"Hi Colleen. I have plenty of green tomatoes on my plants, but I noticed that a few of the tomatoes on one plant look like they're rotting on the vine. The bottom end of each tomato is dark brown. What's wrong? Does this mean I won't get any good tomatoes this year?"
Your tomatoes have blossom end rot. Blossom end rot is the result of calcium deficiency. However, that doesn't necessarily mean that you need to add calcium to your soil. Generally, blossom end rot is due to watering practices (your own or Ma Nature's.) A very typical situation is one in which the soil is allowed to dry out, and then the gardener waters when the plant starts wilting. The plant bounces back (tomatoes are resilient that way!) but some damage has been done. In dry conditions, the plant becomes unable to take up calcium in the soil through its roots. Then, fruit forms, and they soon start showing that telltale rot at the blossom end.
Another less common (but more common this year) situation is when plants are given too MUCH water. We've had a very cool, very wet spring and summer in many parts of the country, including my home state of Michigan. The plants are taking up so much moisture that the amount of available calcium is diluted, and, you guessed it: blossom end rot.
If the issue has been too much moisture, try side-dressing your plants with bonemeal to try to replenish the soil's supply of calcium. There are also a few brands of organic blossom end rot spray on the market. These are calcium sprays that you would spray on the foliage every week or so. It won't save any fruits that already have rot, but it may help prevent rot on future fruits.
If you have tomatoes with blossom end rot on your plants, simply remove them. They will keep growing and using the plant's energy, which would be better put to use producing new -- hopefully rot-free -- fruit.
I hope this helps! Anyone else seeing blossom end rot this year?