I have noticed as my tomatoes are starting to ripen that the fruit on one of my plants is turning dark and looking as if it is spoiling. My plant appears to be healthy and thriving. What is causing this?
If this is on the bottom of the fruit, then it is due to blossom end rot. Here is more information: https://www.gardeningknowhow.com/edible/vegetables/tomato/tomato-blossom-rot.htm