The year that I planted my raspberry canes, I watered them about every 3 days. This turned out to be a little too often and about 3 or 4 of the 50 plantings started to develope root rot. At that point the roots were not interconnected between the plants. I backed off on the watering frequency and that was the end of the problem.

Are you getting any breaks between water intervals? You may need to get the infected plants and their surrounding soil out of the row.

I agree. Raspberries don't need much water at all. I wait until mine begin to wilt for a day or two and then water. Otherwise, don't water at all. I honestly didn't have to water mine at all this year until the drought hit in late June / early July. The occasional rain here up to that point was enough to where I didn't need to water until the drought.