Answer: Blossom end rot occurs at the end opposite the stem and consists of a round mushy black spot. It is essentially caused by a calcium deficiency in the soil and usually shows when there is uneven soil moisture. Potting soil can be unpredictable and often shows deficiencies, so this is a possibility. At this point you might want to try one of the calcium sprays formulated specifically to prevent blossom end rot. Also make sure your fertilizer
includes micronutrients -- for the same reason an occasional top dressing of compost might also be a good idea.
Black spots or dark sunken spots occurring on other parts of the tomato might be related to anthracnose which is a fungus and of course spread by rain or moisture from overhead watering. The usual control for this unfortunately is to remove infected plants. Other preventions are to avoid working in the plants when they are wet and making sure the soil is well drained -- if you are overwatering or if your potting soil is not a free draining one then this could be a contributing factor as well. If this is the problem, you will also need to replace the potting soil before planting tomatoes or other plants susceptible to anthracnose.
Another possible cause could be bacterial spot. Again this is a wet foliage problem but you should probably be seeing spots on the leaves as well as the fruit. Control includes picking and removing affected leaves and fruit and keeping the foliage dry.
You might also want to consult with your county extension (424-9485) for a more exact diagnosis -- especially if you think it could be anthracnose. I'm sorry about your plants.
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