At some point in the season as you inspect your tomato crop, you may have noticed a rotten, sunken spot on the end of the fruit. It looks like a yellow or brown water-soaked spot where the blossom used to be, and over time, it turns into a hard, decaying, dark brown or black patch.
This anomaly isn’t a disease or insect damage, and its cause came long before you discovered what was happening. It’s called blossom end rot (also known as BER) and it frequently affects tomatoes, as well as peppers, cucumbers, melons, and eggplant. It appears on green or ripening fruit, especially with the first flush of the season.
What Is Blossom End Rot?
Blossom end rot is a physiological disorder — that is, a disfigurement of the fruit. It’s not “contagious,” as it won’t spread to the rest of the plant. It’s not a condition that lives in the soil, nor is it caused by fungi or bacteria.
However, blossom end rot tends to affect several fruits on the same plant at the same time. It commonly occurs early in the growing season but can appear at any time, depending on environmental conditions.
It’s often a misconception that blossom end rot is a result of calcium deficiency, and an oft-touted remedy is to add more calcium (by way of lime or crushed eggshells) to the soil.
But blossom end rot is a result of inadequate uptake of calcium from the soil to the leaves and fruits, and there is a difference here.
Generally speaking, most garden soil has a sufficient amount of calcium in it. While calcium supplements can certainly help restore depleted soil and give your crop a boost, the only way to know how much to add is to get a proper soil test and find out what nutrients your garden is lacking. Amending the soil with calcium above and beyond what is needed will not reduce the chances of BER.
The issue that affects BER is the movement of calcium from the soil and through the plant.
Calcium is required for cell growth, so it’s needed in high amounts by the developing fruit. But studies show that there is competition between newly forming leaves and newly forming fruit for that calcium, and stress factors such as excess nitrogen, drought, moisture, and high heat can all reduce or prevent the calcium from reaching the fruit as it grows.
Because the leaves have greater transpiration than the fruits, the leaves tend to receive more calcium, leaving the fruits to suffer.
How to Fix Blossom End Rot
Solution #1: Take it easy with the nitrogen.
Though nitrogen is a vital nutrient needed for plant growth, too much of it (especially when combined with fast-climbing temperatures) can cause foliage to grow too rapidly before various plant compounds have a chance to “catch up” and move enough calcium into the fruits.
Use a balanced, all-purpose vegetable fertilizer or tomato fertilizer at planting time to ensure your plant gets what it needs, but don’t overdo it (especially during early fruiting, when blossom end rot is most likely to occur).
Avoid fertilizing in the peak of summer when temperatures are high, as over-stressed plants may go into semi-dormancy and not make use of the nutrients. (You can read more of my hot-weather watering tips in this post.)
Solution #2: Water consistently and mulch well.
The conditions that cause blossom end rot are closely linked to inconsistent moisture in the soil. Since calcium can only be moved into the plant by a constant, ample moisture supply, underwatering (or going between periods of drought and overwatering) can cause a calcium deficiency as the nutrient is unable to circulate properly throughout the plant.
With tomato plants, remember to irrigate deeply but less frequently (so as not to waterlog the soil). Tomatoes have long roots that benefit from long, consistent watering to make sure the moisture soaks in deep to where the mass of the roots are. Plants grown in containers will need to be watered more often than those grown in the ground or in raised beds.
Drip irrigation or soaker hoses (set on a programmable timer or irrigation controller) work best to deliver a steady stream of moisture to the root zone. I always advise against overhead watering; not only is it an uneven form of irrigation, it promotes fungal diseases on the leaves.
Remember to mulch your tomato beds or containers with at least 3 inches of organic mulch (my favorite is straw) to help the soil retain moisture. Mulch also helps keep water from splashing up onto the leaves, leading to the aforementioned fungal diseases. The less stress you put on your plants, the better your harvest will be!
Solution #3: Avoid disturbing the plant roots.
Try to avoid deep cultivation of the soil near the plant roots after fruit set. If you need to control weeds, hand pull them or scrape the soil lightly with a hoe to remove them. The most ideal way to curb weeds is to apply a thick layer of mulch around your plants to smother any unwanted seedlings.
Any major disturbance of the soil, especially when the weather is hot and dry, can stress the plant and affect the roots’ ability to draw moisture (and thus calcium) into the fruits properly.
In a similar vein, planting tomatoes in cold, heavy soil may cause poorly developed root systems that are unable to transport the needed nutrients to the plant. This may make them more susceptible to blossom end rot.
Solution #4: Amend the soil with the appropriate amount and type of fertilizer.
If you find that season after season, your fruits are continually afflicted with blossom end rot despite your most valiant efforts to water them, it’s time to take a soil test.
Though you can mail samples of your soil to a home testing lab like this one, I find it best to contact your cooperative extension office and ask them to send your soil to a local lab.
A good, reliable soil test will set you back around $50, but it’s money well spent in the long run as it will give you a broad, accurate overview of your soil health and what you can do to improve it. Working with a local lab also means you can tell them you garden organically so they don’t make suggestions for chemical fertilizers you may not want to use.
If your tomatoes already show signs of blossom end rot, there’s not much you can do to “cure” it. Small areas of damage can be cut off, leaving the rest of the fruit to be enjoyed. (Yes, you can still eat it, assuming the non-damaged portion has developed normally.)
Tomatoes with severe blossom end rot should be picked and composted (or discarded). The problem will usually resolve on its own with the next round of fruit as the season goes on.
Though you can help create a more favorable growing environment for your plants by following the solutions outlined above, know that certain varieties of tomatoes (especially those big, juicy slicers) are simply more prone to blossom end rot than others.
If all else fails, try growing cherry, grape, or similar smaller tomatoes, as they almost never have trouble with BER. You can also look for BER-resistant varieties when you order next season’s seeds.
Just be sure to remove any rotting fruit as soon as you see it — you want the plant’s energy to go into growing good, healthy tomatoes!