Apples and autumn are the ultimate match-up. People love to go apple picking, drink apple cider and plan ahead for holiday pies. But for growers who have to plan what to do with their many bushels of apples once they’re off the trees, proper storage is a cause for concern.

Talking about environmental factors that affect apple quality and storability both before and after harvest at the most recent Great Lakes Expo was Dr. Randy Beaudry of Michigan State Extension.

Beaudry explained that even though apple fruit are stored, they are not storage organs – biologically, fruit are designed to be consumed, not stored.

Storage environments can be stressful for fruit due to improper temperatures, moisture levels and more. And apples are variable – they don’t always grow in ideal conditions or are harvested properly. On top of that, marketing of apples can be unpredictable – a cultivar that’s popular one year may not be the next (thanks, TikTok trends). Regardless, growers still need to store fruit.

Stressors that are physical rather than biological are labeled “abiotic.” Physical stressors such as abnormally high or low temperatures, high or low rainfall, high humidity, high and low sunlight and orchard chemistries can all create issues in apples both on the tree and in storage.

“Fruit undergo an amazing transformation in a few short weeks,” Beaudry said. “Fruit trees are subject to dramatic daily and seasonal fluctuations in temperature, relative humidity, water availability, chemicals (natural and otherwise) and sunlight.”

Fruit sensitivity to these abiotic factors shifts throughout apple development:

  • High temperature during bloom = softer fruit
  • High humidity during bloom/early fruit development = less decay
  • Higher temperatures near harvest = soft fruit, more disorders
  • Higher season temperatures = more lenticel breakdown

Just like in humans, stress isn’t necessarily a negative thing. Stresses might be categorized as “good” or “bad” depending on the timing and the stage of fruit development.

Generally, the apple harvest order, starting in late August and running through mid-October in more northern regions, goes like this: ‘Ginger Gold,’ ‘SweeTango®,’ ‘Paula Red,’ ‘Gala’/‘McIntosh,’ ‘Honeycrisp™,’ ‘Jonathan,’ ‘Jonagold,’ ‘Empire,’ ‘Golden Delicious,’ ‘Red Delicious,’ ‘Rome,’ ‘Idared,’ ‘Mutsu’ and then late ‘Fuji.’ The environment plays a huge role in when these cultivars are truly red, though; Beaudry noted how growing degree days continue to increase.

How Temperatures Affect Fruit Quality

Beaudry’s research has found “[higher] temperature had a strong and positive impact on firmness in no less than seven of the weeks, with the most pronounced effects in mid-April (week 16), mid-June (week 24), mid-August (week 34) and mid-September (week 38).”

However, “high temperature during four weeks in a period from mid-August to mid-September (weeks 34, 35, 37 and 38) displayed negative effects on fruit weight.”

“Weather conditions in mid-May (week 20) and early June (week 23) had a strong influence on apple quality characteristics, namely the level of firmness, starch index and weight of fruit, and on fruit susceptibility to infection by grey mold (lesion size-disease development.”

At about week 20, higher rainfall meant fewer fruit, but only then. Higher temperatures during week 20 also meant softer fruit.

These are abiotic stressors, which orchard owners have no control over. Knowing how these stressors can impact the fruit picked weeks later, however, is good knowledge to have.

Look Out for Lenticel Issues

Did you know apples “breathe” while they’re growing? It’s true. They respirate through small, pore-like openings on the fruits’ skin, allowing for gas exchange between the apple and the atmosphere. These openings are called lenticels.

Beaudry referenced “Plain Talk About Apple Lenticel Breakdown” from researcher Dr. Gene Kupferman, a postharvest specialist with Washington State University. Kupferman wrote, “Lenticel breakdown (LB) is a skin disorder of apples that appears largely after the fruit have been packed, thus it is a very expensive problem.” (He noted ‘Gala’ tends to be the most susceptible to LB.)

Mild cases of LB can resemble chemical burn, and the lenticels turn black. In more severe cases, the flesh under the lenticels sinks, resulting in craters under the skin.

In “Factors Associated with Apple Lenticel Breakdown,” Dr. Eric Curry reported, “Some relationship to mineral content (low calcium, high magnesium, high potassium) may be related to the ability of the cuticle to repair itself.”

That’s an abiotic input growers do have control over. Test your soils and your fruits to make sure they’re getting the proper nutrients they need.

Beaudry concluded by saying there is “a distinct need” for long-term studies to better understand the pre- and post-harvest relationships between abiotic stressors on apples – and the impact of a changing climate.

“You have to look at every factor all the time,” he said.