This is the second year of my everbearing raspberry bush. Some of the leaf tips have started curling and are black. I found other sites saying that it was a disease and some that said it was windburn/frostbite. Some of the leaves also have a reddish hue to them. I've read that is a lack of potassium or something and to just add fertilizer to it. Any recommendations? Do I need to dig up the whole raspberry bush or is salvageable?