I Have What I Believe To Be Some Type Of

I have what I believe to be some type of fungal infection that occurs on my feet. It mainly flares up during hot weather when my feet sweat. I am on my feet a great deal, and wear work boots a good portion of the day.
When I remove my boots in the evening and my feet start to dry out, there are patches that get very itchy, and if I scratch them, tiny blister-like places will slightly ooze. Application of cortisone cream seems to keep it in check, but doesn't cure it.
Also, and this seems strange, the problem seems more prevalent when I wear dark socks (washed in cool water, no bleach) as opposed to white socks (washed in hot water, with bleach).