Cob nut disease?

[Ian Pearson]

I planted a couple of bare root cobnut bushes back in January, and they grew away fine. Then I noticed one stem got wilted leaves, then dropped them. Then it spread to a couple more stems. On cutting through the stem, there is a dark stain running right through the centre along its length.
This seems a bit like bacterial canker that you might get on stone fruit, but I didn't think it effected cobs and hazels. I'm cutting hard back to beyond the stain, and using antibacterial cleaner on the secateurs between cuts. Does anyone have any ideas what it might be?

[Ian Pearson]