I got this last year and I think the main problem is not enough air circulation on my seedlings in the basement (which tends to be a bit damp). I'm not too concerned since last year it pretty much went away once I moved the plants outside. I lose some leaves from it though. This really is more curiosity because I'm fairly sure now that I'm moving plants outside it's going to go away again this year.

Anyway, it appears on the underside of the leaf in small lesions of fuzz... slightly whiteish though blends in with the leaf aside from texture. Eventually the leaf curls up and dies.

It also effects the stem and leaves some scarring in patches.

Only have seen it on potato leaf plants, though I don't have enough of a dataset to say if this is just a coincidence or not. My brandywine's last year were the only ones that got it and they were my only potato leaf. This year I've got a lot more varieties and both my brandywines and cherokee purples appear to be effected. Haven't noticed it on the bloody butcher's though.

I'm thinking a mold/mildew, but it doesn't look like powdery mildew as I've seen before. I'll see if I can get a decent picture of the lesions. Any ideas though?