When you say gummiosis do you mean where the lesions leak a sticky substance?
I think that is a symptom of bacterial canker

Yes that's right. Any ideas what I can do to save it?

Its not easy, we have it in two big Prunus avium, we did let a tree surgeon see them and as the trees were generally healthy the advice was to leave them and see.
More difficult for your Stella, I googled it for mine and one or two suggestions came up about removing old leaves etc. Some advised copper sprays......may be worth you having a look at some commercial,landscape and forestry sites.