Blossom End Rot: The Real Cause and How to Stop It

What Blossom End Rot Is

Blossom end rot is a calcium shortage inside the growing fruit, and the usual cause is uneven watering. No fungus, bacterium or virus is involved, so fungicides do nothing for it. University of Illinois Extension calls it a physiological disorder. To stop it, keep soil moisture steady, mulch 2 to 3 inches deep, hold soil pH near 6.5 and avoid ammonium-heavy nitrogen fertilizer while fruit is setting. Fruit that already has the spot will not recover, so every fix is aimed at the fruit still to come.

How to Recognize It

The first sign is a light tan, water-soaked spot at the end of the fruit opposite the stem, where the flower was. University of Wisconsin Extension compares these early spots to small bruises. The spot enlarges in a circular pattern, darkens, and dries into the black, leathery, sunken patch most gardeners know. University of Maine Cooperative Extension notes that secondary black mold can move into the damaged tissue afterward.

Young fruit is the most vulnerable. Maine reports that fruits 1 to 3 weeks old are most susceptible, so the conditions that matter are the ones in the weeks just after fruit set.

Tomatoes are the best-known victims, but the disorder is not limited to them. University of Maryland Extension lists pepper, eggplant, pumpkin, squash and watermelon, and Wisconsin adds cucumber. Maryland also notes that symptoms are most often seen on large plum or paste-type tomatoes and on long pepper fruits.

The Cause: Calcium Does Not Reach the Fruit

Calcium enters a plant dissolved in the water its roots take up. Illinois Extension explains that calcium uptake depends on active transpiration, the steady loss of water through the leaves that pulls more water and nutrients up from the soil. Anything that slows that stream slows the calcium supply. Maine adds that calcium does not move easily within the plant once it has arrived, so a fruit cannot borrow from the leaves when its own supply runs short.

That is why soil with plenty of calcium can still produce affected fruit. Utah State University Extension states that Utah soils generally have plenty of calcium and that adding more is not recommended there. The fault lies in delivery.

Extension sources name the same set of triggers:

• Fluctuating soil moisture, from drought followed by heavy watering. Illinois sees the disorder more often in container-grown tomatoes for this reason.
• Low soil pH or low soil calcium, which is why Wisconsin and Clemson both start with a soil test.
• Too much nitrogen in ammonium form. Clemson Cooperative Extension lists excessive ammoniacal nitrogen as a factor, and Maryland advises against high-nitrogen fertilizers such as urea.
• Root damage. Clemson points to root pruning from cultivation close to the plants.

Fix the Watering First

Every source puts steady moisture at the top of the list. Utah State’s wording is to keep the soil uniformly moist and avoid the stress of fluctuating moisture.

For an amount, Illinois gives a general figure of about one inch of water per week, counting both rain and irrigation. Maryland puts it per plant: a mature tomato may need 2 to 3 gallons of water weekly, applied deeply at least once a week. Either way, the aim is a root zone that never swings from dust to mud. The guide to watering tomato plants turns those amounts into a routine.

Mulch does much of the work between waterings. Utah State recommends grass clippings, straw or similar material 2 to 3 inches thick around the base of the plant to keep the soil cooler and reduce water loss. Clemson adds a second benefit: mulch suppresses weeds, so there is less need to hoe near the stems and less risk of cutting roots.

Containers need closer attention because a pot holds little water in reserve. Check them daily in hot weather. For in-ground beds, our guide to mulching a vegetable garden compares the common materials.

Check Soil pH and Calcium Before Adding Anything

A soil test tells you whether calcium or pH is part of the problem. Wisconsin recommends testing periodically to confirm there is sufficient calcium and states that a pH of about 6.5 is ideal for most vegetables. Maryland’s target range is 6.3 to 6.8, and Clemson advises liming to bring soil to 6.5. University of Georgia Extension, writing for pepper and tomato growers, treats soil calcium as sufficient when the pre-plant test reads medium or high, and more so when soil pH is 6.0 or above.

If the test calls for lime, it has to go on early. Clemson notes that limestone is best applied 3 to 6 months in advance and tilled into the soil. Where pH is already in range and only calcium is needed, gypsum, which is calcium sulfate, is the amendment the sources name. Clemson’s rate is 1 to 2 pounds per 100 square feet, and Maryland suggests mixing 1/4 cup of gypsum into the soil of each planting hole before transplanting.

Skip the amendments if the test shows adequate calcium and a pH in range. In that case the supply is fine and the watering is the thing to change.

Fertilizer: Watch the Form of Nitrogen

Wisconsin and Utah State give the same instruction: use nitrate forms of nitrogen and avoid ammonium-based fertilizers. Clemson names excessive ammoniacal nitrogen among the conditions that raise the chances of the disorder. Illinois also advises against heavy fertilizing during early fruiting, and gives the reason for singling out ammonium: it competes with calcium uptake.

In practice, this means reading the label of a tomato fertilizer for the nitrogen source and holding back on urea and other high-nitrogen products from first flowers onward.

Do Calcium Sprays Work?

Calcium sprays are sold as a quick remedy, and the extension sources are lukewarm about them. Illinois Extension says foliar applications of calcium do not help much because the calcium is poorly absorbed and poorly moved to the fruit. Maryland allows that spraying affected plants with a calcium chloride solution may offer some temporary relief, and Utah State says calcium chloride can reduce symptoms during the season but is generally not recommended in Utah.

Read together, the sources treat a spray as a stopgap at most. It does not replace even watering, and it cannot repair fruit that is already marked.

Peppers and Other Crops

The damage does not always sit squarely on the tip. University of Georgia Extension, in a circular on pepper and tomato, describes it as uniformly dark brown and black and found on the blossom end or on the lower sidewall of smaller, developing fruit. A dark patch low on the side of a pepper can be the same disorder. Maryland’s note that long pepper fruits are most often affected is worth remembering when choosing varieties for a bed that dries out quickly.

The sources give one set of remedies for every crop on the list: steady watering, mulch, a pH near 6.5 and nitrate-based feeding. For a tomato-only plan arranged by what to do this week and what to do before planting, see how to fix blossom end rot on tomatoes. If the damage on your tomatoes is chewed instead of sunken and leathery, the cause is more likely a caterpillar such as the tomato hornworm.