Scion's effects on the tree

We read a lot about the effect of rootstock on a tree. Has there been research into what effects a grafted limb might have on the rest of the tree?

Specifically, I’m wondering if grafting a disease resistant branch onto a tree might improve the resistance of the tree as whole.

If the branch is a major portion of the tree it could help. Can’t see a small limb having much if any effect.

For instance a fire blight resistant top could help prevent rootstock from becoming infected. Once it gets into the roots it’s deadly.

I don’t see anything being transferred from the resistant limb into the rest of the tree to make the others parts more resistant.

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Rootstock, PH , soil type and organisms determine the amounts of nutrients transfered to the tree. That has a major effect on the health of the tree along with water, light, temperature and genetics. I’ve never heard of a grafted branch effecting the health of a tree except in a negative way such as introducing a virus but perhaps it can. It’s an interesting question.

FYI, this somewhat-related topic exists. You will not find current research answering your question there, but a post by bradybb links to a compendium of historic articles, some of which implicitly comment on the matter.

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We know that rootstock can transfer disease resistance up the tree, and not just for soil-borne diseases where they might simply act as a barrier. If they can provide resistance to something like fire blight, they must be transferring something to the branches which is making them more resistant. In that case, why couldn’t it work the other way? The whole tree is connected to the same vascular system; perhaps a branch can transfer something relevant “down” the tree through that.

I’m just brainstorming here, and very hindered by my complete ignorance of how plant immune systems work.

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During my research on cold hardiness I do remember one study that said that it did not work in reverse. That may not apply to disease though.