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Tomato Diseases

In the Garden

What’s That Black Spot on My Tomato? (And How to Fix It)

July 29, 2015
A ripe tomato with Blossom End Rot.

Tomatoes are a finicky bunch. They’re susceptible to an astonishing number of diseases, many of which are nearly impossible to diagnose without an advanced degree in plant pathology– and even then, you’d be surprised how hard it can be to determine what’s causing, say, a wilting leaf. It could quite literally be a hundred different things.

Thankfully, one of the most common tomato ailments is also one of easiest to spot. (Get it? Because it forms a spot? I crack myself up.)

The Dreaded Dark Spot

If you’ve grown tomatoes in pots before, chances are you’ve come across a tomato or two in your time that formed a grayish brown or black splotch on the underside of the fruit, right where the dead flower blossom once connected to the fruit’s skin. You may not have noticed it right away because these spots only form on the underside of the fruit, and the tops can looks surprisingly healthy.

Blossom End Rot- Tops


Blossom End Rot- Bottoms

These dark spots can be small or large, and will often grow in size as your tomato matures. They can be just a little spot just on the underside, or, in extreme cases, they can spread up the sides of the fruit until half of the tomato is affected.

Blossom End Rot- Green

If your tomatoes are experiencing these symptoms, it’s safe to say that you’ve got Blossom End Rot (or BER). It sounds nasty, but before you panic and rip up your plant in frustration, take heart in the knowledge that Blossom End Rot is relatively easy to correct, and even easier to prevent.

BER is a symptom of a calcium deficiency in your plant. While not one of the big three macronutrients that most new gardeners are familiar with– those would be nitrogen, phosphorous, and potassium, abbreviated as NPK– calcium is still a critical nutrient that plants need in order to function at optimal levels of health and productivity. Calcium is particularly critical to plants in the nightshade family, which include tomatoes, peppers, eggplants, and potatoes. If you’re noticing the telltale signs of Blossom End Rot on your tomatoes, then you can bet your plant is having trouble accessing this critical nutrient.

Thankfully, calcium deficiency is easy to correct with a liquid or powdered calcium supplement. While not available at all nurseries, most stores catering to hydroponic growing carry calcium amendments for gardening purposes. Call ahead if you’re not sure if your supply center carries it. Once you’ve got your hands on some calcium, dilute it with water according to the package instructions. Now, many of these supplements will instruct you to spray the calcium-fortified water on your plant’s leaves, but I prefer to use it to drench the affected plant’s roots instead. I might sprinkle some on the leaves as well, but I definitely suggest you pour at least a gallon of the diluted concentrate on the soil around the roots. Repeat the process once more a week later.

Calcium Supplement

It’s important, once you’ve supplemented your plant’s deficient soil with the missing calcium, that you then keep up a regular watering schedule. Under-watering and irregular watering can seriously exacerbate the condition of plants affected by Blossom End Rot. This is why it is so much more common to find BER in tomatoes which have been grown in containers, because they have a harder time holding onto water and generally tend to be under-watered as compared to their in-ground compatriots. A full-sized tomato grown in a properly sized pot (at least 15 gallons) will probably need to be watered once per day in the middle of the summer, depending on climate conditions. Hot sunny weather obviously requires more water than overcast or rainy weather. Windy weather can be deceiving, for while it can help keep temperatures down, wind also has a strong drying effect on the soil.

While it’s relatively simple to diagnose and correct for Blossom End Rot, it’s still always preferable to take steps to prevent the issue so that you never deal with it in the first place. So what can we do to keep BER at bay next time we plant out our tomatoes?

First of all, make sure that you’re fertilizing your tomato adequately when you transplant it to its final destination– be it a pot, a raised bed, or just a plain old hole in the ground. In addition to an all-purpose fertilizer (which typically contained some calcium– check the package to be sure), and any other amendments that you like to use to feed your tomato, I like to add a big handful of a high-phosphorous and calcium supplement, such as bone meal or bone ash. The phosphorous is critical for blossom production, and the calcium will help keep Blossom End Rot at bay. It’s also a great idea to throw some saved, crushed eggshells from your morning omelet in the bottom of your planting hole to build in a little slow-release calcium source for your plants to draw on throughout the season. Continue to fertilize with an all-purpose fertilizer throughout the season for best results, particularly if you’re growing in a pot. Making sure your soil is rich in calcium from the get-go, together with a good watering routine should keep your tomato plants BER-free and growing strong all season long (…just as long as one of the other 100 tomato diseases doesn’t get them!)