Answer: The rotting problem at the tops sounds like it may be caused indirectly by sunscald which is then followed by a fungal infection. There is no particular cure for this except to make sure that your plants are healthy so they have the maximum amount of foliage to shade the developing fruits. (It is not related to Blossom end rot.)
Blossom end rot is ultimately caused by a calcium deficiency in the soil but often only affects the tomatoes when the soil moisture level is allowed to fluctuate. In a dry year many gardeners will have trouble with that because their watering is not keeping the soil evenly moist.
With regard to fertilizing and improving your soil, the best way to keep it healthy is to run some basic soil tests and see what (if any) specific nutrients or amendments are needed. Your County Extension (684-3001) can help you with the tests and interpreting the results. I would caution you about adding wood ashes particularly during the growing season as they may have some drastic (but short lived) effects on the soil which could adversely affect your crop.
In general it is important to clean up, remove and destroy any fallen leaves or vines during the season and in the fall to try to avoid carrying any problems over in to the following year. You will also want to rotate your plantings so that you do not plant tomatoes or their relatives in the same spot for several years. Your County Extension may also be able to diagnose the problem with your Sweet 100's tomatoes and possibly suggest additional control measures.
Enjoy your garden!
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