Blackberry disease

Some photos attached of my blackberry canes which have taken a turn for the worse...

It's been looking pretty healthy - it's shot out some massive canes for next year which look green and vigorous. But several of the branches on this year's canes have gone brown and brittle, with leaves a mottled yellow/brown.

Away from the main crown, another cluster of new canes is coming through - about a foot away - but these new canes have also got yellow / browning. Tempted to pull all of those up. Being further from the main plant maybe they wouldn't be as healthy?

So any tips / thoughts? I assume it's some kind of fungus and I can't imagine treating it with anything (unless organic) would be a great idea. I've cleared out a lot of diseased lower branches so I'm hoping air will circulate a bit better.