Brown spots on tomato blooms are most likely an indicator of blossom-end rot, an environmentally caused tomato problem that destroys the affected fruit. Temperature control, soil nutrient balance, and moisture regulation can reduce or eliminate this problem. Preventive measures like mulching and consistent fertilizer application also help prevent blossom-end rot.
Blossom-end rot starts out as what appears to be a water-soaked area at the bottom of the tomato. This area browns and spreads rapidly. As the fruit matures, the spot will appear leathery and grow along with the tomato, eventually consuming the bottom half of the affected tomato. Blossom-end rot destroys the tomato and renders it inedible.
Blossom-end rot is usually caused by a calcium deficiency in the soil. Sufficient calcium levels support the nutrient demands of tomato plants. Your garden plot may need to be treated several weeks prior to planting to ensure adequate nutrient levels in the soil. Plants may need to be fertilized several times throughout the growing season; make sure you read the fertilizer label and follow the recommendations. Another source of blossom-end rot is inconsistent soil moisture. Allowing the soil around tomato plants to get too dry, or planting tomatoes in soil that does not drain well, both create environments that encourage blossom-end rot.
Once a tomato is affected with blossom-end rot, it is unlikely you will be able to save that particular tomato. Your best course of action is to remove affected tomatoes from the plant. This allows the plant to redirect its energy toward producing new and healthy fruit. It also protects the plant, as affected tomatoes are susceptible to other diseases, like fungal infections, that can compromise the health of the plant.
Resources like Cornell University's Vegetable MD Online and georgiadrought.org both suggest mulching around tomato plants to help regulate the soil's moisture. Regulating the soil's pH level and fertilizing tomato plants regularly are also popular recommendations to support healthy growth. One preventive tool--using calcium sprays on plants--has conflicting information. Cornell University calls calcium sprays "worthless," while North Carolina State University cites it as an effective prevention method.
According to the University of Rhode Island Landscape Horticulture Program, the following varieties are tolerant to blossom-end rot: Jet Star, Burpee VF, Better Boy, Early Girl, Flora-Dade, Floramerica and Walter. If you have had problems with blossom-end rot in the past, consider planting one or more of these varieties in the future.