Why are my tomatoes rotting on the vine?
Hi- fruits that are directly exposed to high ambient temperatures and more than 6-8 hours of direct sunlight can become soft and fail to ripen properly. The extent of injury varies with cultivar, development stage and specific water, soil, and weather conditions. Secondary pathogens, such as bacterial soft rot infects fruits that are first damaged by insects, disease, cracking, heat injury, etc.
Maintaining good foliage cover around fruit clusters, planting tomato plants where they will receive late afternoon shade, harvesting as soon as fruit starts to turn color are some ways to increase the number of usable fruits.