Blossom-end Rot

One of the most disheartening moments in my garden is when I plunge my hand through the tomato leaves, closing my fingers around a juicy red tomato...

... and pull it back to find the underside taken over by a nasty black splotch.

This is blossom-end rot. It occurs on the blossom end of the tomato (duh) and is a result of inadequate calcium at a critical stage during the fruit's growth. This deficiency means the cell walls are not sturdy enough, and results in a soft spot on the tomato, which then begins to rot while the tomato is still ripening.

What causes the calcium deficiency? It can be a lack of calcium in the soil itself, but it can also be a result of inconsistent watering or soil that is too dry or too wet, all of which affect how calcium is absorbed by the plant. Basically this disease is one that needs prevention, not treatment, since there's nothing you can do to reverse rot! If you get a lot of rainfall, unfortunately you may not have control over moisture levels, but in general it is something you can deal with by being diligent.

Fortunately the problem doesn't seem to be widespread in my tomatoes. These Amish paste tomatoes are an indeterminate variety, so they are continually flowering and producing new fruit throughout the season. So even though a few were deprived of calcium at one particular time, others seem to have gotten the nutrients they need and are ripening just fine.