Re: black spot under toenail... very worried ! ! !
Consistently wearing open toe shoes will allow for dirt, dust, etc to collect around and under the nail plate. Dirt, dust, etc., contain large amount of bacteria which can become a full blown infection if you do not clean your nails really well every night. This means cleaning out any debris under the free edge and scrubbing the nails with a soft toothbrush and warm soapy water every day to eliminate any bacteria from deep within the nail folds.
The characteristics of pseudomonas bacterial infection is a green to brown to black spot under the nail plate. The nail plate will turn soft from the waste of the infection which is the discoloration - this is composed mainly of iron compounds. You can 'kill' the infection by keeping the nails clean and dry at all times; however, if it is already black, you may very well need medication from your doctor to totally kill all the bacteria. The stain will remain and will grow out with the nail plate.