Blossom-End Rot: Myths and Facts
Few things are more disappointing than picking that first ripe tomato only to find a sunken black spot on the bottom! Blossom-end rot (BER) can affect tomatoes, peppers, summer squash, eggplant, and occasionally, watermelons. Gardeners seeing the fruits of their labor in such condition look for a quick fix to the problem, but most do not understand the real cause of BER. It is time to debunk some myths and give gardeners the facts they need to combat this all-too-common garden problem.
Myth 1 – BER is caused by calcium deficiency in the soil.
Fact – BER is a physiological disorder in developing fruit of tomatoes, peppers, squash, and watermelons. It starts as a water-soaked spot, usually on the blossom-end of the fruit, although it is common on the sides of peppers and can resemble sun scald. It progresses to a brown, then black, sunken, leathery appearance and may range from a small spot to over half the fruit. Secondary infection with black mold is common.
While it is true that fruit with BER have lower calcium levels in their cells than healthy fruit, the deficiency of calcium in the tissues is caused by a disruption of water uptake! The disruption in water uptake causes damage to the tissues, which limits the amount of calcium moving through the fruit. The blossom end is last to receive nutrients, so the little calcium that is transported through the fruit is absorbed before it reaches that area.
Myth 2 – Adding eggshells to the soil will prevent or cure BER.
Fact – Ground eggshells are rich in calcium carbonate (CaCO3) and are a fine thing to add to the compost pile. (Whole or crushed eggshells do little to nothing to add usable calcium because it is locked up in the shell.) Adding them to the soil, which probably has enough calcium already, will not prevent or cure BER. BER is seldom caused by lack of calcium in the soil; it is caused by a plant’s inability to absorb or transport calcium.
Arizona soils tend to have plenty of calcium in the form of calcium carbonate, also known as calcitic lime or garden lime. Gardeners in areas with acidic soil add calcium carbonate to “sweeten” the soil by raising the pH. The pH of Arizona soils is already quite high enough, so we certainly do not need to add more calcium carbonate. Fun Fact: Calcium carbonate is responsible for the formation of caliche (kuh-lee-chee), the cement-like sedimentary layer found in many Arizona soils.
Myth 3 – Epsom salt will prevent or cure BER.
Fact – Epsom salt is magnesium sulfate (MgSO4). It contains no calcium. In solution (when it mixes with water), the magnesium ions compete with calcium ions for uptake by the plant and can cause a calcium deficiency! Even if your soil is deficient in magnesium as shown by a soil test, Epsom salt is not the best choice for supplying it.
Epsom salt is highly water-soluble meaning it is quickly flushed from the soil. However, it is a salt and can accumulate in soils that do not drain well, soil in containers, and soil in raised beds. Plants that are suffering from salt stress look as if they need more water, but the roots are unable to absorb water even from saturated soils.
Sodic (salty) soils are unhealthy for plants, but that is another post! Save the Epsom salt for soaking your tired feet after a long day of gardening.
Myth 4 – Spraying a calcium solution on tomato plants will prevent/cure BER.
Fact – Calcium and other nutrients can be absorbed through leaves in small amounts. The skin of fruit is too waxy and thick to allow very much absorption. Nutrients in leaves do not travel to fruit. Water carries nutrients from the roots through the xylem (part of the plant’s circulatory system) to leaves, stems, and fruits. This is the best way to get calcium into the plant.
Causes of BER
The primary causes of BER are high soil salinity, high temperatures, high light intensity, drought, high levels of ammonium (from nitrogen fertilizers) and potassium, and low humidity, and (rarely) calcium deficiency in the soil. Several of these factors often occur together. Here is how to avoid BER.
Have your soil tested by a lab. It is impossible to develop good soil without first knowing the composition of the soil you are trying to improve. You can order a soil sampling kit on Amazon with instructions on how to collect a sample. You then send the sample to the lab. You can have a local lab test your soil. IAS Labs in Phoenix charges $75 for a comprehensive soil test and recommendations for correcting any deficiencies or imbalances. Texas A&M offers soil testing starting at $12 per sample with the option to add testing for micronutrients, salinity, and soil texture. In a future post, I will compare the results from having my soil tested by all three labs.
Add nutrients based on the results of the soil test. Excessive ammonium and potassium compete with calcium for uptake. High levels of nitrogen cause rapid growth of foliage. When foliar growth out-paces root growth, plants are stressed because the roots are unable to supply enough nutrients. Since leaves receive nutrients before fruit, the fruit will be starved for calcium resulting in BER.
Building your soil with organic matter such as compost containing well-rotted manure will provide a slow release of nutrients. Commercial fertilizers tend to provide a quick, but short-lived, supply of nutrients. Nutrients need to be in balance for plants to thrive. The old saying, “shoots, roots, and fruits” is an easy way to remember how the basic components of fertilizer, nitrogen (N), phosphorous (P), and potassium (K). Nitrogen boosts the growth of stems and leaves. Phosphorous stimulates root growth. Potassium is needed for blossom and fruit development. Too much any nutrient can interfere with uptake of other nutrients.
Plant at the right time. Tomatoes and peppers planted before the soil has warmed in the spring are more susceptible to BER in the first fruits produced. Wait until soil is consistently 65o to 70o F to plant seedlings.
Manage watering to avoid dry or water-logged soil. A common gardening question is, “How much should I water my garden.” Unfortunately, there is not a one-size-fits-all answer. Soil texture and composition affect how well soil drains, so the amount of water needed in one area can be quite different than what is needed elsewhere. Mulch, shade, weather, and the type of plant all affect watering needs. In general, the soil at the root zone should be moist enough to form a loose ball when squeezed.
A soil moisture probe is a valuable tool for judging the soil moisture level in the root zone. Soil should be kept moist, not soggy. Water-logged soil restricts oxygen reaching plant roots and the plants essentially drown. Dry soil not only causes plants to suffer dehydration, it prevents the uptake of nutrients. If your garden is watered by a drip system on a timer, remember to adjust it based on weather conditions and how moist the soil is as determined with a soil moisture meter. The Reotemp soil moisture meter is sturdy and has a 15 inch probe making it possible to check deep-rooted plants.
Salts in soil prevent nutrient uptake by restricting water movement in plant roots. Watering affects soil salinity. Frequent, shallow watering, especially in poorly drained soils, containers, and raised beds, can cause a build-up of salts in the soil. Periodic deep watering will help flush salts from the root zone.
Shade plants during the summer. When the UV index rises, people use sunscreen. Plants do not have that option, so we need to provide shade. Even plants that are listed as needing “full sun” will suffer under the intense Arizona sun. Shade cloth is available with different degrees of UV blocking ranging from 20 to 90 percent. For most purposes, 40 or 50 percent is appropriate. Shade cloth reduces both light intensity and heat.
Increase humidity levels. Low-humidity environments increases transpiration (loss of water through leaves). The more water is lost through the leaves, the more water a plant must take in through the roots. Although increased water uptake would seem to increase the amount of calcium moving into the plant and into developing fruit, it pulls calcium away from fruits and into the leaves. This, of course, leads to BER.
The desert southwest is an arid environment. The average humidity level in Phoenix is less than 40 percent most of the year, dipping to an average of 20 percent in June. Most garden plants prefer a humidity level of 40 to 60 percent. Mulch, shade, and proper watering can all help create a more humid microclimate.
Remove affected fruits. BER commonly affects the first fruit of the season, especially in squash. As soon as you notice fruit with BER, remove them so the plant will put energy into producing new fruit. The next fruit produced will be less likely to be affected. The first fruit of the season are more likely to develop BER than later fruit.
Select varieties that are resistant to BER. Some varieties of tomatoes are less susceptible to BER. Round fruited varieties are reported to be less prone to BER than plum and pear-shaped varieties. Cherry tomatoes are not affected by BER. Seed catalog descriptions are a good resource for finding BER resistant varieties of tomatoes, peppers, eggplant, and squash.
BER does not have to ruin your harvest! Test your soil, select resistant varieties, plant when the soil is warm enough, manage nutrients and watering, and provide shade and mulch for healthy, productive plants. Happy gardening!
References
Center for Agriculture, Food, and the Environment. (2018, December 27). Solanaceous, Blossom End Rot. University of Massachusetts Amherst. https://ag.umass.edu/vegetable/fact-sheets/solanaceous-blossom-end-rot#:~:text=Identification,invaded%20by%20secondary%20black%20mold.
Goldberg, N.P. (2013, February). Blossom-End Rot Guide A-231. New Mexico State University College of Agriculture, Consumer, and Environmental Sciences. https://aces.nmsu.edu/pubs/_a/A231/welcome.html
Hossain, M.M. & Nonami, H. (2012). Effect of salt stress on physiological response of tomato fruit grown in hydroponic culture system. Horticultural Science (Prague), 39 (I), 26–32. https://www.agriculturejournals.cz/publicFiles/57891.pdf
Plant Disease Diagnostic Center. (2018, February). Blossom End Rot. Cornell University College of Agriculture and Life Sciences. http://plantclinic.cornell.edu/factsheets/blossomendrot.pdf
Siderman, B. (2018, January 18). Growing Vegetables: Managing Blossom End-Rot [fact sheet]. University of New Hampshire Cooperative Extension. https://extension.unh.edu/resource/growing-vegetables-managing-blossom-end-rot-fact-sheet-0
Williamson, J., Kluepfel, M., Blake, J.H., & Keinath, A.P. (2018, December 13). Tomato Diseases & Disorders. Clemson Cooperative Extension Home & Garden Center. https://hgic.clemson.edu/factsheet/tomato-diseases-disorders/