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Rapid apple decline remains mystery for pathologist

By DOROTHY NOBLE
AFP Correspondent

BEDFORD, Pa. (April 11, 2017) — During the Appalachian Fruit Growers winter meeting, Dr. Kari Peter, tree fruit pathologist at Penn State’s Fruit Research and Extension Center, described sudden decline and death of apple trees.
In her first season at Penn State in 2013, a three-acre plant pathology research block that had been planted in 2011 suffered a massive die-off.
Aliette, Rampart and Ridomil were used to save the trees; none worked.
Mysteriously, the symptoms observed did not fit the usual diseases that cause tree collapse such as fireblight infection, Phytophthora and tomato ringspot virus.
The problem in Biglerville continued in 2014, and the issue was discovered with other orchards in the state.
Her lab began working with the Pennsylvania Department of Agriculture.
More recently, Cornell, North Carolina State, Massachusetts University, Virginia Tech, plus tree fruit growers and the USDA in Beltsville, Md., have joined the effort.
So far, there have been no clear answers as to the cause of the sudden collapse of the trees.
The research, however, has identified diagnostic characteristics, and has been able to rule out several factors.
Young, 2- to 8-year dwarf trees are most susceptible.
Several varieties, including Fuji, Gala and Golden Delicious have been affected.
The M9 (Nic29 and 37) rootstock has been most affected.
The trees in Biglerville came from multiple sources.
To date, semi-dwarf rootstocks have not been associated with the issue.
Necrosis begins at the graft union. Severe shredding of the bark occurs around the graft.
Cankers appear. Necrosis proceeds up the tree trunk.
The affected wood is usually solid, not spongy.
Upon examination, Peter noted, “The rootstock looked great.”
She added that many suckers remained, and the root system itself was healthy.
However, the leaves on the trees begin to appear pale yellow, then reddish. That can indicate tree girdling, Peter explained. The tree can be dead within two weeks.
Even with a full load of fruit, the tree can collapse.
Total collapse has been observed from late July through September.
An orchard block of trees can be a mix of healthy, declining and dead trees dispersed fairly evenly throughout the block.
Peter reported that many indicate that winter injury may be the cause, as the stress of severe temperature fluctuations weakens the trees and increases their susceptibility to diseases.
Although she acknowledges that 2014 and 2015 were exceptionally challenging, she noted that observations in her problematic apple block prompts her to question the winter injury theory.
The research has revealed numerous facts that can eliminate several scenarios.
For instance, while the symptoms of fireblight are similar, bacterial DNA has not been detected.
Insects such as borers have been present, but Peter said borers likely entered after the tree weakened.
Phytophthora has not been isolated in samples. Nematodes have been found in soil samples, but in insufficient numbers to cause significant damage. Neither Phytophthora nor nematodes appear to be a cause.
Because many sites have no history of previous apple plantings, a replanting issue has been ruled out.
Latent virus infection and herbicide residue are being investigated aggressively.
Peter has surveyed Pennsylvania growers for specific data, and is collaborating with other researchers on this perplexing rapid apple decline, also known as sudden apple decline.
She told the fruit growers that this problem may be a complex of issues.
In the meantime, she suggests to minimizing stress to the trees by applying the appropriate nutrients and managing diseases and insects.
Also, consider irrigation, especially with M9 rootstock trees.
Painting the trunks white or using white tree guards to prevent southwest injury may limit winter injury.
Peter said she would keep growers informed as more data develop.