Blossom End Rot in Tomatoes

Blossom End Rot in Tomatoes

Most gardeners have probably experienced blossom end rot, particularly in tomatoes. It is a frustrating disorder because it is physiological and not caused by a pathogen or insect that can be treated with a spray. Blossom end rot mostly affects tomatoes and peppers but can also be found in eggplants, watermelons, cucumbers, pumpkins, and squash. Blossom end rot affects the blossom side of the fruit, which is the side opposite the stem. The blossom end develops a water-soaked spot that turns blackened and leathery, then rots, making the fruit inedible. Particularly in peppers, it can be confused with sunscald because the rot can occur slightly to the side of the blossom end of the pepper, not always directly at the blossom end.

Some tomato varieties are more susceptible to blossom end rot, particularly many paste types, and it can even occur internally with no visible blemishes outside. Other varieties are more resistant (see below). Blossom end rot frequently occurs early in the season and can later disappear with subsequent fruit set.

What Causes Blossom End Rot?

​Many people believe if you add calcium to your soil when planting your tomatoes you will avoid blossom end rot. Although this idea is based on fact, it is not the whole story. Some add a calcium-rich fertilizer specifically designed for tomatoes, others add eggshells (which take too long to break down to be helpful, so compost them instead), or even Tums, which contain calcium carbonate. Although a localized calcium deficiency is the cause of blossom end rot, specifically, a lack of calcium in the fruit, it is rarely due to a lack of calcium in the soil. Most soils have plenty of calcium, get a soil test done to be sure, but it is not always available to be taken up by the plants. Unfortunately, many factors can prevent calcium uptake by the plant. Most commonly, alternating dry and wet soil conditions can prevent calcium uptake but also overly wet or dry soil for a long period, too high or too low pH, high concentrations of certain nutrients such as ammonia, magnesium, or potassium, soil that is too cold, too much salt in the soil, or damage to a plant’s roots, either from infection or mechanical disruption. 

​Water-soluble calcium enters the plant via transpiration, the process by which plants lose water above ground, usually via the leaves, which causes moisture to be pulled, along with minerals and nutrients, such as calcium, from below ground. Calcium is most easily transported to the leaves, but any water that enters the fruit also brings along calcium and other nutrients. If most calcium goes to the leaves, there is no way to later get it from the leaves to the fruit. Only water that moves from the ground directly to the fruit will bring calcium into the fruit. Therefore, overfertilizing, especially with ammonia-based fertilizers, will encourage leaf growth, and make the lack of calcium in the fruit even worse. Anything that also limits transpiration, such as too high or even too low humidity along with windy, hot weather makes less calcium enter the fruits.

How Do You Control Blossom End Rot?

​The two easiest things to control are how much and how often you water and how much and what kind of fertilizer you use. You cannot control how much rain your garden receives but you can water regularly (I prefer drip irrigation or soaker hoses) during dry periods, at least 1 inch per week. Use nitrate-based fertilizers, not ammonia-based, or better yet, use compost which releases nutrients more slowly, and do not over-fertilize with potassium or magnesium. A soil test is helpful to know what you should and should not add. A soil test will also tell you if you need to alter the pH of your soil. A pH of 6.5-6.8 is ideal, significantly above or below that level can make it difficult for a plant to take up nutrients. Proper soil drainage will also help prevent waterlogged soil and prevent salt from building up in your soil. Also, avoid cultivating the soil close to the roots, instead, mulch to prevent weeds and foliar disease.

Tomato Varieties That Resist Blossom End Rot

​Certain tomato varieties are more resistant to blossom end rot than other varieties. For example, pear/plum-type paste tomatoes are notorious for developing blossom end rot, I stopped growing the Roma variety specifically for this reason. Other varieties I found published that are particularly susceptible to blossom end rot include Big Boy, Castle King, Fantastic, Independence, Supersonic, Surprise, Whopper, and Wonder Boy (https://ipm.illinois.edu/diseases/series900/rpd906/). I have not grown any of these varieties, so I have no personal experience as to whether they are resistant or not. Varieties reported to be more resistant include Celebrity, Fresh Pak, Jet Star, Manapal, Mountain Pride, Pik Red, Sunny, and Winter. I grow Celebrity every year and have never had blossom end rot, not even in the summer of 2023 when we were in a drought or the summer of 2024 when it rained constantly. I have also grown Jet Star and do not recall blossom end rot being a problem.

References and Resources

1. Joy, Ann and Hudelson, Brian. Blossom End Rot. Wisconsin Horticulture Division of Extension. 2024. https://hort.extension.wisc.edu/articles/blossom-end-rot/
2. Reports on Plant Diseases. RPD No. 906 - Blossom-End Rot of Tomato.  University of Illinois Extension. 1999. https://ipm.illinois.edu/diseases/series900/rpd906/
3. Tomato Pests, Diseases, & Physiological Disorders. Johnny’s Selected Seeds. https://www.johnnyseeds.com/growers-library/vegetables/tomatoes/common-tomato-pests-diseases-disorders-overview.html#4