Cleaning under bird feeders with rain/water

I've been feeding regularly since March using hanging feeders mounted on a pole and while I've cleaned them every 2 weeks I've never really cleaned the ground beneath them. I read today that the RSPB says to disinfect hard surfaces under the feeders as otherwise diseases can be spread which started to concern me a bit. My garden is concrete with a slight slope so water always runs down to the back and the feeders are situated in the middle above the concrete and the seeds tended to fall right underneath. So far I've always assumed that when it rains the ground will be washed/cleaned anyway and so it shouldn't be a problem, but I also didn't have any ground feeding birds until recently. A colony of sparrows have started visiting for about a month and half of them like to jump about under the feeders in order to catch seed/crumbs/find insects and they seem to have stopped doing that this week. This made me think that either they realised it wasn't worth foraging the ground (as I have trays to catch seed) or they became sick through contamination on the ground. Hopefully it's not the latter, but am I incorrect in thinking that rain or an occasional hose-down is good enough to prevent disease transmission?