent. But in terms of soil health there is not a lot of concern.
Q During what times of the year do growers cover their orchards?
A Generally almost all the industry will use nets that are deployed immediately after pollination and then they’ll stay there until after harvest. I’d be concerned about nets over the top of trees during pollination.
There are some growers that are very interested in using retractable nets. They want that flexibility to open the nets up near harvest if the weather’s good.
This year we started a project to look at the practice of retraction, and whether there’s a trade-off between improving red colour and increasing sunburn risk. We’re retracting nets at 14 days and seven days before harvest to look at how it improves red colour, but we don’t have those results quite yet.
Q Is anyone using draped nets?
A Draped nets have been used off and on. You see them a little bit. There was a lot of interest about two or three years ago, but it hasn’t been widely taken up by the industry.
It’s convenient in that it’s easy to put on and take off with machines, but there are concerns that the material isn’t as durable as growers want it to be, for the cost. Some in the industry have tried it out and then they’ve gone away from it.
Q You’ve done extensive research on different coloured nets. What are your main findings?
A We looked at red, blue and pearl – a semi-transparent material. What we found was that netted – regardless of colour – had a greater impact than between the colours themselves. So netting was the main effect and then the change in the colour was more like fine-tuning.
For example, blue net reduced red colour, because it transmits less red and far-red light into the canopy. The grower where we did that trial wondered, well, what can we do with the blue net? So they moved it to Granny Smith, and it works very well and they’re very happy with blue net on Granny Smith. And then, the red and the pearl work well with a bicoloured variety.
I would say there are really no significant advantages to using coloured net. So I’d be surprised if it’s widely adopted. Black is very similar to blue, and white is good for coloured varieties. I think that kind of covers the range in any case of what you’d want in responses.
Q What do you consider to be the optimal shading factor?
A We recently finished up a project on optimal shading factor and we’re working on publishing that now. We tested 10%, 17%, and 24% compared to a non-covered control. Those shading factors are the manufacturer’s specifications.
For Honeycrisp the 10% and the 17% were sufficient for reducing sunburn without affecting red colour, whereas the 24% was too much shade and red colour was affected. In Granny Smith 10% was too low – we had a lot of blushing in the apples and a little more sunburn – whereas the 17% and 24% was more optimal for Granny Smith.
One recommendation I would have to any grower is, when you’re testing a product, sometimes there’s a difference between the manufacturer’s specification and the actual shade factor. Also, shade factor can change over time with settling of dust, and in our case, ashes from nearby forest fires can cause an increase in shading. So growers need to be at least aware that the shade factor can increase over time, and you might need to wash the nets or do something to clean those nets.
Q Do you think young trees benefit from protective netting?
A In initial studies, when we did work with the different coloured nets, we were working with trees that were struggling to fill their canopy space, and we saw about a 25% increase in growth and also an increase in lateral branching. So the trees were more robust and stronger underneath nets. I think there is an advantage to putting those nets out immediately after planting.
Q Are there other advantages of nets that you’d like to highlight?
A In Washington State we have a lot of water availability. Obviously, that’s not the case in other parts of the world. One thing that netting can do is reduce total evapotranspiration by the trees, so growers working with drip irrigation will be able to save water.
Trees stay healthier and wetter underneath nets. From a water-conservation standpoint there are potential benefits for areas of the world where water is either more expensive or more restricted.
