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Garden Q&A: Blossom end rot a calcium deficiency

Jessica Walliser
Blossom end rot on a tomato

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Saturday, Aug. 23, 2014, 7:43 p.m.

Question: My tomatoes are ripe on the top, but they are rotten on the bottom ends. What is happening?

Answer: I believe your tomatoes are suffering from one of two possible issues: blossom end rot or a fungal disease. Because we've had such a wet season, I suspect the latter may be the issue, but I will describe both and allow you to determine which you think is the probable culprit based on my description.

Blossom end rot is an affliction that begins as a small sunken canker at the bottom of the fruit and quickly develops into a large, sunken, black lesion. Unlike disease issues, blossom end rot is not caused by a bacteria or fungus, nor is it something that is caused by an insect pest, despite what many people commonly think. It is a disorder caused by a lack of calcium in the developing fruit.

During the growing season, tomatoes develop at a very rapid rate, and they use a lot of calcium in the growing process. When there is not enough calcium present, the fruit tissue breaks down into the sunken lesion you see at the bottom. Many times this lack of calcium is not caused by an actual soil deficiency, but rather by inconsistent watering.

Because calcium moves into a plant via water, if the plants are allowed to dry out between waterings, a calcium deficiency is the result. Blossom end rot is especially common in container-grown tomatoes or during years of inconsistent rainfall.

The reason I suspect blossom end rot is not the issue is because we've had plenty of consistent rainfall this summer. In fact, the frequent rains that have occurred throughout the growing season are creating the perfect recipe for many fungal and bacterial diseases.

And so, without seeing the fruits, my guess would be that your tomatoes have developed one of several common fungal issues. My tomatoes, and the tomatoes of many other local gardeners, are suffering from various bacterial and fungal blights and leaf spot issues. At this point in the season, there really isn't much you can do about them. Organic fungicides based on Bacillus subtilis (Serenade) or copper are an option, but when things are this wet, there is little we can do.

To help stave off fungal issues in future years, there are some cultural practices every gardener should be following each and every season:

• Always rotate your crops. If possible, do not plant tomatoes in the same place year after year. Some fungal spores can overwinter in the soil.

• Space plants properly to allow for plenty of air circulation. Fungal spores love wet conditions.

• When irrigating, try to keep the foliage as dry as possible.

• Some diseases are soil-borne. To keep the spores from splashing up onto the leaves, mulch the plants immediately after planting. Use a 2- to 3-inch layer of compost, straw or hay to mulch them.

• Avoid over-fertilization, especially the chemical kind. Tomato plants fed excessive amounts of fertilizer produce lots of green growth that crowds the fruits and traps humidity against them, encouraging fungal diseases to take hold.

Horticulturist Jessica Walliser co-hosts “The Organic Gardeners” at 7 a.m. Sundays on KDKA Radio. Her website is

Send your gardening or landscaping questions to or The Good Earth, 503 Martindale St., 3rd Floor, D.L. Clark Building, Pittsburgh, PA 15212.

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