That's Blossom End Rot. It's a physiological (not infectious) problem. Try to keep even moisture to the plants, and not too wet - an organic mulch (hay, compost, etc) will moderate changes in soil temperature and moisture. A consistent drip irrigation system will help, also. Plus, calcium is supposed to help prevent this - there are commercial sprays that work against Blossom End Rot (Calcium and other minerals, no pesticides). Many people think that putting crushed eggshells in the garden before you plant tomatoes will help with this, but for now, moderating the fluctuations in water, and using one of those sprays are your best options.

Thanks for the feedback! I've tried to be careful with them, but it's getting late in the season to be growing them I guess, isn't it? The heat really dries them out. I cut the 5 bad tomatoes off the plant tonight, and I'll watch the rest to see whether they start to show signs of rot. Thanks again!

You can get a 50lb. bag of Garden Lime (Calcium magnesium carbonate - crushed limestone) at HD for about $10. This will last you a long time and it's a good cheap source of calcium for your potting mixes. Put a tablespoon in each gallon of mix and you will have a sufficient calcium source for most of your plants needs, calcium builds strong bones ;-)

Now, about the watering. BER will occur IF you water too much or too little because either will change calcium absorption. It's possible to get BER if the plants wilt even once. It's just going to take you a year or two to get the hang of how water your container plants need at various times of the year.

Tom

Tom advice is right. This problem usually happen with paste varieties. I am growing a few paste tomatoes now that are doing really well, Chico, Roma and Plum Regal. I water every day in the morning and my mix has lime in it.

The bowl has the roma and the one on the side is plum regal

Silvia