potato blight?

I planted potatos in ground that was in pasture last year. Decent soil tilled in the clover and we dug a trench 12" wide each row about 3" deep, planted the whole potato seed and covered with topsoil DH brought in from a friend who owns a nursery. Our soil is clay and we are zone 6. Everything looked great and it seemed all the plants were up, then yesterday i notice yellow and black on the tips of some of the plants leaves. It looks like they are curling up and dying with black crust. They are from Johnnys Seeds and are Yukon Gold and something else(not near my notebook) I have 350' of potatoes planted. What do I do? Last year I planted from the local market and they were fine....in another spot in the garden. We strive for organic gardening so if there is an organic solution that would be preferred. Most of the plants are not yet affected. TIA