—by Matt Milkovich
For the past two or three years, some apple trees in the Northeast have been showing signs of decline: off-color foliage, weak shoot growth, bark sloughing off the shank — and in the worst cases, trees setting fruit but then collapsing midseason.
Cornell University fruit crop physiologist Jason Londo and other scientists noticed that the declining trees showed damage at the rootstock shank but not at the graft or scion. They suspected cold damage to the rootstock was causing the decline, exacerbated by recent mild winters with large swings in temperature.
Londo’s lab has been studying the problem, and he shared some of his findings with the International Fruit Tree Association in Rochester, New York, in February. He hasn’t yet proven that cold damage to rootstock shanks is causing tree decline, but his findings appear to be moving in that direction.
Londo described symptoms of the collapse phenomenon he is focused on. In the early stages of decline, the leaves start to turn chlorotic. The bark around the shank becomes thick, cakey and loose, and there are often slugs, beetles, borers and fungi underneath. The rootstock shank appears almost entirely girdled, except for a small bit of connective tissue extending from the scion down to the roots. That strip of tissue is all that remains of the vasculature supporting the canopy. Then, additional stressors such as heat or drought can tip such a tree into full decline, he said.
To understand how cold damage could contribute to this phenomenon, Londo’s lab spent the past couple of years artificially freezing trees and ungrafted rootstocks. As they gradually lowered the temperature, they discovered that rootstocks generally take damage and die before scions. A grafted apple tree is “two genetic individuals fused together,” he said, and it’s likely that at different points in winter, one is more cold-hardy than the other. When they froze Honeycrisp grafted to Geneva 214, for example, minus 13 degrees Fahrenheit killed the rootstock, but the scion didn’t die until the temperature reached minus 31 degrees.
After being damaged in freeze chambers, some of the trees were planted in a greenhouse, where they often grew for a while then collapsed — mirroring the symptoms of decline in the field. Londo’s team also planted freeze-damaged trees in outdoor research orchards and will monitor them for collapse in the coming years, he said.
Cold damage in the field seems to occur more often during mild winters with large swings in temperature — a thaw followed by a freeze, for example. In early winter, the tree, or more specifically the rootstock shank, doesn’t have a chance to harden off and enact its freeze defenses before being hit by a sudden, acute cold snap. The trees are vulnerable to cold damage in late winter, too, when they’re coming out of dormancy, he said.
Snow covering the rootstock shank poses another complication. Snow acts as an insulator and keeps the shank warmer than the rest of the tree, making the shank even less likely to harden off, Londo said. That’s fine if snow stays all winter, but if it melts before temperatures take a precipitous drop, that can set the stage for damage.
After surveying 25 scions and about two dozen rootstocks, Londo concluded that scions are generally less reactive than rootstocks are to temperature swings. Also, some rootstocks gain cold-hardiness more slowly in the fall or shed it more quickly in the spring than others. Budagovsky 9 and G.11 develop significantly less cold-hardiness during milder winters, while G.41 and G.257 are “cold hardiness overachievers,” proving very hardy in all years, he said.
“In a cold winter, all the rootstocks we currently use are safe,” Londo said. “Mild winters are where we have to be worried.” •
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