What Causes Black Rotten Spots on Tomato Bottoms?

Symptoms

• Blossom-end rot presents as a dime-sized brown or black spot on the base of tomatoes. The spot then increases to the size of a half dollar or larger. The resulting lesion leaves a cavity in which other insects or diseases can enter the fruit and cause further decay of the fruit. Fruit that matures early in the season is more susceptible than those that are produced later in the summer. Fruit that is approximately one-third to one-half of their mature size are most often affected.

Causes

• The primary cause of blossom-end rot in tomatoes is a calcium deficiency. It does not necessarily indicate a deficiency of calcium in the soil, but a deficiency in the plant or fruit. Calcium is needed in high quantities for developing fruit for proper cell wall construction. Inadequate water supply or damage to the roots can also contribute to the formation of blossom-end rot. Periods of drought should be addressed with supplemental irrigation and hoeing, or cultivating of weeds should not be done close enough to the base of the plant as to cause damage to the roots.

Prevention

• To prevent blossom-end rot in tomatoes, prepare soil properly. Adjust the soil's pH to between 6.5 and 6.7 so the calcium present in the soil will be more available to the tomato plants. Use nitrogen fertilizer with nitrate as the available form of nitrogen. Do not exceed recommended rates, as too much fertilizer can impede calcium uptake and in turn cause blossom-end rot. Mulching around the base of tomato plants can also help prevent blossom-end rot by reducing the amount of moisture evaporation.

Control

• If all preventative measures have been taken and blossom-end rot still occurs, there are remedies. Ensuring that the tomato plants receive 1 1/2 inches of irrigation per week when the plant is producing fruit. This helps maintain an even moisture level and reduces the incidence of blossom-end rot. Sprays containing chelated calcium can be applied at a rate of 4 lbs. of calcium per 100 gallons of water. These applications needs to be done repeatedly as the tomato does not easily absorb calcium in this manner. Lime and gypsum can also be applied around the base of the plant to increase the supply of calcium. Know the pH of the soil though, before adding lime.