Q: My wife noticed this on a blackberry. It looks like a fruit is growing from the cane.
A: It’s a blackberry knot gall, caused when a tiny wasp inserts eggs on the blackberry cane. The wound causes the plant tissue to swell and make protective cells for the eggs and larvae. The gall does little harm to the plant and you may not see one again for years.
I’d leave it alone, or maybe cut it off to take around as a conversation piece with friends.