Experts and BHG readers answer.
fungus on tomatoes
It sounds like you might have encountered Late Blight. Late Blight is an airborne fungal disease thats most common after cool, wet weather. Late Blight causes dark leaf spots. The leaves usually turn yellow and drop off. The best way to control any fungal disease is good hygene and crop rotation, but tomato blights are hard to get rid of once they are established in the garden. Pull up and get rid of vines. Dont put them in the compost pile because they can live there unless you turn the pile often and it really heats up. The fungus can overwinter in the soil, so rotate your plantings every year. You may be able to keep blight down by mulching heavily and never watering your tomatoes from the top. Water the soil at the base of the plant, dont let the water splash soil on the plants, and space plants far enough apart for good air circulation. Good luck.