Black Spots on Tomatoes Could Be Blossom-End Rot (BER)

As you pick your first tomatoes of the season on a beautiful summer morning, you panic when you discover a gruesome black spot on some of the fruits, opposite the stem. This is called Blossom-End Rot, so named because the black spot on the tomato occurs on the end of the fruit which contained the flower blossom. This disease can also appear in peppers and watermelons.

During a dry period or drought, water your tomato plants with a hose, watering can, soaker hose or drip irrigation. This should keep them alive and somewhat productive, but there’s no substitute for a soaking day-long rain. When that downpour finally comes, everything in your garden perks up and greens up, fruits and vegetables begins to ripen and move a little closer to edible. That’s because the roots finally have enough water to transport nutrients through the stem of the plant to where the fruits and vegetables are forming.

What causes Blossom-End Rot in Tomatoes?

Blossom-End Rot is caused by insufficient calcium supplied to the tomato, pepper, or watermelon fruit as it’s forming. This may be caused by a calcium deficiency in your soil, but more likely it will appear after a period of drought – the roots fail to obtain sufficient water, decreasing the ability of the plant to transport calcium up to the developing fruits, and the fruits become rotted on their basal ends.

In many cases, Blossom-End Rot is confined to the first few fruits to ripen and is more of a cosmetic issue than a disease like tomato blight which requires drastic action. Fungicides and pesticides will be of no use in fighting Blossom-End Rot, as it’s not a fungus that spreads from fruit to fruit or fruit to plant. However, as the fruit is in a weakened and vulnerable state, secondary infections from pests and disease may occur. It’s wise to immediately destroy any fruits which show signs of the disease.

Other causes of Blossom-End Rot

Another commonly overlooked reason for Blossom-End Rot is cultivation too close to the plant. This may destroy portions of the roots, limiting the plant’s ability to take up water and minerals.

Tomatoes planted in cold, heavy soils or planted out too early in spring often have poorly developed root systems. Since they are unable to supply adequate amounts of water and nutrients to the plant during times of stress, blossom end rot may result. This is a good reason to pinch off the first few flower buds which appear after transplanting, as it will encourage deeper root penetration and therefore a better supply of nutrients, especially during a drought.

Soils that contain excessive amounts of soluble salts (for instance from overuse of synthetic fertilizers) may predispose tomatoes and peppers to Blossom-End Rot, because the availability of calcium to the plant decreases rapidly as total salts in the soil increase.

How to avoid Blossom-End Rot

• Water tomatoes, peppers, watermelons and other plants regularly – make sure they get at least one inch of water per week in absence of rain.
• Compost liberally when transplanting and side dress with more compost thirty days later. Tomatoes love compost and if you’ve been putting egg shells in the compost pile, there should be sufficient calcium around the root zone to supply all of the calcium the fruit needs. Compost will also help the soil retain moisture.
• Once the plant is established, don’t cultivate too close to the base to avoid damaging surface roots
• Mulch the soil around your plants to keep the moisture in the soil from evaporating during dry periods
• If Blossom-End Rot appears year after year in the absence of drought, it may point to a calcium deficiency in your soil and steps should be taken to increase calcium levels

More information on Blossom-end rot is available from the Cornell University Vegetable MD online.