How to Fix Blossom End Rot on Tomatoes for Good

The Short Answer

To fix blossom end rot on tomatoes, pick off the damaged fruit, then keep the soil evenly moist with about one inch of water a week and a layer of mulch. The rot comes from a shortage of calcium in the growing fruit, and in most gardens the soil has the calcium but the plant cannot move it. Fruit already marked will not recover, and University of Georgia Extension calls the condition irreversible, so every step here protects the fruit that sets afterward.

Blossom end rot is a physiological disorder. No fungus or insect is involved, so fungicides and insecticides do nothing for it.

What It Looks Like and Which Plants Get It

University of Wisconsin Extension describes the first stage as water-soaked spots that resemble small bruises, most often on the bottoms of fruits. The spots enlarge and become dark brown to black, sunken and leathery. University of Illinois Extension describes the start as a light tan lesion at the end of the fruit where the blossom was.

It tends to show on the earliest fruit. University of Georgia Extension notes that symptoms appear during the first fruit set, because the shortage happens before anything is visible and growers find out too late.

Tomato type matters. University of Maryland Extension sees it most often in large plum or paste-type cultivars and rarely in cherry tomatoes, and Illinois reports that long-fruited Roma types are more susceptible than round varieties. The same disorder affects peppers, eggplant, squash, pumpkins and watermelons, which the broader guide to blossom end rot causes and fixes covers crop by crop.

Why the Fruit Runs Short of Calcium

Calcium has one route through a plant. Georgia’s circular explains that it moves only in the xylem, carried by the flow of water from the roots up toward the leaves, and that it has no ability to travel from the leaves to the fruit through the phloem. Water goes where transpiration demand is greatest, which means new shoot growth. A fruit transpires less as its waxy outer layer develops, so it sits at the back of the line.

Anything that slows that flow of water can leave the fruit short. Utah State University Extension lists drought, cool cloudy weather, root rot, and root damage during transplanting. That is how a soil can test high in calcium and still produce rotted fruit. Illinois sees the disorder more often in container-grown tomatoes, where soil moisture fluctuates most.

Fertilizer plays a part as well. Illinois warns against heavy fertilizing, especially with products that supply nitrogen as ammonium. Wisconsin gives the same advice from the other direction: use nitrate forms of nitrogen in place of ammonium forms. Maryland adds a caution against high-nitrogen fertilizers such as urea.

What to Do This Week

1. Remove the affected fruit. Maryland’s advice is to take them off immediately when symptoms appear. They will not heal, and Illinois says to discard them.
2. Measure the water. Illinois gives the general figure of about one inch of moisture per week, from rain or irrigation combined. Maryland puts it per plant: a mature tomato may need 2 to 3 gallons weekly. A rain gauge or a straight-sided can in the bed tells you what the weather has already supplied.
3. Water evenly. The aim is to avoid the swing from dry to soaked. Split the week’s water into regular sessions sized to your soil, and check below the surface before each one. The routine is laid out in how to water tomato plants.
4. Mulch. Illinois notes that mulch conserves soil moisture and evens it out, and Wisconsin recommends mulching to retain moisture during dry periods.
5. Ease off the fertilizer. Stop any high-nitrogen or ammonium-based feeding while fruit is setting.

Containers need the closest watch, since Illinois ties their higher rate of blossom end rot to fluctuating soil moisture. A drip system for the vegetable garden on a timer keeps the supply steady in beds and pots alike.

Check the Soil Once, Before Planting

Soil calcium and pH are worth confirming, but only once, and before the season. Have the soil tested, and compare the pH with the targets extension services give. Maryland says to maintain soil pH in the 6.3 to 6.8 range. Wisconsin calls a pH of about 6.5 ideal for most vegetables. Georgia treats soil calcium as sufficient when a preplant test reads medium or high, and more so when the pH is 6.0 or above. The article on soil pH for the vegetable garden explains how to read a test report.

If the test shows a need, add calcium before planting. Wisconsin suggests bonemeal or lime. Maryland’s option is gypsum, which is calcium sulfate, at 1/4 cup mixed into the soil of each planting hole. Illinois makes the same point from the other side: test first to see whether the soil needs any calcium at all.

Root health belongs in the same preseason check. Utah State recommends deep soil with good structure so roots can reach water and air, and a raised row or bed where the soil is shallow. A well-built mix helps on both counts, and the guide to the best soil for raised bed vegetables covers the recipe.

Fixes That Do Not Work

Remedy	Verdict	Why
Foliar calcium spray	Little help	Poor absorption and poor movement of calcium to the fruit (Illinois)
Spraying larger fruit	No benefit expected	Past golf ball size, the waxy skin is believed to be quite impermeable to water (Georgia)
Fungicide	None	The disorder is physiological, with no pathogen to control
Extra nitrogen	Raises the risk	Extension advice is to avoid excess fertilizer, especially ammonium forms (Illinois, Wisconsin)
Calcium added to soil that tests adequate	Not needed	The shortage is in delivery to the fruit

Georgia sums up the state of the evidence on foliar sprays as very inconclusive. Maryland allows that spraying affected plants with a calcium chloride solution may offer some temporary relief, which is the most any of these sources claims for a spray. Money spent on a spray is better spent on mulch or a timer.

Stopping It Next Season

Three choices at planting time lower the odds. Choose varieties with care if the problem returns every year: Utah State recommends selecting tomatoes with lower susceptibility, and round-fruited types are less prone than long paste types. Handle transplants gently, since root damage at transplanting is on Utah State’s list of causes. And set up the watering before the first flowers open, because Georgia identifies the critical period as running from first bloom set through fruit development.