NDSU Extension - Stark & Billings County

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Rotten Tomatoes

One of the greatest joys in summer is to find the first ripe tomato in your garden. That said, one of the greatest disappointments in summer is to discover your first tomato is rotten!

This is blossom end rot—the #1 threat to tomatoes today.

Blossom end rot (BER) is a complicated problem, but it is most often associated with a lack of calcium in the fruits. Without enough calcium in their cell walls, the tomato fruits collapse at their bottoms.

BER is also associated with drought, salinity, high temperature, low humidity and high light intensity (all may occur in North Dakota).

What’s the answer? We need to get more calcium to the fruits.

Don’t focus on the soil. Most garden soils in North Dakota have plenty of calcium. Home remedies such as adding eggshells and Tums do little good; adding Epsom salts may worsen the situation. 

Focus on watering. The uptake of calcium depends on the uptake of water. Irrigate regularly. Avoid extremes of waterlogged and droughty soil. Mulch to maintain consistent levels of moisture in the soil.

Cultivate shallowly. Don’t damage the roots of your vines. We need these roots to absorb calcium.

Do not overfertilize. Avoid fertilizers containing ammonium nitrate (including 10–10–10). Ammonium ions compete with calcium ions for uptake by roots. Calcium nitrate is a better choice. Fertilizers high in potash are also undesirable because potassium ions contend with calcium ions for uptake.  

Avoid lush plants. Tomato leaves compete with the fruits for calcium. Lush, leafy vines “steal” the calcium ions before the fruits can get them. As a general rule, don’t sidedress a vine until its first fruits set. Pinch suckers.

Grow resistant varieties. Plum- and pear-shaped tomatoes are more sensitive to BER than round varieties. Cherry tomatoes are resistant to it. Among round, standard tomatoes, Celebrity, Jet Star, Mountain Fresh Plus and other Mountain tomatoes have shown some resistance to BER.

Spray calcium? Researchers debate the usefulness of calcium sprays. If interested, mix 4 tablespoons of calcium nitrate per gallon of water. Spray fruits, not leaves, two to three times a week. The key time is when tomatoes are dime-sized or smaller.

Let’s get busy. Remove any damaged fruits. The first cluster of fruits is most often affected.

Subsequent clusters of fruits are less susceptible. Perhaps this has to do with the vines and fruits being in a better balance; perhaps it has to do with the expanding root network mining more calcium for the plants.

Focus on keeping some moisture in the soil and you’ll find good times ahead in the tomato patch.

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