You have described a disease called \"blossom end rot\" and it is caused by a lack of calcium available to the plant. Most often due to inconsistent watering. The cure is to water plants evenly (consistently) and only when needed. If the soil is too acidic (below 6.0 if you check it with a soil acidity test) than add limestone which has calcium in it. Using a fertilizer too high in nitrogen can also make calcium unavailable to the plant. I use a slow release plant food like Osmocote in all my potted plants both vegetables and flowers because it is a balanced plant food with all the micro nutrients. No need to toss out your tomato plants, but you should remove the infected fruit. There is still time to correct the problem with proper watering (feel the soil by poking a finger down one inch - if dry than add water until it runs from drainage hole) and proper fertilizing. Keep growing, Marianne Binetti