Judith, I have heard mixed opinions on that advice. Some think it dates from an era when people planted wild black raspberries and blackberries which would then pass viruses on to their reds, or some of the nearly-native blacks had poor virus resistance. But, for buying virus-free, virus-resistant plants today it is outdated. I have taken this view and have them all planted closely. I have seen a few viruses on my plants but never the same virus on the blacks and the reds and all are still going seven years later. I would say it is best to not plant them directly next to each other.

On the other hand many cooperative extensions still advocate not planting them close together. Maybe there is something they know.

Do make sure to remove any wild plants you can get your hands on; I think everyone agrees that they are going to be the primary source of the virus that can sicken your planting.

Scott

Thanks Scott. Luckily, I don't have any wild blackberries around here, as I am in the city.
My blackberries are planted in the ground - I heard this cultivar is not supposed to sucker(Chester). I hope 'they're' right about that, since my roses are pretty close by. I was planning to keep the raspberries in a large pot, due to the fact that they spread like wildfire, and they would be placed about three feet away. Am I making a big mistake?
I need more land....
Judith

What sort of symptoms does one see from a virus? Are Heritage Red raspberries susceptible to powdery mildew?

I agree with Scott, my raspberries got rust from the wild blackberries growing 10 feet away. I am trying to kill the wild berries without killing the raspberries. Blue label round up does NOT KILL THEM. I am putting clear plastic down to let the sun burn them.