I’m doing some research in hope of locating a successful ‘local’ apple or pear cultivar for commercial propagation & production. It was suggested I look for an ‘established fruit tree’ in the vicinity of the potential orchard, because it will have ‘become adapted to that climate.’
My question: being that fruit trees are susceptible to both virus’ and bacteria, can/ do they develop antibodies, or have any internal method of fending off such pathogens? If so, can they actually acquire an immunity to either?
My assumption is ‘their progeny,’ with a new/ different mix of genetics, may appear to ‘adapt’ or survive local conditions, but the fact a fruit tree has survived in any environment is a testament to it’s natural resistance and not to it’s ability to ‘adapt’ or in any way become immune to local pathogens. Make sense 
Traditionally, people used different heirlooms in different areas, because they knew they grew well there. They didn't have all of the high-tech pesticides that we do, so they had to concentrate on varieties that did well there. Remember there are some varieties that came from England but didn't do well in England. They became heirlooms elsewhere. Some do well in several places, but not well in others.
John S
PDX OR
Viron, My belief and understanding is that trees must take many small steps to unleash better resistance strategies, could take many many generations to evolve. Some principles can adapt in very little time as per one generation to figure out when to go dormant based on the growing season, and yes, this means identical twins are not exactly the same even though the DNA may be identical.
Take advantage of what you already know grafting in order to combine the best of two proven worlds (ie the proven resistances of what apples are better at and pears are better at).
I do think I understand you and hope you me? What I don't of course know is what the disease pressures are exactly (your new location). If fireblight is an issue and you need a good stem builder for the pear family that are more nailed by fireblight, naturally and of course, find a resistant apple clone. Currently the only apples that I know would work grafting to pears are Palmetta (one I tested) and Winter Banana, but I don't know if more compatible apples that are more fireblight resistant exist anywhere.
I reported that compatibility test result to Corvallis OR recently. They with many other species of trees that are resistant to various disease is invaluable to you.
I also know a stressed relationship of a graft that is even slightly incompatible can have dire consequenses to the tree. Nothing worse than stressing a tree 365 days a year unless everything is perfect like it is around Yakima. Therefore it is a good thing to stay away from dwarfing stocks. I see this in Alaska when I travel to the interior and it is very interesting. Trees are less likely to condition to the cold environment when "stressed" as such on a dwarf stock.
This I pulled from a science link published something new about plants:
"Notably, the LD protein is involved in the “vernalization” phenomenon, an example of epigenetic “memory” of previous environmental changes (6). The term “vernalization,” known for about a century (7), refers to triggering the flowering and reproduction process after the exposure to cold weather."
Viron, because I show the same interests as you I read lots of the free 'how to go about doing things' via patents and science papers. Any project is relatively a predictable method of going about it when narrowed down to pome fruits. For example the cherries group inside prunus, some of the different species of cherries carry a highly specialized way of detecting and sniffing out the presence of viruses. This occurs in prunus that is specific to the variety (clonally) and the family or species. Viruses also culture to specific forms, mutate, and jump species, all of which is going on at the same time in the host. Some clones within a species of prunus like 'shirofugen' flowering cherry are already very "hypersensitive", meaning, they sniff out a PDV or PRSV very quickly and starve the virus by going into self destruct mode.
Apples-Pears do not (that I know of) have economic losses from any viruses, they can have them, when they do they are not looked into that much. They can be destructive to grafting such as with some failed quince graft discoveries in nurseries. Therefore it is always better to purchase clean and verified source material. Wild material can be + infected material. It is least likely present in young seedlings.
The internet quotation I previously posted I had never intended to apply to local selections. I intended to produce some recent molecular ongoing discoveries into plant memories that were actually observed by scientists generations ago. The ability of woody plants to start recognition of a new location to adapt and even pass on the information to next generations.
When it comes to other destruction from fungal or bacteria the above host-pathogen strategies are very similar. The stem building method (referred to in previous post) works for many problems. Hybrid stems stand a better chance to combat than otherwise. The competition for food reserves seem to be processed by hybrids faster, the pathogens get less. However this hasonly been again "observed". No molecular proof.
Viruses are the only pathogens known to have sometimes the ability to pass through from plant to seed. Generally, subject to new discoveries.
Hi Rooney, I guess there’s a reason the National Clonal Germplasm Repository for pears is in your backyard and not mine Yes, Fireblight’s a killer over East… I have accumulated quite a file on ‘blight resistant’ pear cultivars, my main focus. It’s said none are ‘immune,’ only varying degrees of ‘resistance.’ Having planted an Asian and European pear of my own last spring, both are fruiting nicely ..though I’ve already lost a scaffold limb on the Asian to Fireblight..
I appreciate your advice on the stress of a ‘dwarfing rootstock’ and remain a fan of standard rootstock, cuz I like to prune 
..and the orchard I envision would consist of stand-alone (untrelloused) trees.
In my question above, I was more curious about an actual systemic ‘immunity’ than climatic adaptation… Or, can/ do fruit trees have the ability to form antibodies against viral or bacterial infections? Do they have an immune system ..guess I could actually google that… Can they ‘fight off’ viruses, or simply live with them? Perhaps a difficult to describe stupid question 
Your second response ..seems close to an answer. So, some fruit tree species have evolved responses to ward off various pathogens ... with an internal response, triggered by attacking pathogens.. such a response would be internal - an inherited capability.
The trigger to my question was to discern whether a ‘localized’ fruit tree can acquire a resistance ‘on it’s own,’ not having ‘inherited it,’ but from the use of ..an immune system, thus giving it (and hopefully it’s scion’s) the same immunity?
I realize, on some of the 300+ year old homesteads over here, someone could have planted hundreds of ‘pear seeds’ ... whereas a couple may have acquired, through their mixed genetics, an immunity or resistance to local pathogens, thus survived. If I were to ‘clone or propagate’ an orchard of such pears, it’s not that they’d ‘adapted to this climate,’ but that they possessed a blend of genes that fended off the local pathogens, right? As in ..the pear could look and taste like crap, but grows great 
Thanks, (all) Rooney.. I enjoyed your well researched answers to perhaps a stupid question....
Sorry but I guess I could have given a more simplistic approach by just saying no, plants don't have our kinds of immune systems or therefore are incapable of developing or circulating antibodies.
Nothing is ever a dead ended thought though. For instance bacterial fireblight could conceivably be used and injected into bodies of rats, then can develop antibodies. The hard part is how to deliver the fireblight antibodies to pear trees. (millions of dead rats for a good cause isn't bad either)
Lots of our current understanding of what computers have done for us in DNA sequencing have left us looking at many things so new that they are just beginning to be taught at the universities. 
RE: pathogens, Click the "show more" button inside the read of this 2014 article,
Sound Of Munching Caterpillars Puts Plants In Defense Mode
RE: plant memories, Since 2014 above, already there had been discoveries leading to protein structure inside that are the basis of memories (aka associations). The first finding involved seeing the protein inside of plants that began to fold upon stimulation of a fingers touch. When repeated the same touch day after day the plant began to learn there was nothing to worry about and as a result never folded. When touched in a different pattern to the same plant, folding of leaves again resumed.
The above experiments that are "real" are the beginning of discovering of what I think are "the equal" compared to our immune systems. Both the above examples are the products of RNA, a sub particle and the manufacturing of DNA. RNA is the random configuration of amino acids to perform complex and random shapes which sort of are not picked up by predators but are listened to by the plant. For example I have a feeling that there are a thousand sections sub-divided inside a tree with unique and different configurations of shapes of proteins (all random).
DNA will be 100% the same, but each unique, each will respond for a solution of successful episode in some yet mystical way, and report back to the adaptive training system that must exist.
Further indications that this can be possible is strengthened by observations made way back over 100 years ago, primarily as far back as studies in Russia by Michurin. A proper word that may define is "mentoring". Where it can be observed that one grafted branch of a northern sister of a shorter summer tree would learn the length of the year. Then when grafted to the 100% identical body back in a more southern location, many times faster, years later the autumn preparation of losing leaves reverts back to what is found to work best.
There is much to read upon the same by Karl King inside his collection and translation of Michurin on the 'bulb and rose' web site when it is online, which it sometimes isn't.
Viron: I recently searched for 'winter banana' apple as a possibility for fireblight resistance to build pear on, but unfortunately it is susceptible. If you are interested in testing my 'palmetta' I am in the process of fall air layering some of those at this years grafts to pear.
Phil was at the Leach gardens several years ago and in a slideshow had presented the way to seek for fruit tree resistance to apple scab. (your nemesis is fireblight but I don't see why it will make any difference)
Pollen collections are labeled from varieties thought to have resistance and bred to apples that have no resistance. The new seedlings are tested on a scale of about 1-10 for grading. This will indicate which donors of pollen carry the resistant trait.
Earlier you had a question in regards to viruses. In nature viruses stay with the fruit tree till death. Suppliers like in Corvallis do try to clean viruses by artificial temperature schemes, speed growing, and grafting the most brand new cells (micrografting). To determine if a tree has any particular virus, is to test it in a lab, or use an indicator graft, but this is a different question, and sometimes important to not start with a virus. (always sterilize pruning tools!)
Indexing starts with scionwood from a hypersensitive tree to the virus of concern. This is grafted on the subject tree. If the graft does not fail very fast then no virus is detected.
Breeders keep coming up with answers for new virus threats all the time. To give you an idea here is one recent answer to PPV virus,
Even though plants don't have immune systems in the way animals do, there must be reasons that some are susceptible to diseases and insects, and others are not. Some of that must have a genetic basis, and if so, mutations in either the germ line (reproductive cells, mutation results in next generation change) or by formation of sports (somatic mutation). For example, variegated plants are taken over by nonvariegated mutations that form, resulting in more vigorous growth.
Another type of change could be sports that become triploid, from a diploid tree. When I was reading up in the literature on triploid apples, I found one article from Italy about triploid variations from diploid apples. If I think of it and can find it, I'll re-read it and see if my memory is right.
Some things that might affect a plant's resistance to disease, are thickness of waxy layers, alterations in cell constituents, change in cell receptors. For trees such as American Chestnut, sports from near-killed trees might be more resistant due to mutations at the cellular level. There are genetic engineering efforts ongoing to create such changes in American Chestnuts on a permanent basis, using a single gene from wheat. GME Chestnuts for blight resistance
One more concept, and I'll quit blathering. There have been looks into whether substances such as aspirin can prime plant defenses against disease or insects. If a plant produces aspirin - or salicylic aspirin, similar to aspirin - and a mutation results in increased production, that could make it more resistant to disease. I know there is debate about the idea of spraying plants with aspirin water, but there is also serious research into the concept. USDA Research into aspirin for plants " pre-treating tomato plants—a relative of potato—with salicylic acid can prevent phytoplasma infections from occurring or at least diminish their severity." I've read that some plants increase their production of salicylic acid in response to injury or infestation, but I don't know how reliable are those reports.
Daniel: Good plugin concerning chestnuts. I never knew that any solution had ever a happy ending over the many manhours expended!
regarding: "thickness of waxy layers, alterations in cell constituents, change in cell receptors."
I would also never had thought of it myself until your comment that maybe thickness of cell walls are probably bigger barriers to the spread of disease and blights, as triploids and tetraploids have thicker leaves than diploids in apple and supposedly pear. Thicker leaf skin could leed to intuitive resistive strains against fireblight for pome fruit or chestnut blight in the case of chestnuts that way. So this could be novel!
I had run across some State of Michigan information and limited experimentation concerning this fireblight bacterium to apples (ie bud-9 rootstock).
"Fireblight infections in canopy stop when Bud.9 is the rootstock"
This observation is 5 years old. So I wonder if since then anything more as far as the mode of action as bud 9 being a vulnerable cultivar and yet as a rootstock it imparts fireblight resistance to above the graft. I guess in strange ways plants sure do know the importance of throwing out lots of variables. Maybe we are the ones they as plants are co-dependent on. In that regard I am wondering if it (b9) will also (with appropriate interstem) improve cultivars of pears.
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