Strange canker disease on Plumeria
Hi folks. Over the past month I've noticed a strange canker developing on my Jeannie Moragne which is in it's third year from cutting for me. About a month ago I noticed some splitting on the trunk, figured it was just due to the trunk growing. Later, I noticed a sunken area - it covered about half the trunk, and seems to be gradually spreading to the point where I'm afraid it's going to girdle the tree (about three feet tall) and kill it.
It looks as if the living tissue in the outer bark has died down to the wood core. I noticed the same thing on a Kaneohe Sunrise cutting I had about two years ago near the soil line but when I raised the level of the plant in the pot it seemed to heal over. I took the attached pictures, and then looked around at my other plants and noticed something similar on a three year old Jungle Jack's Inner Peace.
Does anyone have any ideas of what this might be and how to treat it? I googled plumeria diseases and could only find the usual about rust, mildew and comments about how disease resistant plumeria normally are. No mention of this strange sort of girdling canker. I'm guessing I'll have to cut off the tree above the canker and re-root later in the summer but I'd really rather not.
And by the way, it's not sun scald as only a small portion of the trunk gets sun and we're just now getting out of our miserable June Gloom this week. And on the Inner Peace it seems to be starting on the underside of the branch.