Apple trees (pommiers) here in the Loire Valley are tortured by mistletoe (gui), which is a parasite. Le gui sends "roots" called suckers (suçoirs) down into the wood of apple tree branches and slowly sucks the sap and life out of its host. It's known to live on poplar, willow, locust, almond, linden, and certain pine trees as well. I'm talking about European mistletoe; there is also a variety of mistletoe that occurs only in North America. (Here's a link to a series of posts about mistletoe that I've done over the years, with photos of course.)
Here's what the big apple tree in our back yard looks like these days. It appears to be pretty old (but we don't know how old because we've only lived here for 16 years) and it is definitely in a weakened state. It's full of gui. It has lost several big branches over the past five years, and it is really lopsided and starting to lean. We figure we'll have to have it taken out before too much longer — or just wait for it to fall over. The trunk is badly split, and mushrooms grow around its base. That's a bad sign.
And here's a smaller apple tree out by our back gate, in front of the garden shed. It's also full of gui. From what I've read, one way to remove the gui is to "dig" it out by cutting the green part of the mistletoe plant off at the base and then scraping the surface of the wood it's been living on to try to remove all the suçoirs. The danger is that the scraping leaves a wound that can be attacked by other parasites and pests, or simply by humidity, rotting the wood and further stressing the tree.
This is a tree that has finally been killed by, I assume, a combination of gui and the weather. It grows on the far side of the pond that's outside our back gate. Monsieur Denis the elder, who owns or at least used to own most of the vineyards around our property, once old me that this was the tree that produced the very best pommes out of all the dozens of pommiers growing in and around the hamlet. He's handed the grape-growing business business off to his son and DIL now, and he has such debilitating mobility issues that we never see him out in the vineyard any more.
And finally, here's another solution to the gui problem: radical pruning. Our neighbor who lives most of the year in the Paris area just had a man over to prune the four or five apple trees that grow on her land. She told me in September, when she spent a few weeks in her house in the hamlet, that she was disappointed to see so few apples on the trees in 2018. I assume some professional told her that this is a way to try to bring the the trees and apples back. I hope it works.
I'll be curious to see how these trees fare in the spring. Maybe our trees need the same pruning. The strange thing is that we have two fairly large apple trees in our yard that have no gui growing in them at all. So I think certain varieties of apple are more susceptible than others. Another pommier, just outside our fence, is so full of gui that it can't possibly remain standing much longer. Our pear tree died last winter, but there was no gui in it. And our neighbors across the street lost two big pommiers last winter too.