Fungicides that are safe to use on tomatoes include copper octanoate and copper sulfate, MANCOZEB, chlorothalonil and maneb. Gardeners most often use the copper-based fungicides for early blight, though they are also useful for late blight.
MANCOZEB is a fungicide based on zinc, and it fights both late and early blight. Apply it as soon as the seeds start to germinate, and apply it every week or so. Discontinue its use at least five days before picking the tomatoes.
Maneb and chlorothalonil are both man-made fungicides that tackle early and late blight. Use maneb until five days before harvesting, while it is safe to use chlorothalonil right up until the harvest.