A mysterious phenomenon is taking down dwarf apple trees across the Northeast. After ruling out the usual suspects, researchers hope a widespread grower survey will help them identify the elusive culprit or combination of factors causing trees in full fruit to suddenly collapse.
“It’s typically younger trees. They will seem healthy up until a certain point. The leaves get yellow and then more yellow, and the next thing you know the tree is dead in two weeks, completely girdled,” said Kari Peter, a plant pathologist at Penn State University who first described the devastating disorder she calls rapid apple decline, or RAD. It’s also referred to as SAD, for sudden apple decline. “It’s startling, the sudden die-off.”
Peter first encountered the phenomenon in her own research block in 2013, shortly after she arrived at Penn State. A variety of fungicide treatments failed to save the trees — Fuji, Gala, Golden Delicious, all growing on Malling 9 rootstock — and the problem returned in 2014. As she began to investigate, she heard that other Pennsylvania growers were seeing similar tree collapse.
“It’s pretty widespread. I don’t think it would have shown up on our radar if it wasn’t significant. When you have acres of trees that are just collapsing, you pay attention,” Peter said.
RAD is characterized by decay that appears to move up the trunk from the graft union, while the rootstock remains healthy. It looks like fire blight, but lab analyses for that pathogen have consistently come up negative, she said.
Once the symptoms are spotted, there appears to be no way to save the trees. “It was a very expensive adventure that first season because everything but the kitchen sink was thrown at these trees and they just collapsed,” Peter said.
Similar problems have now been reported in Ontario, New York and North Carolina as well. More than 20 cases were documented by Ontario officials in 2016, said pathologist Michael Celetti. To him, it looks similar to the damage apple trees sometimes experience a few years after a really cold winter, in which the cold damage to the trunk enables the growth of weak pathogens, such as black rot. And 2014 was a very cold winter.
“There is something different going on, but when we do isolations from the cankers at the bottom of the trees that were collapsing, we’re getting the same type of weak pathogen we usually get in cold injury,” Celetti said. Last year was different because the damage seemed to be associated with the graft union, he said.
New culprit or a perfect storm?
In Pennsylvania, Peter is less swayed by the winter injury theory. That’s because she first saw the RAD phenomenon in 2013, before the cold winters of 2014 and 2015. It also seems an unlikely culprit because, in her own orchard, the rows containing Fuji, Gala, and Golden Delicious trees all suffered from RAD, but the Crimson Crisp planted in buffer rows did not.
But she’s looking into every theory. And it’s quite possible, if not likely, that a combination of factors is at play, she said. “All we can do right now is rule stuff out,” Peter said. “And there’s no common denominator because all the trees are coming from different sources.”
So far, the orchards she has studied have tested negative for fire blight, Phytopthora and tomato ringspot virus.
Diagnostic tests in Ontario also ruled out fire blight and Phytophthora. Tests did turn up various weak pathogens, but it’s unclear if they contributed to the decline in some way or just took advantage of already weak trees, Celetti said.
Other theories being floated included some sort of replant disease or site-specific problem. But that doesn’t fit either because her own orchard and many others were planted on former row crop fields or pasture, she said. And it seems unlikely that a site-specific problem like poor soil nutrients or residual herbicide would have such dramatic effects on some trees while their neighbors remain healthy.
“It’s a real pickle,” she said.
And while many factors could contribute to the symptoms, it’s also likely that growers across the region could be conflating several different disorders under the new name of rapid/sudden apple decline, said David Rosenberger, a retired pathologist who spoke about the disorder at the Empire State Horticultural Expo in January. Fire blight in the rootstock, herbicide injury, winter injury, boring insect damage and drought stress could all be causing or contributing to the symptoms reported, he said at the meeting.
“It’s a bunch of different things being lumped together in my opinion,” Rosenberger told Good Fruit Grower. “Rootstock blight is our major cause in the Northeast, but there are some other things going on that nobody understands as well. People in the field claim they are seeing a lot of trees dying in a way they haven’t seen before.”
Conversely, because the disorder Peter is documenting manifests like fire blight, it may not be new but simply overlooked in the past.
“This may have been around longer than we realized, but we’ve had some crazy stressors here in the Northeast with the drought and two crazy winters, so it may just have compounded everything, and that’s why we are now seeing problems,” she said. “Or why we might be seeing it is because the production of fruit has changed so much. In the high-density system, we’re pushing trees to do a lot of work in a very short time, so that may be one of the factors contributing to it, but it’s still a mystery.”
Connecting the dots?
The next steps to solving this mystery is to move from anecdotal reports of tree collapse to collecting comprehensive data. Peter is working with other pathologists and extension scientists across the region to distribute a survey to growers this season to track the frequency of the phenomenon. That should help researchers understand if certain cultivars, chemicals, orchard sites or specific stressors play a clear role in RAD.
Meanwhile, pathologists at the U.S. Department of Agriculture research facility in Maryland are using genetic analysis tools to see if an as-yet-unknown virus could be responsible.
“We’re still trying to understand what could possibly be in the tree itself. There may be something there that’s never been described before, so it’s not on our radar,” Peter said. “Plant viruses can be this wild card.”
As the investigation effort grows this season, Peter, Celetti and Rosenberger all stressed that while the disorder is a serious, costly problem in the orchard blocks where it’s found, it’s not a widespread problem that poses a threat to the industry.
“It’s not every orchard, it’s not every Gala orchard on M.9, which makes you scratch your head because they all went through the same winter,” Celetti said. “There’s a lot of pieces to this jigsaw puzzle, and the more small pieces there are, the harder it is to put the pieces together. We don’t even know if we have all the pieces.”
“I don’t want to make people feel like this is a looming disaster; this is something we want to keep tabs on so we can give people better guidance to avoid it in the future,” Rosenberger said.
Unfortunately for growers, the guidance at this point is simply to minimize stress to trees through careful irrigation and nutrient management, Celetti said.
“Bottom line, there is not a clear-cut answer to what’s going on,” Peter said. “After this season, I’ll know more hopefully about if there is a common denominator and just how pervasive this is.”
—by Kate Prengaman
Signs of rapid apple decline
Diagnostic characteristics of rapid apple decline, also called sudden apple decline:
—Impacts young dwarf trees of multiple varieties and rootstocks, although M.9 is the most commonly affected.
—Necrosis that appears to start at the graft union and moves up the tree.
—Severe shedding of bark around the graft union with cankers present.
—Rootstock appears healthy with many suckers present.
—Trees with a full load of fruit collapse.
—Total collapse observed from late July through September.
—Dead or declining trees dispersed throughout a block with other trees that remain healthy.
Experts recommend reducing stress to protect trees:
—Ensure trees are getting proper nutrients.
—Paint trunks of young trees white or use guards.
—Prune out fire blight infected shoots as soon as possible.
—Remove dead or dying trees during the dormant season.