Washington State feels the heat

How climate change is impacting pome-fruit growers. By Anna Mouton.

Prof. Lee Kalcsits holds the Endowed Chair of Environmental Physiology for Tree Fruit at Washington State University’s Wenatchee Tree Fruit Research and Extension Center. His team researches the interactions between trees and their environment to improve stress tolerance and fruit production.

Kalcsits spoke to Fresh Quarterly about the Washington State experience of climate change and how growers and researchers are responding.

Q. How is climate change affecting Washington State growers?

The biggest issue has been greater sunburn pressure — we’ll go through 20–30 days in a row where there’s sustained sunburn pressure. In the past, that wouldn’t normally have occurred. Earlier bloom and maturity on average, and then earlier overall higher temperatures expose the fruit to higher temperatures during that final ripening stage when it’s really sensitive to sunburn.

Water shortages are not as big an issue. A large portion of the land used for tree-fruit production takes water from the Columbia River system. That river is relatively stable and not forecasted to change substantially under climate change scenarios.

But the amount of snow in the Cascade Range that provides water for a lot of the pear-production regions and the Yakima Valley is forecast to decrease by about half. There will be years when the snowpack will not be sufficient to meet irrigation needs, and water restrictions will be put in place.

Q. Are warmer winters likely to become a problem?

I was part of a group that did some research on modelling. It was led by Dr Kirti Rajagopalan, a climate modeller, and she found that in areas where it was traditionally colder, we would actually get more chilling. Those areas are so cold that temperatures often fall below the chilling window during the winter, whereas under climate change, temperatures would rise and more frequently persist within the chilling window.

We could see less chilling in the southern area of Washington State, where it’s warmer. But overall, the conclusion was that there’s not going to be a lot of pressure on fulfilling chilling requirements for most apple, pear, and cherry cultivars.

Q. What are growers doing to cope with climate change?

Washington State growers have always experienced sunburn — kind of like in South Africa. I think what we’re seeing is growers adopting multiple strategies for sunburn protection rather than relying on just one.

Netting isn’t really effective anymore at preventing sunburn when the temperatures reach about 37 °C. The more days that happen in a season, the more benefits there are in using multiple protection strategies. Growers might combine netting with a cooling system when temperatures get really high.

Water-based cooling systems require more precise control than irrigation. The idea is to have them on for 15 minutes — enough to get the trees wet. Then, you turn them off and let the water evaporate to cool the fruit. And then you turn it back on just as it dries and do it all over again. So, it cycles throughout the day.

You either need labour to do that manually, or you need a control system. Installation costs of evaporative cooling systems are about USD 2 000 an acre [R 95 000 per hectare]. Nets are much more expensive. We’re up to about USD 10 000 an acre [R480 000 per hectare] for netting.

Q. Are you researching ways to mitigate climate change impacts on fruit production?

Our research has focused on providing the strategies and the protection to prevent sunburn during the season, but then giving the fruit the best conditions to mature and develop good colour and quality at harvest.

We’re doing work on retractable netting. We finished a project where you have the netting on until late in the season and pull it away to provide the best conditions for colour development.

And we’ve done some work on de-leafing. So, the canopy protects the tree all season, but then you remove as many leaves as possible near harvest to promote red colour development.

My programme’s done a little bit of work on reflective mulches, but other researchers have already looked at that. Reflective mulches are pretty much standard on all bicolour cultivars in Washington State.

Q. How is climate change affecting long-term planning in the tree-fruit industry?

I think cultivar choice isn’t necessarily being considered much right now — cultivar is market-driven. Growers obviously want to choose cultivars that do well in their environment, but few cultivars don’t do well in Washington State. So, there’s not a whole lot of thought put into the climate change equation for cultivars.

I think it’s probably the same way for orchard systems. Most companies have their preferred way of growing trees, and they’ll do it that way and make it work.

In the long term, we need to develop cultivars that will do well in future climates. Breeding programmes are 25–30-year efforts to produce cultivars that will be suitable for future conditions, but the climates in which we select cultivars now are different to what they will face when they’re planted 25 years from now. So, we just need to be careful that we’re selecting adaptable cultivars.