From the Mailbag: My tomatoes and squash are rotting at the tip!

July 22 - P.K. Runkles-Pearson asks:

"I hoped you might be able to answer a question about a problem I've had with my tomatoes and squash this year. I'm not sure it's the same problem, but they are similar. The paste tomatoes (Super Marzano) are getting a brown soft patch on the bottom tip of the fruit. The brown patch spreads and eventually I have to pick the fruit and throw it away. The summer and winter squash plants are setting fruit, but the tiny fruits (1" or less) are turning brown, shriveling, and dying. If the fruits get past that one inch stage, they seem to thrive and grow normally."

What you're seeing on the tomatoes is called Blossom End Rot. Technically this is caused by a calcium deficiency; but in practice it's almost always due to sub-optimal watering habits. Overwatering is usually the culprit - calcium is rather water-soluble, so overwatering flushes most of it out of the plants' root zone. Severe underwatering can also lead to blossom end rot; but in our climate it's fairly difficult to underwater tomatoes to that degree, since they are somewhat drought tolerant.

There are sprays you can purchase that will help prevent blossom end rot, but the real solution is to solve the calcium shortage. First, rake some limestone into the soil around the plants (dolomite lime will work, and is easier to find - but if you can get calcitic limestone use that instead). Next, start paying attention to how you water your plants. Tomato plants do quite well when they receive an inch of water a week in our maritime climate (you may want to provide a little more than that when we have one of these unusual stretches of 90°+ weather, like we're seeing right now). If you're watering with a sprinkler, try putting out a few tin cans or other containers to help you judge the amount of water that's going down.

Note that these preventative measures will not remove blossom end rot from already-afflicted fruit - but it will help your currently-developing tomatoes to stay healthy.

The squash "rot" is actually normal - that's what happens to the fruit when pollination doesn't occur. As the bees locate your plants, this should become less frequent.

More information on Blossom End Rot and other tomato problems can be found in my article Tomatoes, Part Two: Problems. Another great resource - provided you make a mental adjustment for Keith's midwestern climate - is The On-Line Tomato Vine's Problems page.