Apple storage disorders and recommendations for four UMN varieties

Madeline Wimmer, UMN Extension educator and Cindy Tong, UMN Professor & Extension postharvest horticulturist

Images: A collection of apple disorders caused by or worsened by storage conditions. This includes 1) soft scald;  2) bitter pit, a physiological disorder related to calcium use in the apple fruit, which can start in the field, and worsen in storage; and 3) diffuse flesh browning, which result from storage temperatures that are colder than an apple can tolerate. Photo credit: Photos 1 and 3 under images are credited to Cindy Tong. 

The Minnesota apple industry primarily sells apples through direct sales at farm stands, farmers markets, community supported agriculture (CSA), and on-site agritourism events like U-Pick and farm festivals. It’s not uncommon for elementary school classes to visit a local orchard in the fall to learn about apples, or for families to make a trip to an orchard on a Saturday afternoon to purchase apples, pumpkins, and other complementary goods. 

This means that most apples grown in Minnesota are stored for shorter amounts of time than states with a larger wholesale industry. However, even short term storage takes consideration for apple care after harvest like how quickly the apples are cooled before entering storage, along with the final storage temperature and humidity levels set to prevent storage disorders and maintain quality apples.

Storage disorders are physiological issues that result from interactions between the environment where the apples are being stored, the condition of the apple at harvest, and metabolic changes that occur within the apples while they are stored. 

How likely it is for an apple to be affected by a storage disorder can be impacted by the variety, as well as the seasonal conditions that impact its quality. During some years, for example, apples may be more prone to russeting, which decreases fruit integrity in storage because the skin has been slightly damaged by micro-abrasions.

This table lists four popular University of Minnesota apple varieties, which storage disorders they are susceptible to, and how long they can be stored at 32° and 40° F.

Data from Cindy Tong, UMN Professor & Extension postharvest horticulturist.

Different storage disorders:

• Bitter pit: Sunken spots on the skin with additional brown, hollow (corky) spots within the flesh (see photo 2 above). 

Related to cell wall breakdown within the apple flesh due to issues with calcium getting into the fruit. Symptoms can show up when apples are still growing on the tree, but often worsen in storage. There are a few ways that growers can try to manage bitter pit prone apple varieties like Honeycrisp, most of which was covered in the final fruit update in 2024.

• Diffuse flesh browning: A condition in which a brown discoloration spreads throughout the apple flesh (see image 3 above). 

It results from apples being stored in temperatures too cold for what the apple can tolerate, and is more likely to occur in larger apples. Some research has indicated that large swings in orchard temperatures can increase the risk of diffuse flesh browning (1).

• Lenticel breakdown: Symptoms can look similar to bitter pit on the fruit's skin as the lenticels collapse, though this condition does not extend as much into the apple flesh.

Lenticels are small openings that allow for gas exchange on the surface of the apple flesh. They are similar to stomata found on the leaves and can be somewhat compared to the pores on our skin. Lenticel breakdown is more likely to show up on the shaded side of apples. This issue tends to be more problematic for long-term storage.

Additional reading about lenticel breakdown:
Plain talk about lenticel breakdown (Good Fruit Grower)

• Soft scald: Apple skin develops spots that are dark and sunken, and can extend into the flesh (see photo 1 above).

Similar to diffuse flesh browning, soft scald occurs when apples are stored in a temperature colder than they can tolerate. Unlike diffuse flesh browning, soft scald begins on the apple skin. Honeycrisp and McIntosh are two varieties that are susceptible to soft scald.

• Soggy breakdown: May look similar to diffuse flesh browning, but has defined borders between healthy and affected parts of the apple flesh

This cold-induced issue is very signature to Honeycrisp in particular. Symptoms look like spongy, small brown spots within the apple flesh.

Other storage considerations:

While the above conditions are classified as disorders, apple fruit quality can be impacted by a number of factors that can either lead to disease issues or minor changes in fruit texture and flavor. 

• Humidity: Having adequate moisture levels (i.e., relative humidity levels between 90-95%) helps to prevent apples from dehydrating and shrinking over time, which can impact the fruit firmness and the skin quality. This is especially important for growers investing in longer storage periods. Too much moisture can also negatively impact fruit, especially when the risk of pathogens is high.
• Excluding diseases: Be sure to routinely clean storage bins and environments to prevent the development of diseases like blue mold (Penicillium sp.) in storage.
• Monitor stored apples and equipment: Check in on stored apples regularly and check the temperatures and relative humidity to make sure everything is working properly.
• Harvest apples at the correct stage for the anticipated storage duration: Apples being stored for longer periods of time will do better harvested slightly earlier than those planned for short-term storage. This should be based on the iodine-starch test. Apples that will be stored for longer periods (greater than 4 months) should be harvested between stages 3-4 on the Cornell scale.
• Stepwise cooling: Some apple varieties that are very prone to cold damage can benefit from first conditioning apples to 50° F and gradually cooling the apples down to the final storage temperature (37-39° F).

1. Tong, C. B., Chang, H., Boldt, J. K., Ma, Y. B., DeEll, J. R., Moran, R. E., Bourgeois, G., & Plouffe, D. (2016). Diffuse Flesh Browning in ‘Honeycrisp’ Apple Fruit is Associated with Low Temperatures during Fruit Growth. HortScience, 51(10), 1256-1264. Retrieved Sep 18, 2024, from https://doi.org/10.21273/HORTSCI11179-16