Q&A: Blossom-End Rot

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Photo: blossom-end rot on grape tomatoes.

Photo: blossom-end rot on grape tomatoes.

Question: I think my tomatoes have blossom-end rot. There is a dark spot on the bottom of the tomato. What should I do? What other plants get blossom-end rot?

Blossom-end rot is a disease that strikes tomatoes, peppers and watermelons. It is not caused by a pest but instead by growing conditions. You don't need to worry about it spreading from plant to plant, though of course plants of the same type may develop it on their own. Blossom-end rot effects the fruit of the plant, whereas diseases like tomato late blight effect the foliage.

You can recognize blossom-end rot by a small wet area at or near the bottom of the fruit. This becomes darker and larger as the fruit develops, and it takes on a leathery look. On peppers, the spot will look tan. In tomatoes and watermelons, it becomes black. Photo: blossom-end rot on grape tomatoes.

A lack of calcium causes blossom-end rot. It could be that there's little calcium available in the soil, or, frequently, fluctuations in water hinders the uptake of calcium.

To prevent blossom-end rot, have your soil tested and if it shows to be lacking calcium, follow the recommendations for amendments. Generally this will involve applications of lime and/or gypsum a few months before planting time.

According to Charles Averre and PB Shoemaker, plant pathologists with North Carolina State University, you can aid already-planted tomatoes by "spraying foliage and stems with anhydrous calcium chloride (4 pounds per 100 gallons of water per acre) four times on a weekly schedule beginning when the second forms, or when symptoms first appear." They note this treatment is only for tomatoes, and to follow package directions carefully. If fruit already shows symptoms, it can't be saved, but you may be able to prevent the disease from striking new fruits as they form.

Careful watering goes a long way to prevent blossom-end rot. Too much or too little water, or periods of drought then deluge, increases the chances your plants will develop the disease. In hot, dry weather, water your plants. Mulch the soil to conserve moisture. Try not to disturb the roots of the plants, including those small roots near the soil's surface, when weeding. Damaging these small roots inhibits a plant's ability to draw up water and nutrients.

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