It’s happened to all of us – you’re so excited that your tomatoes are finally developing fruit. Then, you notice a brown or black spot forming on the bottom. Read on to find out what causes bottom rot on tomatoes, what to do about it, and how to prevent future issues.
Tomato bottom rot, also known as blossom end rot (BER), is a physiological disorder in fruiting plants originating from a lack of calcium.
Because it’s not due to bacterial or fungal disease or insect damage, fungicides and insecticides aren’t effective. Instead, examine your growing conditions to remedy the issue.
If you check on your tomato plants and notice a spot on the blossom end of the fruit that looks like a bruise, you may wonder, “Why do tomatoes turn black on the bottom?” Blossom end rot is one of the top tomato problems, and luckily it’s simple to fix.
- Why do Tomatoes Turn Black on the Bottom?
- What Causes Bottom Rot on Tomatoes?
- Calcium Deficiency in Tomatoes
- Watering Problems May Lead to Tomato Bottom Rot
- Weather Factors that Affect Tomato Plants
- Imbalanced Soil pH
- Excess of Nitrogen in the Soil
- How Soil Temperature Affects Tomato Growth
- Root Damage May Induce Tomato Bottom Rot
Why do Tomatoes Turn Black on the Bottom?
Whether you grow tomato plants in a container or in the garden, a variety of diseases and insects can affect your harvest. While it is important to keep little black bugs off tomato plants, as well as hornworms, there are other issues that must also be addressed.
The first sign of BER or bottom end rot is a small bruise on the bottom of the fruit that grows and takes on a water-soaked appearance. The dark spot usually appears on green tomatoes about to start ripening.
It’s most common during the first fruit set or when plants are under moisture stress. While there’s not much to do about the tomatoes already affected, rest assured that your whole tomato crop isn’t ruined.
Use the following plant care tips to stop tomato bottom rot and prevent it in the future. First, find out what the source of the problem is.
What Causes Bottom Rot on Tomatoes?
Tomato bottom rot results from a lack of calcium. While your soil might be calcium deficient, it’s more likely that plants cannot absorb enough calcium due to environmental factors.
Calcium is an essential macronutrient responsible for holding the plants’ cell walls together and sending signals to activate enzymes and coordinate cellular activities.
Without adequate calcium levels, the cell walls break down, starting with the newest fruit and leaves. Plants absorb calcium from the soil through transpiration. The roots take up nutrients and transport them to the new growth.
Excess water vapor is expelled through pores in the leaves called stomata. When this process is slowed by environmental factors like drought, cold soil temperatures, or root damage, plants cannot access nutrients in the soil.
Calcium Deficiency in Tomatoes
In most cases, problems with calcium uptake are due to environmental issues rather than a lack of calcium in the soil.
However, amending your garden soil with compost and fertilizer each spring before planting helps prevent plant disorders like tomato bottom rot. Bone meal, garden lime, gypsum, kelp meal, and rock phosphate are excellent calcium sources.
If you’re concerned about your garden soil’s nutrient levels, contact your local extension service for a soil test. These professionals also recommend appropriate soil amendments to bring your garden back in balance.
An excess of calcium in the soil is rare. It would inhibit plants from absorbing magnesium and potassium. It’s a prevalent myth that Epsom salts and eggshells prevent tomato bottom rot.
While eggshells do contain calcium, they decompose too slowly to have a noticeable effect. However, they’re an excellent addition to your compost and deter slugs when placed around the plant’s base. Epsom salts contain magnesium sulfate, not calcium.
Watering Problems May Lead to Tomato Bottom Rot
Most often, a plant’s inability to absorb enough calcium is due to moisture stress. Dramatic fluctuations in soil moisture inhibit transpiration and lead to problems like BER.
When the soil dries out too much during dry periods and plants wilt, it’s tempting to overcompensate by deeply soaking them. However, the damage was already done when the plant couldn’t absorb enough calcium from the dried-out soil.
When fruit that developed during this time matures, you’ll likely see the telltale spot on the blossom end of the fruit. During a period of dry weather when your tomatoes begin fruiting, keep the soil consistently moist to a depth of six inches.
Overwatering also prevents plants from absorbing nutrients and oxygen from the soil. During heavy rains, ensure your tomato plants have adequate drainage.
On average, tomato plants should receive about an inch of water per square foot of soil each day. It’s best to use a soaker hose to avoid splashing water on the plants’ leaves to keep tomato plant leaves yellowing to a minimum. Organic mulch holds in soil moisture.
Weather Factors that Affect Tomato Plants
The risk of BER is highest during especially dry periods or after prolonged or heavy periods of rain followed by lengthy spells of dry weather. However, it’s possible to reverse blossom end rot after it’s started.
BER is most prevalent during the first fruit set of the season. Although tomato fruit that’s already affected cannot be salvaged, if you provide your plant with an immediate calcium boost and remedy your soil culture so that the roots can absorb calcium, the rest of your tomato crop will be fine.
The fastest way to give your plants extra nutrients is with a foliar spray. Just like plants “exhale” excess water by the process of transpiration, they also take in water and nutrients through their leaves’ stomata.
Most retail garden centers sell spray treatments with calcium nitrate for BER. Alternatively, try making a DIY calcium spray for your tomato plants.
Milk is a magnificent source of calcium, and chamomile supplies essential nutrients like calcium, potassium, and sulfur.
To make this tomato leaf spray, boil the water and steep the chamomile until the water cools. Stir in the powdered milk, and strain the solution into a clean spray bottle.
Spray the plants until the liquid is dripping off the leaves. Reapply once per week for three weeks after dry periods or at first fruit set.
Imbalanced Soil pH
Soil pH affects plants’ ability to take in nutrients through their roots. Most fruits and vegetables thrive with a soil pH between 6.2 and 6.8. If it’s too acidic, nutrients like calcium, magnesium, and phosphorus become inaccessible.
Highly alkaline soil reduces the availability of phosphorus and most micronutrients like boron, copper, iron, manganese, and zinc.
To determine your soil pH, contact your local extension service for a soil test. Based on the results, the extension service professionals recommend appropriate soil amendments.
Excess of Nitrogen in the Soil
When your plants are struggling, it’s tempting to give them more fertilizer. However, that could actually worsen BER. The excess of certain nutrients makes other nutrients less available to plants.
When there’s too much nitrogen present, plants’ roots don’t take in enough calcium and potassium. When you transplant your tomatoes outdoors, add one tablespoon of organic, all-purpose fertilizer to the bottom of the hole.
Throughout the growing season, fertilize your tomato plants every four to six weeks with a fertilizer higher in phosphorus and potassium than nitrogen.
If possible, use a tomato fertilizer with nitrate-based nitrogen rather than ammonium-based nitrogen. Ammonium competes with calcium for intake.
To ensure that your tomatoes get adequate nutrients, try making your own tomato fertilizer.
Mix the ingredients in a large bucket with a lid, then let it cure for at least two or three weeks.
Apply a quarter-cup of fertilizer around the plant’s base when transplanting your tomatoes in the spring and once every four to six weeks throughout the growing season. Store your fertilizer mix covered in a cool, dry location away from direct sunlight.
How Soil Temperature Affects Tomato Growth
If you’re experiencing an unseasonably cold spring, wait a bit longer than usual to transplant when cherry tomato gardening or planting larger varieties. Cold soil inhibits plants’ ability to take in nutrients. The ideal soil temperature for growing tomatoes is between 65-70℉.
To warm the soil before transplanting, cover your tomato bed with a layer of dark-colored mulch or a sheet of black plastic for a week. If your area has consistently cold spring weather, consider using an insulating plant protector like Wall-O-Water.
Extreme heat also increases the risk of BER. Temperatures are above 90℉ reduce root growth and mass, restricting the flow of water and nutrients to plant tissues.
To protect plants during periods of hot weather, keep the soil consistently moist but not oversaturated. Adding mulch around the plant’s base keeps the soil insulated.
Avoid pruning or fertilizing the plants in extreme heat to avoid prompting more new growth. Provide extra shade using a tarp or canopy.
Root Damage May Induce Tomato Bottom Rot
Root damage is often a contributing factor to tomato bottom rot. When roots are injured by weeding tools, late staking, or pest activity, they cannot take in water and nutrients.
Use caution when weeding around your tomato plants. Apply a three- or four-inch layer of mulch to suppress weed growth. Or, try planting leafy greens like lettuce and spinach around your tomato plants to act as a living mulch.
It’s best to put your tomato cages or stakes in place before the roots extend very far. Staking your tomatoes when mature increases the risk of BER as well as bacterial or fungal disease.
Many beginner gardeners wonder, “Why do tomatoes turn black on the bottom?” Blossom end rot, also called tomato bottom rot, is a relatively common problem that’s usually easy to remedy.
Since it’s generated by a lack of calcium, not bacterial or fungal disease, the solution lies in proper plant care practices. To ensure a healthy crop of ripe tomatoes, follow the gardening tips listed above.
If you think your soil pH or nutrient levels are imbalanced, contact your local extension service for a soil test.
Proper maintenance techniques can reduce the likelihood of BER and stink bug damage on tomatoes, so keep an eye on your plants to treat problems quickly.
We hope you found these tomato care tips helpful and would appreciate it if you’d share this article about what causes bottom rot on tomatoes with your fellow green thumbs on Pinterest and Facebook who’ve ever wondered, “Why do tomatoes rot on the bottom?”