Plant Palette

Plant Palette

Blossom-End Rot

Photo of Jennifer Schultz Nelson

Jennifer Schultz Nelson
Extension Educator, Horticulture

If I were a gambler, I would have made a lot of money this week betting on the subject of homeowner's questions received in our office. We had at least one call every day on blossom-end rot in tomato. There was one day that it seemed like every other call was someone asking about blossom-end rot! It seemed appropriate to address this problem in today's column since it seems like a good portion of Macon County's tomatoes are suffering.

People who have never seen blossom-end rot are pretty shocked at first, even panicked. It looks like the tomatoes are literally rotting on the vine. Those that are aware of our Master Gardener Help Desk may call, wondering what sort of horrible disease their tomatoes have. In fact, blossom-end rot is not a disease. It is a physiologic condition caused by environmental conditions in the soil. It can also affect eggplant and peppers in addition to tomatoes.

The simple, but misleading answer as to what causes blossom-end rot is calcium deficiency. This is only part of the story. Calcium is a nutrient that is essential for normal cell growth, no matter whether you're an animal or a plant.

In a developing tomato, the tissues are undergoing rapid growth and development. So they need a lot of calcium to support this growth. If the supply of calcium is not there, the tissue breaks down. The characteristic early stage of blossom-end rot, with its dry, sunken, and leathery region develops where the tissue has broken down.

This leathery area may be relatively small, or may consume one half of the fruit or more. Secondary fungi move into this leathery region, leading to the more advanced actively decaying stage of the disease.

Usually, the calcium deficiency that leads to blossom-end rot is not due to a lack of calcium in the soil. More often, the calcium deficiency is caused by the failure of the plant to take up the calcium present in the soil. There are several reasons this may happen.

One very likely cause this summer is the hot dry weather and associated fluctuations in soil moisture levels. Calcium is transported into the plant while it is dissolved in water. Extreme swings in soil moisture between wet and dry will disrupt the movement of calcium into the plant. If this disruption occurs during fruit development, blossom-end rot is a likely result.

Probably the most effective management strategy for blossom-end rot is to regulate moisture levels through use of mulch. A layer of shredded leaves or grass clippings is a great choice to help hold in the moisture and slow evaporation in the heat of summer, plus it is very economical. Also remember that actively growing and fruiting plants need about one inch of water per week.

Another likely cause is over-cultivation of the soil near the tomato plant. Cultivating too deep and too close to the plant will break some of the roots. If enough roots are disturbed, this disrupts calcium uptake by the plant, and blossom-end rot develops. Be cautious when cultivating to control weeds near tomatoes. This is another reason why mulches are a good choice–they will help to prevent weeds from growing near the tomato plant, reducing the need to cultivate.

A less obvious cause of blossom-end rot is other ions competing with calcium in the soil. A great example is the ammonium ion, which is a component of ammonia-based fertilizers, and a source of nitrogen essential for plant growth. The ammonium ion competes with calcium for uptake by the plant. Even if there is plenty of calcium and moisture present, if there are too many ammonium ions around, the calcium will not be absorbed by the plant.

While you will likely see rapid growth of the plant, you have a good chance of blossom-end rot developing if fruits form. When too much nitrogen is applied to tomatoes, it is also likely that you will get lots of lush green growth, but no tomatoes. A better nitrogen source is nitrate, which will not compete with calcium for uptake into the plant. But the same holds true that over-application of nitrogen, even as nitrate, will push green growth, but likely inhibit fruit development.

There are some sources that suggest spraying plants with calcium chloride, or adding lime to the soil. This should only be done when a soil analysis indicates a true calcium deficiency is present in the soil, and then only at the recommended rate. The effectiveness of foliar applications of calcium is debatable. It is very likely that the plant will not absorb the calcium to a great extent, and that if absorbed, it will fail to move to the areas within the plant that it is most needed.

There are also some home remedies such as using egg shells and dry milk to add calcium to the soil. While these home remedies will not hurt the soil, there is some doubt surrounding their effectiveness at preventing blossom-end rot. Personally, I wonder about the scent that might develop from using dry milk in the garden after it gets wet and is warmed by the sun for awhile.

If you have other gardening related questions, call the Master Gardener Help Desk at 877-6872, from 8:30 a.m. to 4:30 p.m. Monday through Friday. There is no charge, and we would love the opportunity to help you!

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