I got a little sidetracked, I (Holly) had a final exam. Let's talk tomatoes. Everyone loves to grow them and it is very satisfying to see them out there in the yard, doing what they do. What isn't satisfying is to go out there and see some fruit with a black patch on the bottom.
<cue the sad trombone>
This is called blossom end rot and it isn't a disease but rather a physiologic problem with the plant: It isn't uptaking calcium correctly.
"WHY!?" I hear you cry as you drop to your knees and shake your fists at the heavens...
There are several reasons but the most common ones for around here are cool temps in the soil and inconsistent watering.
I know that for those who aren't garden tinkerers, knowing when and how much to water is one of those things that probably kills more plants than anything else. When do you water? How much do you water? And how do you water?
First, tomatoes don't want to be stressed. Don't let them get too dry. If you can stick your finger into the soil and it's bone dry more than a 1/2" down, it probably needs water. If you can't stick your finger in the soil because it is too hard and packed... Yeah, it needs water. Leaf droop, wilted looking ... all of that means it needs water. BUT you don't want to water it too much either. Stress is stress.
If you are growing in pots, try not to fuss too much. Make sure the pot is plenty big with great soil and a tomato focused fertilizer, if you can.
Here's the kicker: provide the plant with bio-available calcium when you plant it in its spot for the summer. You can also provide this to cabbages, peppers and berry plants, too.
I have some pulverized sterilized homemade eggshell powder to give with each tomato plant sold. Not that much is actually required!
(btw: photo credit to the Missouri Botanical Garden. They are great. Their stuff is more technical and I use it for research. They have a wonderful photo database. No name credited to the specific photo so to whoever: thanks!)