Some of my tomatoes (mostly Roma) have what looks like black mold inside. The tomatoes look perfect on the outside, but they are black in the center pulpy area. Any ideas on what this is and how to prevent it?
Even though you don't see any spots on the outside of the tomatoes, what you describe still sounds like blossom-end rot or even a small area of sunscald. All that is necessary is a small area, which allows a secondary decay-causing organism to enter the fruit. Decay begins quickly after the organism gains entry into the tomato. Sunscald results when plants are exposed to a combination of heat, humidity, and intense light. Blossom-end rot is associated with a calcium deficiency, fluctuations in moisture, and heavy applications of nitrogen fertilizer. It may not be a lack of calcium in the soil, but rather inconsistent watering that causes the calcium to be unavailable to the plant. The plants need a uniform supply of moisture and mulch to maintain that moisture. Any use of high-nitrogen fertilizer should be discontinued.