You have worked hard to lay out your garden. You have painstakingly plated your rows in straight lines. You even have nifty plant labels so that all passers-by will know what varieties you are keen on. Then you notice a black speck on the bottom or side of your tomato or squash.

You try to wipe it off, but alas, the dreaded blossom end rot has reared its ugly head on your home ground. This shot across your bow does not mean you have diseased plants or poisoned soil. Blossom end rot can occur even in the nicest of gardens.

Blossom end rot (BER) is an abiotic (non-living) disease that affects the fruits of primarily solanaceous plants like tomatoes, peppers, and eggplants, and cucurbits like watermelons, squashes, and cucumbers.

As it is a physiological affliction (a collapse of the cell membranes), pesticides are useless in treating it. Many novice gardeners think it is a fungal pathogen and try to treat it with fungicides.

Many seasoned gardeners, and most professionals, know that it is caused by a deficiency in calcium. What many of these same individuals do not fully understand, however, is that it is not so much about the quantity of calcium in the soil as much as the availability of a plant to use it when needed.

How Does Blossom End Rot Affect Plants?

The good news about BER is that it is not a communicable plant disease. One affected plant will not transfer BER to another plant, nor will one infected fruit cause another to develop it even on the same plant.

Even better news is that it is reversible and a plant that develops fruit with BER can be restored to health and produce healthy fruits within a relatively short span of time. Fruit that is already affected by BER, however, is not salvageable; it is not reversible for the individual fruits that have developed it.

Blossom end rot can appear unexpectedly and from a variety of causes: a sudden onset of a draught, a disturbance of a plant’s root system by improper cultivation or animal pests, or mere “jumping the gun” on the planting season and transplanting when the soil is still too cold to safely do so.

However, all the possible causes of BER can be reduced to a combination of a lack of water supply for the roots to uptake and a lack of available calcium at critical times during fruit development.

Calcium serves many roles in proper plant health. Think of it like mortar—it helps a plant with cell wall membrane stability, aids in its transport processes, and helps to extend a plant’s primary root system. A disruption in any of these functions can lead to BER.

Symptoms of blossom end rot

Blossom end rot is easily identified by its sometimes large, light-to-dark, brown-to-black lesions, generally found on the bottom or lower side walls of the affected fruit. It also tends to affect a plant’s first fruits, though it can appear at any time during a plant’s maturity.

The lesion formed by BER will be dry, though it can become a moist, rotten area if not discovered before other pathogens take advantage of the weakened tissue. These sunken dark spots appear during the fruit’s development, anywhere from when it is one-quarter to half of its mature size. The spots then continue to grow in size. Fruits that have BER will ripen faster than fruits without it.

Though there are not many, there are a few other diseases of fruit that can be mistaken for BER. Buckeye rot is a fruit disease that causes a large, dark, lesion on some fruits, but its cause is phytophthera. In some instances, cucumber mosaic virus can also appear as similar to BER in cucurbits.

On peppers, spots caused by sunscald or anthracnose may at times be mistaken for BER as well. However, these lesions are lighter in color and are not as often misdiagnosed. When in doubt, take a picture or the fruit itself to your local cooperative extension or garden center for positive identification.

Prevention and Control of blossom end rot

One of the best ways to prevent BER starts with a soil analysis. Knowing what your existing nitrogen, phosphorous, potassium, magnesium, and calcium levels will do more to prevent the onset of BER than arguably any other single factor except availability of water. Most soils naturally have sufficient amounts of calcium without having to add any supplemental quantities.

However, if a true calcium deficiency is found as the result of a soil test, the element can be added to the soil by amending with lime or gypsum. Lime will raise the soil pH, so if that is not desirable, defer to using gypsum or other natural calcium suppliers such as bone meal, ground egg shells, clam shells, or oyster shells.

The pH of soils can fluctuate and testing should be done on an annual basis at least to determine which amendments should be used that season. If BER is discovered during the growing season and no soil analysis was performed previously, gypsum should be applied at a rate of one to two pounds per 100 square feet to increase the amount of available calcium.

Some well-meaning but incorrect resources claim that a foliar spray of calcium will correct BER. This is folly; for it to be usable for the plant, calcium must be absorbed from the roots, through the stems and leaves, and into the fruit of a plant via the xylem of the stem walls.

If the introduced calcium cannot be absorbed in this manner, it will not benefit a plant suffering from BER. As fruit begins to develop, the waxy, outside layer also prohibits absorption through the fruit’s walls, again making a foliar or contact spray ineffective.

Blossom end rot can also just as easily occur in soils with sufficient calcium as soils without it. In this case, soils that have high levels of soluble salts can decrease the amount of calcium that is available for absorption. Too much potassium and magnesium specifically block the uptake of calcium in plants. Too much nitrogen can lead to abundant leaf production at the expense of proper fruit development.

There are instances where, in an effort to give plants an additional feeding, one can in fact be causing the conditions for BER to occur. Conversely, fertilizers that are low in nitrogen but higher in potassium, such as superphosphate, can help increase the absorption of calcium.

BER can usually be prevented by adhering to a few proper cultural practices around susceptible plants. Start by selecting healthy plant stock with a well-developed root system if possible. A poor root system will not be able to take up calcium sufficiently, even if it is present in the soil. There are cultivars of vegetables that are less susceptible to BER than others; check seed catalog, your local garden center, or cooperative extension if you need help in finding them.

Make sure to observe the “frost-free” planting dates in your area. These can be found online or by contacting your local cooperative extension. This will help to avoid prematurely planting your seedlings, which could damage the root systems and prevent nutrient uptake. Ensure that the soil you will be planting in is well-drained.

Waterlogged soils do not allow for your plant’s roots to breathe and they will rot, causing the plant to die. The easiest way to determine proper soil moisture is to grab a handful of soil a few inches below the surface.

Squeeze the soil into a ball. Sufficiently moist soil will form a ball. If the soil is too wet, squeezing it will release water like a saturated sponge; too dry and it will just crumble and not adhere into shape. There are moisture meters available to test this, but getting your hands into the soil is usually the best way to determine moisture.

Once your proper site has been selected and the soil has been appropriately amended, if needed, a few other practices will go a long way towards keeping BER at bay. Add one to two inches of mulch around each plant.

In addition to the benefits of weed control, this will help to regulate fluctuations in both soil temperature and soil moisture, both which will benefit the root system. When weeding, planting, or otherwise disturbing the soil around any plant susceptible to BER, take care not to harm the root structure of your plants. Damaged roots of any kind do not function properly to transport necessary nutrients.

Finally, make sure that moisture is evenly applied and given when needed. Make sure that in the absence of rain, your plants get at least one inch of irrigation per week at a minimum. This will make sure that whatever nutrients are in your soil can be utilized by your plants and therefore reduce the chance of BER taking hold in your garden.