Most plant diseases are specific to one host or species, so while dogwoods are susceptible to cankers (such as Crown canker, Phytophthora cactorum), they are caused by different organisms than the cherry canker (Pseudomonas syringae), so no need to worry about that particular disease.
A trowel in the hand is worth a thousand lost under a bush.
Go back to below all the plain green, even if that means taking off some variegated bits too. Go a bit at a time starting from the top of each green-leaved shoot if you're not sure, and keep cutting back a bit further until there's no plain green left.
I'm not so sure that it isn't a sucker. Could the knobbly bit where the two branches come from be a graft?
Doncaster, South Yorkshire. Soil type: sandy, well-drained
I’ve no idea, @Jenny... I thought that reversion would have affected the entire right hand branch, from the whole stem upwards, yet about 2/3 of the branch is variegated and the rest is green. Does reversion only happen in increments then?
It can be weird. I have a yellow chrysanthemum that occasionally produces a shoot with pink flowers, but sometimes the reverted shoots have some flowers that are half pink, half yellow, divided straight across the middle.
Doncaster, South Yorkshire. Soil type: sandy, well-drained
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Glad that cherry canker won’t affect my cornus.
I thought that reversion would have affected the entire right hand branch, from the whole stem upwards, yet about 2/3 of the branch is variegated and the rest is green. Does reversion only happen in increments then?