It might be Cedar-Quince Rust. I'm not aware of a treatment, but that's not my expertise, so I'll defer to others. I had an Eastern red cedar in my backyard with a large, similar looking patch on the main trunk. The patch was most obvious when the trunk was wet. Here's how the tree looks now.
Yeah, didn't turn out too well for the tree. I figured I might as well make use of the dead spar.
I'm familiar with Gymnosporangium forming perennial cankers/galls on junipers but have never seen it on the main trunk as pictured above. This is not a good thing I'm sure.
In any event, treatment on junipers is typically not recommended and usually focuses on removing these hosts and applying fungicide to rosaceous hosts.
Nursery and orchard owners have been known to be rather aggressive and persuasive when it comes to removing this life cycle host as well.
Quite right JD, eradication of the alternate host is a legitimate preventative for macro-cyclic rusts. What's interesting and potentially confounding is the assignment of "primary" and "alternate" or "secondary" status. For practical purposes, those labels are assigned with the crop plant being "primary" and the less commercially important host being "secondary". Sure, up here in northern New England, the rosaceous host (apple, quince, etc) is considered primary and the juniper we call "redcedar" is secondary. However, in those places in the mid-South that harvest veneer logs from that same species of juniper, well, juniper is the primary and all of the wild/naturalized apple is secondary and subject to eradication attempts.
Just wanted to make the point that "primary" and "secondary" designations arise from commerce more than biology.
The short answer is to engage in phc that is consistent with general phc plus a bit sunnier canopy. The long answer is very long. The most important pieces of information are that:
I defer to others about which rust it is. I cannot tell the difference between, e.g., the apple, quince, and pear rusts. I am not sure that the exact variety matters and once again defer to others about that.
The rust spreads very far on wind.
The likelihood of alternate hosts within its range approaches 100%. In my area, flatwoods plum and hawthorne are common in natural areas.
UV kills it, but the orange blocks UV to an extent. Strategic and minimal thinning may thus be helpful. In general, expect more rust in shaded areas, so often on lower branches or in forests rather than fields. Thinning/culling adjacent trees and thinning the upper crown may help in some cases. The "system" needs consideration as a whole before deciding whether to thin, where, and how much.
It is normal for a cedar to have rust. One of mine does and I suspect the others do too. Here are pictures:
The first photo is whole tree about 8 months after pruning. The goals were to:
1. Thin branches that were too close together on the main trunk for the long term.
2. I hope to get larger branches for aesthetic appeal, so removing some ought to redirect growth into the remaining branches.
3. I reduced a couple branches that were competing with the top for dominance. I tried to flatten them simultaneously and get some motion into them. I still have one more to reduce, coming high and straight at the camera. To really get structured arsthetic motion I likely have to prune back to first interior growth. I may or may not do that and time remains on my side to decide that.
4. One branch was girdling the main stem half way up, so it was removed, leaving a noticeable gap.
The second photo shows rust at the base of the trunk. Gilbert points to it...
The third photo is a 1.5-inch diameter branch with rust.
The fourth photo is what I would call a rust canker on .25-.5-inch stem growth. Please correct my terminology as needed. The swollen area has various dusts and flappermcflappies that are orange and change with progression of season/moisture. Right now there is no orange to be seen.
The area around this cedar is in transition. I was first able to work on it just before loss of low limbs from adjacent vegetation shade would have happened. The tree has more sunlight due to clearing. It also has more sunlight to lower branches from having pruned it so. It also has presumable moderate stress from soil compaction - the car tracks are new. The "system" is okay for pruning to promote uv degradation because I was going to do it anyways for aesthetic and structural reasons, and the leaf flush of the whole tree - including low limbs - was very good. The compaction and site change point a bit towards not pruning, but I chose to prune.
Does any of this seem to bear on your tree, @surveyor ?
Yes, I would say that the Cedar is in the heavy shade under mature hardwoods, but not with a congested branch structure. It is a sort of run of the mill red cedar that only my sister seems to have much regard for (she loves cedars). There is a pear tree about 100' away. She is concerned also that the cedar may infect her magnolias which she planted along moms border, some within about a 100' also. I suppose I should mention that there is a 6-8' tall cedar that had the top half dead and brown, with the bottom half green and healthy, she seems to have saved, by cutting the top completely off. She is treating the orange rust with a copper anti-fungal spray she has on hand. Thanks to all for your suggestions and comments.