Answer: There are a number of problems that can cause black or dark spots on the leaves, but in all cases you would remove and destroy the affected leaves as soon as they occur to prevent reinfection. Also, avoid overhead watering especially at night when there is no sun to help dry the leaves quickly. Rainy cool weather helps spread many foliar problems but overcrowding and lack of air circulation can also contribute to the problem, so you may want to consider pruning a bit if the vines are very thick. It is usually better to water tomato plants less often but deeply rather than a daily light sprinkling. The deeper watering encourages roots to grow deeper to where the soil tends to stay more evenly moist and this will assure the plants are not subjected to moisture stress -- very important if the season is a dry one. It also saves on water. The skim milk and water mixture you've heard about is sometimes used to prevent blossom end rot in tomatoes. Blossom end rot, or catfacing, is caused by lack of calcium to the fruit and the skim milk supposedly helps this. However, it won't stop or cure a fungal disease. It's late in the season so the best approach for you is to remove the affected leaves and hope for warm weather.
Best wishes with your tomatoes!
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