Something I never understood is why trees seem to be ill so rarely. There is a gazillion infectious agents on Earth from viruses to fungi, mammals are barely keeping up even with our adaptive immune system and fever. Plants lack all of that and yet some trees live for centuries or millennia when undisturbed.
I realize that are many factors skewing the observation: trees may be sick without us noticing, most of the trees die very early and we observe ones that get mature, some trees are in fact visibly sick etc. What I don't know is if these factors combined explain it all or not. My gut feeling tells me that without an adaptive immune response trees must be sick way more often than we see, but I have no proof.
I imagine the hard walls of plant cells helps keep bacteria and virii at bay, but there's plenty of fungi that infest trees. Oak wilt is a rather pernicious local threat.
The programming language Rust is named after a type of fungus that attacks plants.
It's out there, but trees don't run around screaming, so it's not something that people tend to notice.
It may be that you don't see the illness/infections unless you are looking for them or until the trees die off in sufficient numbers. I look for interesting things growing near me while walking or hiking and around here I see galls from various insects like wasps and aceria, leaves covered in bright gymnosporangium rusts, and colonies of wooly adelgids. In the summer the deciduous trees tend to get powdery mildew, and I know there are some more harmful fungi in the region which seem to be damaging living trees in a way that is not common, but also not new.
Sometimes you can see tress dripping sap from cuts or other damage, which I understand to be part of how they defend against disease. My parents had a tree that I remember dripped for many years before falling over in a storm. They also had a cherry tree that had a ring of dried sap bubbles on the stump of a large pruned branch.
If you see some mistletoe up on a branch, that's a parasitic plant. If you see an insect in the bark, it's probably munching away. If you saw a human being consumed by that stuff, you'd think they were monstrously ill.
Dutch Elm Disease killed a whole bunch of trees in Edinburgh, Scotland.
I know a lot of the old Elm trees in Princes Street Gardens, beneath Edinburgh Castle, were replaced by Limes. Of course the problem is more widespread than than just the one city, or the one country:
I realize that are many factors skewing the observation: trees may be sick without us noticing, most of the trees die very early and we observe ones that get mature, some trees are in fact visibly sick etc. What I don't know is if these factors combined explain it all or not. My gut feeling tells me that without an adaptive immune response trees must be sick way more often than we see, but I have no proof.