My potatoes were very happy unitil about a week ago. Now they have yellowed and are dieing. I thought it might be because they have bloomed and are ready to harvest, but when I dig around the plants the potatoes are very small. I can email photos if it will help.

What kind of soil were you growing them in? Is it a very loose medium?

Yes the soil is very loose and compost was added last year and in February before the seed potatoes were planted. Are you aware of any type of fungal disease that strikes potatoes? Maybe something similar to the Early Blight that strikes tomatoes. That seems to be what this looks like. I tried to upload the photos and it said the file was too big. However, it was a very small file.

Try using fish emulsion and corn meal and dry molasses on your potato bed.
The extra nutrients will greatly increase beneficial aerobic soil microbes in the soil, while "greening" up the plants. Plus the extra potassium from the molasses will help develop the potatoes.
The corn will encourage beneficial fungi to act as a generic organic soil fungicide.