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Blossom End Rot of Tomatoes

One of the most common problems is the appearance of a water-soaked or slightly sunken dark colored spot near the blossom-end of the fruit. This problem is called blossom-end rot.

The malady starts as a water-soaked spot that darkens and enlarges into a widening circle until the first fruit begins to ripen. The affected area may remain small or it may cover one-third to one-half of the surface. As it gets bigger, the skin shrinks and the surface becomes flattened or sunken, dark and leathery. The fruit does not get soft rot unless bacteria invades the spots.

Blossom-end rot occurs when plants grow quickly and luxuriantly during the early part of the season and then are subjected to a period of dry weather when the fruits are still young and green. Abundant rain, on the other hand, may smother the root hairs and cause the rot to occur during sudden hot weather.

A calcium deficiency causes tomato blossom-end rot. When there is an excess of such soluble salts as ammonium, magnesium, potassium, or sodium the effective concentration of calcium salts available to the plant decreases more rapidly than that of other salts. Both excessively dry and excessively wet weather can adversely change the ratio of calcium salts.

To help avoid blossom-end rot, spread some super phosphate fertilizer, fertilizer that is high in phosphorous and low in nitrogen. Work it into the soil before you set out your tomato plants. You can use calcium sulfate, better known as gypsum, in place of the super phosphate. Apply either fertilizer at one to two pounds per 100 square feet of soil area. Have the soil checked for acidity.

Maintain a uniform moisture supply in the soil by mulching and irrigating when necessary. Do not hoe or cultivate closer than one foot to the plants to avoid root pruning. Mulch the plants soon after they have been set out using leaves, clean straw or compost.

For additional information, contact your county office of the NDSU Extension Service.

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