Apples “breathe” while they’re growing, respirating through small, pore-like openings on the fruits’ skin, allowing for gas exchange between the apple and the atmosphere. These openings are called lenticels.

Just like our skin, apples can have issues with their outsides, often with something called lenticel breakdown (LB). Dr. Gene Kupferman, a post-harvest specialist with Washington State University, has noted, “Lenticel breakdown is a skin disorder of apples that appears largely after the fruit have been packed, thus it is a very expensive problem.”

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.

A team from the Department of Horticulture at Michigan State University (Alex Engelsma, Nobuko Sugimoto, Özge Horzum, Emily Lavely, Nikki Rothwell and Randy Beaudry) has been studying this growing concern. They presented their research, “Breaking down apple lenticel breakdown: Spray exclusion, lenticel injury characterization and fungal diversity” at the most recent Great Lakes Expo, with an update at the American Society for Horticultural Science conference in July.

The researchers noted that awareness of lenticel injury and breakdown as a problem both pre- and post-harvest has been rising. They reported LB is likely due to several factors: the apple variety, new pesticide and fungicide chemistries, changing environmental conditions, harvest dates and storage length and practice – and probably more.

As both awareness and a lot of yield loss is due to lenticel injury, work needs to be done to characterize the different types of injury and how they occur. This knowledge will hopefully lead to practices and solutions for controlling lenticel injury and LB.

The MSU team conducted three tests: orchard spray exclusion, image analysis and fungal diversity assessment.

  • Orchard spray exclusion:

In July 2023 and 2024, bags were placed on ‘Honeycrisp’ and ‘SweeTango’ fruit. Two types of bag material were used: mesh (actual plastic material) and paper bags. A total of about 200 fruit were analyzed for each treatment across five orchards, either with no bag, the mesh bag or the paper bag.

Orchards 1 and 2 were in Hart, MI, and orchards 3, 4 and 5 were in Traverse City, MI.

“We found that bagging fruit in paper bags reduced the total disorders significantly in both cultivars,” the team said. However, the mesh bags significantly reduced total disorders in Honeycrisp only.

While placing bags on all the fruit in an orchard is not feasible, figuring out where LB affects the fruit most often and using bags could help reduce the incidence of damage.

  • Image analysis:

The researchers found that light and electron microscopy are useful tools in better characterizing the different types of lenticel injury and breakdown. Their work looked at the differences between healthy and unhealthy lenticels, very closely observing ‘Evercrisp’ apples.

  • Fungal diversity assessment:

While trying to figure out which fungi were most often present in lenticel injuries, to see if there was a correlation between a fungus and LB, ‘Evercrisp’ apple tissues and lenticels were DNA sequenced. In unhealthy lenticels where disease was present, there was a much higher abundance of Alternaria than in the healthy tissue.

By and large, Alternaria dominates the fungal community in unhealthy lenticels. Orchard owners working to keep that fungal pathogen in check may see a reduction in lenticel injury.

Going forward, the research team wants to better quantify lenticel disorders in packinghouses; characterize the different kinds of lenticel injuries; and apply various types and rates of wetting agents with a typical cover spray to see their impact.

by Courtney Llewellyn