Blossom-end rot
Blossom-end rot, a common disorder in tomatoes, is usually visible as quarter-sized black spots on the underside of the fruit. The result of calcium deficiency, it damages fruit by causing premature ripening. If soil pH is below 5.5, add lime before planting. An adequate supply of calcium in the soil alone, however, may not prevent ruin of the fruit. An even supply of soil moisture is necessary to make calcium available to plant roots. A calcium chloride spray applied to leaves of affected plants is a popular prescription, but this method likely has limited effectiveness because calcium reaches the fruit through the roots, not foliage. Prevention is the best route. Mulch with organic material and irrigate during dry spells to prevent extreme fluctuation in soil moisture. Remove any affected tomatoes; the plant may rebound and eventually produce undamaged fruit.