Q: For two years my tomatoes have been getting a black rotting spot on the bottom just before they ripen. The damage is always on the bottom, no matter whether the plant is staked or the fruit touches the ground. I assume that this is caused by a type of fungus and have sprayed with fungicide without success. Any ideas on what’s wrong? — Vince Lamberti, Pittsburgh, Pa.
A: Blossom end rot, which is what this condition is called, is the result of not a fungal disease but a calcium deficiency. That doesn’t mean your soil needs a quick shot of calcium — most soils have plenty. Blossom end rot almost always happens when the plant can’t get enough water, which carries the calcium into the plant. You may also see the problem on peppers, except the spots are tan and look a little like sun scald. There is no quick fix, beyond mulching the plants to conserve moisture. Use a good organic mulch — shredded leaves or high-quality hay or straw, not plastic. And make sure the tomatoes get an inch of rain or the equivalent once a week. (Set tin cans in the path of your sprinkler, and see how long it takes to fill them with an inch.) Do those two things, and small tomatoes forming now will probably be fine later in the summer. If you see any fruit with even slight signs of the problem, pull them all off. They will never amount to much, and removing them diverts calcium into the healthy fruit.
Once you’ve mulched, you shouldn’t have to hoe around the plants anymore, and that’s good, too. Cultivation near the plants destroys roots, and that can bring on blossom end rot. So can too much water, oddly enough. Excess water suffocates root hairs, reducing the plant’s ability to absorb calcium. If your soil drains poorly or is in a low-lying area, that could be part of the problem, especially when there’s plenty of rain. Nitrogen fertilizer (or too much rich compost or manure) can also contribute to blossom end rot. Tomatoes grow like weeds and almost never need fertilizer of any sort. Nitrogen-rich fertilizer makes lush, oversize plants, and that sets the stage for a calcium shortage when fruit begin to form.
It wouldn’t hurt to get your soil tested, if you haven’t in a while. If the test shows that the pH has fallen below 6.5, add enough lime to bring the soil back into the 6.5 to 6.8 range. Lime contains calcium, which helps, but the main advantage is that calcium and other mineral nutrients are more available at that pH range. The lime takes two to three months to become effective, however, and fall is the ideal time to work lime into the ground, after the crops are harvested. You might also try switching varieties. No tomatoes are immune to the blossom end rot problem, but some are more susceptible than others, notably the plum and paste types. Cherry tomatoes rarely suffer from the condition.