To grow tomatoes without the black scabby circle on the bottom, keep your watering consistent and don’t damage the tomato roots by tilling too close to the plants.
“I had these really beautiful tomatoes,” the caller told me, “but when I went to pick them there was a black scab on the bottom. What is this, and can I still eat the tomatoes?”
This was a typical August call into GardenLine, my weekly radio program. At one time or another most gardeners will have a similar experience. The dark spot is a condition called “blossom-end rot.” At one point we thought that it was the result of insufficient calcium in the developing fruit, but now we know that stress is the issue. If the roots are too wet or dry, or the plant is under stress from high winds, for example, this black scab is likely to appear.
Excessive fluctuations in soil moisture can also result in that ugly brown scab. Tomatoes grown in containers that are allowed to get dry before being soaked may also develop blossom-end rot. Another potential cause? Roto-tilling or cultivating too close to the plant. Tilling the soil too deeply, or too near the stem will cut up the root system and stress the plant.
To prevent blossom-rot, keep the soil evenly moist but not wet. A layer of mulch around the plant can help; dried grass-clippings, chopped leaves or thick layers of hay work well. If you control weed growth using a hoe or roto-tiller, stay at least two feet away from the tomato stem.
Ignore those who tell you to spray with calcium, add egg shells to the hole or sprinkle your garden with 
Tums. Read all about how some myths are hard to kill: Coffee for Roses: …and 70 Other Misleading Myths About Backyard Gardening
And if you do have blossom-end rot this season, take heart. The first fruits are most severely affected, and the tomatoes are still edible. Most tomato plants grow out of it as the plants get larger. Cut off the ugly parts, garnish with homegrown basil, and repeat the gardeners mantra: “Next year, it will all be perfect.”