So, one of the problems that you may witness in your tomatoes is blossom end rot. This condition is usually caused by uneven watering or lack of calcium in your soil. You can see a brown and mushy spot where the blossom of the tomato flower would have been.
Sadly, you will most likely have to remove the affected fruit, as it is likely to rot further as the fruit continues to ripen. However, you should be able to fix the problem for future fruits.
First, water your plants on a regular schedule. Try to ensure they are receiving an even level of moisture, and water deeply at the base of the plants as opposed to from above, this will help ensure the water gets down to the roots of your plants.
Photo from http://thegoodthing.org
Secondly, there are varying suggestions on how to do it, but you need to get the plant some more calcium. Try adding eggshells and epsom salts to your soil and water to increase calcium content in your soil.
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