Blossom End Rot

When tomatoes are slow to ripen it is due to the cool summer, but even delayed, the taste of a ripe tomato off the vine is still worth the work a gardener does all summer long.  One of the most frequent questions I get in the office is, “Why are my tomatoes developing a brown or black leathery spot on the ends?”  This condition is called blossom-end rot. 

Blossom-end rot is caused by a lack of calcium in the developing tissues of the tomato, but is not necessarily caused by a lack of calcium in the soil.  The condition is a result of slowed growth and damaged roots caused by any of several factors including excessive nitrogen fertilization, high soil salinity and high levels of magnesium and potassium in the soil as well as any gardening task that may have damaged the roots.  The reason for most blossom-end rot questions that come into my office are variations in watering.  Extreme variation in soil moisture, such as letting the plants begin to wilt, then soaking them until the garden becomes a mud hole, is a contributor to the problem.

So how can blossom-end rot be avoided?  Dr. Bob Gough, MSU Horticulture specialist, says, “Plant cultivars that resist blossom-end rot.  Mulch your plants to keep soil evenly moist.  Organic mulches work better than black plastic for controlling blossom end rot.  Cage your tomatoes or let them sprawl on the ground; don’t stake them and don’t prune them too much.  Lastly, calcium nitrate sprays applied about a month after transplanting can effectively reduce symptoms where this problem is severe.”  Tomatoes with blossom-end rot are still edible.  Just cut of the damaged part and eat the rest.

For more information or other horticulture questions you can call the Roosevelt County Extension Office at 787-5312 or visit our website at or visit Dr. Bob’s garden guide at




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