
Getting the plants wet for long periods of time when fungal spores are present in the planting area is enough and would promotes development of leaf spots in zinnias as well as other plants. Clean sanitation practices and anything that makes the fungi "uncomfortable" will help minimize the chances of getting these infections. Not enough info was given to classify the leaf spots. Possibilities: alternaria leaf spot or cercospora leaf spot. I have seen some copper fungicides for cercospora useful on hydrangeas, like Bonide Liquid Copper Fungicide RTU Spray.