Plenty of things can kill young apple trees, but growers across Eastern North America are increasingly reporting a sudden tree collapse that seems unusual, not appearing to align with any of the usual culprits such as fire blight, winter injury, drought stress or crown rot.
“What we’re getting is death right at the rootstock, just before harvest,” said Ontario grower Tom Ferri. The rootstock itself remains healthy, ruling out rootstock blight, and “if it was winter injury, we’d see it in the spring,” he said.
While some growers and researchers struggled to understand the phenomenon — now known as rapid/sudden apple decline or RAD/SAD — looking for new pathogens or a perfect storm of systemic stresses, others believe it is simply misdiagnosed.
“It’s just plain winter injury,” said Michigan State University Extension horticulturist Phil Schwallier. “Sometimes you get winter damage this year and it takes three years to kill the tree. They fight, but eventually they lose.”
One of the first people to study RAD, Penn State University pathologist Kari Peter, says it’s definitely more complex than just winter injury. She found a new virus linked to collapsed trees a few years ago, but after further study, she doesn’t think that’s a smoking gun, either.
This year, Pennsylvania growers are reporting tree death she attributes, in part, to the stress from saturated soils after last year’s heavy rains.
“There’s a weakness we haven’t quite found,” she said. “Winter injury is a stressor, extreme rainfall is a stressor, but why are the trees suddenly more susceptible?”
On that question there’s more, but not total consensus: New orchard systems with dense plantings and dwarfing rootstocks that are aggressively pushed in their first few production years are more vulnerable to a wide variety of stressors than are older systems.
Winter injury, drought stress, pests and pathogens may be longstanding foes, but the resilience in the system and the climate may be changing.
“We’re pushing these trees harder than we’ve ever pushed them before, so the trees are already stressed,” Schwallier said. High rates of nitrogen keep trees growing late into the season instead of hardening off, and weather extremes such as unusually warm falls make it even worse. “It sets you up for a disaster, and sometimes Mother Nature delivers,” he said.
Pennsylvania grower Mark Boyer first encountered the mysterious tree death he now calls RAD in 2013, in a block of Galas planted in 2010 on Malling 9-Nic.29, M.9-337 and regular M.9.
The trees were stressed by a dry fall their first season, and by 2012 he was pushing them pretty hard. He noticed in 2013 that heavy nitrogen would kill the trees — like trying to speed up growth when the tree was already on life support and pushing them over the edge into a seemingly sudden collapse.
Gala on M.9 seems to be the most common block for RAD reports across the East, but it’s also been reported on Honeycrisp, Ambrosia, Fuji and others. Healthy-looking trees rapidly collapse midseason, with necrosis in the scion wood just above the graft union. The rootstock, on the other hand, appears healthy and pushes up suckers.
Every test that Boyer had the Penn State lab run came back negative. Over the subsequent years, he kept fighting for the block and replanting to fill gaps, but ultimately lost about 17 acres.
“This sounds pretty extreme, but it’s like people living with HIV. With proper treatment, they are fine, but with that compromised immune system, if they get pneumonia, it’s bad. The trees can’t bounce back, they can’t take these multiple blows,” Boyer said. “Was there some winter injury in there? Sure. But the compromised immune system is the key to it. They live a perfectly normal life for a few years, just to see this massive loss.”
He thinks so-called latent viruses may be making the trees more vulnerable to stressors. The new virus Peter found in 2017 was found in his block, along with others typically considered secondary or latent.
“We as an industry have let ourselves get into this position because we as buyers of trees have not asked for virus testing,” he said.
Researchers have yet to prove if viruses are playing a role in RAD, by making them more susceptible to other stressors, but it’s both possible and hard to prove, retired Cornell pathologist David Rosenberger wrote in this summer’s issue of the New York Fruit Quarterly.
He reviewed the various factors that could be contributing to RAD, including winter injury, which can be exacerbated by high fertilization and glyphosate exposure; rootstock blight; drought stress; latent viruses; and new pathogens.
“If new pathogens only weaken trees, perhaps by making them more susceptible to winter injury, then it will take even longer to sort out exactly how important they are because pathogenicity tests in the absence of interacting stresses may fail to show their importance,” he wrote. “It behooves growers to request and insist upon trees that are propagated using virus-free scion material.”
Same problems, new vulnerability?
It makes sense that growers such as Boyer want to place the blame, at least in part, on a pathogen, but PSU Extension horticulturist Rob Crassweller said he thinks the compromised immune system in that analogy is actually coming from the growing system itself.
“Growers need to be aware that it’s probably not any one factor, and it’s a combination of how we’ve gone about changing our production system,” he said.
It reminds him of the 1980s, when Mid-Atlantic peach growers were struggling with an unexplained decline problem that turned out to be a complex of poor site preparation, pushing the trees too hard and cold damage. Better basic horticulture solved the problem, and he thinks apple growers could benefit from the same things.
“We’re putting them 18 inches apart, we’re trying to crop them immediately,” he said, leaving the trees without the resources to build resilience. “Instead of trying to get the hot varieties in first, we’ve got to look at getting good trees in the ground and getting the site prepared.”
Cornell horticulturist Terence Robinson said most of the data he’s seen fits completely with winter injury — although it’s rare in some places, so growers may not be as familiar with it. Looking back at weather data, he’s looking for a big, sudden drop, 50 or 60 degrees without a snow blanket, that shocks and damages the trees near the graph union, just above the soil.
“These 3- to 5-year-old trees seem most at risk. There’s something about these younger dwarf trees leaving little reserves for the roots,” he said, especially on M.9 roots. “The next spring, they leaf out fine but start to collapse in the heat of July.”
In his review, Rosenberger wrote that winter injury appears to explain many but not all of the reported cases of RAD. “Based on available evidence at this time, I suspect that our current tree decline problems result from a complex of interacting factors that will be very difficult to disentangle and which may differ from one orchard to the next,” he wrote.
What’s in a name?
Trees decline for lots of reasons, but the problem with a catchy new name such as RAD and SAD is that suddenly more growers think their decline is the newly identified decline.
That makes it difficult for researchers trying to make sense of the phenomenon, Peter said. First, she and her collaborators at the Pennsylvania Department of Agriculture have to rule out everything else. This year, that’s included growers with serious nutrient deficiency, subpar trees to start, root rots, and fire blight.
“There’s lots of tree decline this year, but I am not going to call it all rapid apple decline,” she said. “I see the name as fitting that scenario you can’t quite place anywhere else.”
Schwallier and Robinson caution that the new name creates more confusion and a sense of a spreading problem, when it’s actually the same stresses that growers have contended with forever.
In Ontario, however, integrated pest management specialist Kristy Grigg-McGuffin said the new name is important to understanding the risks inherent in how apple orchards have changed. Agriculture and Agri-Food Canada and the Ontario Ministry of Agriculture, Food and Rural Affairs have formed a task force to study this issue, she said, reports of which began after a hard winter in 2016, followed by drought.
“People will say we’ve had winter injury forever, but we’ve changed our system and we’re growing really stressed trees,” she said. “I think it’s a complex of stressors that’s taking down the trees. Everyone is having the same phenomenon, but with different factors.”
So, what’s a worried grower to do? Minimize stress when possible in young blocks: That seems to be the standard recommendation from researchers around the region. •
—by Kate Prengaman
Related: What’s killing these trees?