Apple trees are dying, growers are concerned and researchers are resolute in determining what’s causing the decline and death of previously healthy apple trees.

Dr. Kari Peter, associate research professor of tree fruit pathology at Penn State, discussed the issue at an Apple Decline Summit at Penn State’s Fruit Research & Extension in Biglerville, PA.

In 2019, Peter and Virginia Tech University hosted a rapid apple decline (RAD) summit. “Years have passed, apple decline has evolved, and I’ve become more enlightened about the causes,” Peter said.

She urges growers to not diagnose every dying apple tree with RAD. “That’s something separate,” she said. “I see apple tree decline as a wheel barreling through the orchard, taking out trees, and there can be multiple causes. It can be a combination of causes, and there may be causes we don’t know. Apple tree decline is not just RAD. It’s apple tree decline.”

Because rootstock seems to be a common factor in apple tree decline, Peter asked growers to describe what they were seeing in their orchards. She was particularly interested in rootstock and tree symptoms.

One Pennsylvania grower first observed decline in 2013 on M.9, M.9 T337 and Nic29®. Another Pennsylvania grower noticed apple tree decline in two locations on B.9 green stock. His first observation was that leaves were turning yellow, but a soil fungicide treatment seemed to stabilize the trees. The grower then saw other rootstock with issues but wasn’t concerned because losses were within the normal range.

However, the sudden collapse of several hundred trees in a single five-acre block was concerning. This time, trees wilted and leaves turned brown. The same block also had an ambrosia beetle infestation. Among the trees affected were EverCrisp® and Rosalee® on G.11 and B.10.

At the Appalachian Fruit Research Station in West Virginia, a 2011 planting of Granny Smith and Golden Delicious on M.9 showed signs of decline. Gala started to decline within the first three years after planting. Enterprise and Crimson Crisp on B.9 planted in 2009 declined over the past five years. Most recently, decline has occurred in a 2018 block of GoldRush and Enterprise, both on B.9.

One Virginia grower reported problems with Gala and Fuji on Nic.29. Another Virginia grower noted issues with Gala on M.9 and M.37 and CrimsonCrisp® on M.9 and Nic.29.

A grower in North Carolina said apple decline in his region first occurred in 2016. At that time, it affected any dwarfing rootstock including Geneva® series, B.9 and M.9 T337. Cultivars primarily affected were Fuji and Honeycrisp. Over the past two years, he saw decline mostly with Honeycrisp under three years old on M.9 and M.9 T337, usually in early spring.

This past summer, the same grower noticed decline in Honeycrisp, EverCrisp, Gala, Fuji and Pink Lady®. In his orchard, decline was random, sometimes occurring early in the season, sometimes later.

A New York grower reported decline on all M.9 clones as well as B.9, G.11, G.935, G.16 and G.41. Although he didn’t note any particular cultivar trends, he saw problems in first leaf through 13th leaf blocks. He explained that most decline started mid-block in a well-managed, mature orchard. Scion bark discoloration was one of the primary signs.

Growers report Gala on various rootstock seem to be most affected by apple tree decline. Photo courtesy of Peggy Greb/ARS

He also stated that the initial observations of a serious problem were in western New York in 2013, which is when a major black stem borer infestation occurred following a 2012 drought.

A Connecticut orchardist noticed tree decline in the late 1990s/early 2000. He noted that trees affected were healthy standard seedling apple trees on M.7, M.26, M.9 and other dwarfing rootstocks. At first, only Macoun seemed to be affected, then other cultivars showed signs – primarily sloughing bark and cankers. He also noted that yellowing symptoms first appeared in summer when fruit was setting.

Even farther north, a Massachusetts grower noticed trees occasionally dying; then, last year, he saw significant bark sloughing off the rootstock shank. Several rootstocks were involved, including Geneva rootstocks G.41, G.11, G.935, G.214 and M.9. Varieties included Honeycrisp, Gala and some new releases from New York.

Another Massachusetts grower observed rootstock bark sloughing primarily with G.11. He recorded Honeycrisp on Nic.29 as the worst, while EMLA rootstock wasn’t as bad. Also bad was M.9 T337, with a little bit on B.9 and G.41 with Honeycrisp, but not G.41 with Gala, Crimson Crisp and EverCrisp. He also noticed ambrosia beetles. He emphasized the fact that it was odd to see 20-plus-year-old trees decline.

New Hampshire and Rhode Island growers reported issues similar to those in Massachusetts.

Apple decline is also evident in the Midwest. In Michigan, a grower saw decline in first leaf Gala on M.9 T337. Another Michigan grower observed decline in Gala and in Fuji on Nic rootstock following extreme cold in 2013, 2014 and 2015. More recently, Honeycrisp on G.11 had bark sloughing similar to what Massachusetts growers observed. Trees affected included first, second and third leaf.

At the University of Illinois, cultivars affected include Evercrisp, WineCrisp®, Northern Spy on G.41 and early Fuji on Bud9. There are also concerns about decline in B.10® rootstock on numbered trial cultivars in second and third leaf.

Wisconsin reported problems with mature MacIntosh on M.9. Numerous Canadian growers are reporting issues similar to what U.S. growers are seeing.

The most problematic rootstock and scions as reported by growers? M.9 clones including T337, Nic 29 and Pajam.

“In 2023, we noticed vulnerability with several varieties,” said Peter. “Geneva rootstock that had more problems were G.11, G.41, G.935, G.214 and G.16.” Various cultivars on B.9 and M.26 also had decline.

“We’re seeing the decline symptoms with dwarfing rootstocks in high-density plantings,” said Peter. “Symptoms are similar, particularly on G.11, and appear at the rootstock shank below the graft.”

Peter will host an all-day apple decline session at the Mid-Atlantic Fruit & Vegetable Convention in Hershey, PA, on Jan. 27. The meeting will also be available via Zoom.

Future articles will outline details about possible reasons and solutions for apple decline.

by Sally Colby