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Apple growers first became interested in grafting scions onto rootstocks when they realized that dwarfing rootstocks could tame vigor, bring trees down to manageable size, and cause them to bear fruit earlier.

The question, “How do rootstocks do that?” was never fully answered. And a new question is becoming important: “What else might they do?”

Some of the new rootstocks coming from the breeding program in Geneva, New York, are able to confer whole-tree resistance to the pathogen complex called replant disease.

In the future, growers may be able to choose a rootstock that would greatly increase calcium uptake and help cure bitter pit or scald or reduce internal breakdown in stored apples. Can we fix Honeycrisp’s calcium and iron chlorosis problems with rootstocks?

Can we change the nutritional profile of an apple by putting it on a different rootstock?

Can we choose rootstocks that will perform better in the dry, alkaline soils of the West and others that fit better on wetter, acidic soils?

Can we grow larger fruit by choosing rootstocks that increase uptake of potassium or sodium? Some rootstocks take up 20 percent more than others.

In an interview with Good Fruit Grower, Dr. Gennaro Fazio, the geneticist with the USDA Agricultural Research Service Plant Genetic Resources Unit in Geneva, New York, talked about recent experiments there in which scientists looked at some fundamental attributes of rootstocks. He also reported on the work in the spring issue of New York Fruit Quarterly. “Right now, we are basically screening the rootstocks we have to find what they can do,” he said. “The next step would be to ­actually breed rootstocks for the traits we want.”

Rootstocks alter scions

Scientists know that rootstocks are able to affect production of proteins in the scions, making them less vigorous or less susceptible to the complex called replant disease, he said, but they don’t know how they do it.

“Why do the tissues of a Gala apple scion grafted on an M.9 rootstock behave differently than the same tissues grafted on B.9?” he asks in the New York Fruit Quarterly report.

“Somehow, signals transmitted by the rootstock machinery are detected and interpreted by the scion machinery to give visible effects on ­photosynthesis, growth, and productivity.”