Post Harvest Specialist Peter Toivonen on Soft Scald

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I follow AAFC scientist Peter Toivonen into the depths of the Summerland research center. In the basement is a series of cold chambers. Some are for plant propagation and some for fruit storage trials. Toivonen opens one of the latter and I am immediately struck by a smell from my childhood. The rich odour of ripening apples (some are actually rotting) reminds me of the fruit we stored in the ‘box room’ at our house in rural Victoria. Although we called them ‘winter apples’, without the refrigeration or the controlled atmosphere storage available in today’s packing industry, the apples would not last through the winter and would end up in the compost by mid-February.

As a post-harvest specialist with AAFC, Toivonen has spent much of his career studying ways to improve apple storage. He’s developed the DA meter into a reliable tool to determine the best time to pick Ambrosia and Honeycrisp for maximum storage. He has worked with a spectrometer to test for the optimum time to pick cherries, so they can arrive at export locations in top condition. And Toivonen believes he has recently solved another storage puzzle, how to avoid soft scald, an injury that develops when apples are kept in cold storage. “My experiments are showing that it is related to water management practices,” he says.

Toivonen points to four bins of apples on the cold room floor. The first have had consistent irrigation all summer and look almost fresh-picked. The second bin contains apples from trees that had reduced water from June to July and they too look fine. The third group had water cut back from July 15 to August 15 and some have small brown spots are appearing on the skin. The fourth group are from trees that had restricted water from August 15th to harvest. Some of those are more than two-thirds brown colour and are giving off that telltale rotten apple smell. “That’s the effects of soft scald,” Toivonen says.

“Soft scald is a chilling related injury,” explains Toivonen. “Most apples are tolerant of very low temperatures, but we find certain batches of both Ambrosia and Honeycrisp are susceptible when we put them into cold storage.

“My hypothesis, and it has been demonstrated this year, is that if the apples are under stress in the field just four or five weeks before harvest, some of the tissues in the apple will be weakened,” Toivonen says. “When you put those apples into cold storage, say at half a degree, they get chilling injury.”

It’s a year by year thing depending on the conditions, Toivonen says, adding that losses to soft scald average 5% for BC. But he says that this year the damage could be much higher. “I am hearing that some orchards may see 20% damage in this year’s fruit. I believe that there have been some insurance claims already.”

If the orchard is very susceptible, you can see soft scald developing in storage within a month, but normally it’s 2-3 months. One strategy has been to condition the fruit at 15 or 20’C for a week before they are put into storage. Another practice is to store them at a warmer temperature. “Sometimes 3 or 4’C storage alleviates the problem,” Toivonen says.

The condition has been studied for some 40 years but no one has made the connection to water management, and indeed even for Toivonen himself, it was by chance. “I was studying two orchards in the Similkameen that were particular hot spots for soft scald and one of the orchards was sold,” he recalls. “There was a new manager who followed different irrigation practices and the next year there was very little soft scald.

“The following year we went back to the other orchard and they had changed their irrigation as well. There was no soft scald and I had no experiment,” he chuckles.

“I really made the connection when I was comparing two types of ground covers placed to help the apples colour up in the last weeks of the growing season,” Toivonen recounts. He was looking at Extenday, which is permeable and lets water through and a silver mylar product, which doesn’t. "Water from the micro sprinklers just pooled up on the surface of the mylar," he says.

Those dry trees showed signs of water stress and Toivonen says the fruit suffered soft scald and soggy breakdown (both chilling injuries) while in storage.

“So from there I came to the conclusion that it is late season water stress that is related to soft scald,” Toivonen says.

“This year we had great cooperation from growers that allowed us to look at reducing irrigation at different times through the season,”says Toivonen. “From the results downstairs it is very clear that late season reduced watering is directly related to the development of soft scald.”

Reducing irrigation is practiced to conserve water in our hot, dry valley. Growers also do it to increase crop quality. “If you do it from the middle of June to the middle of July there are quite a few reports that indicate you get increased dry matter content and improved fruit quality including red blush,” explains Toivonen. “It reduces the amount of foliage that can steal nutrients from the developing fruit.”

But it is that last phase of growth when the tree needs the most water of any time in the growing season to keep the fruit from going into stress, Toivonen explains. “What happens is the surface of the fruit gets heated and the tissue gets compromised although you don’t necessarily see anything, it’s not like sunburn,” he says. “When you give the fruit a second stress of the cold storage it starts to break down.”

“It is the two stresses that finally cause the tissue to deteriorate,” Toivonen says. “If the apples are kept at 3’C it may not show up ever, but at 5’C it will.”

There can be too little water, but also too much. “I was contacted by a colleague in New Zealand and they have a problem with soft scald on Ambrosia too,” Toivonen says. “But before their harvest there are heavy rains, the soils are saturated, and the roots don’t work. They get water stress because of too much water.”

“You have to keep your mind open that it is not drought, but water stress,” Toivonen emphasizes. “Even here, there may be growers who have too much water in their orchards who may be having the same problem.”

The researchers have a proof of concept that stress leads to a problem. Now they need follow-up trials to develop best practices for growers.

“We are learning that post-harvest quality is really linked to production practices,” says Toivonen. “Once the fruit has been picked, post-harvest can only maintain quality, it can’t improve it.