A perfectly ripe, home-grown tomato is a thing of beauty and unparalleled flavor, the perfect combination of acid and sweetness. Mother Nature seems to plot against us, marring our perfect fruits with various maladies. One of these is Blossom End Rot. There is a lot of misinformation out there, so here are the facts, straight from institutions of higher learning.
Blossom End Rot on tomatoes is a common problem. It presents first as a discolored, water-soaked patch on the bottom (“blossom end”) of the fruit, opposite the stem. With age, the watery spot becomes firm and leathery. Such spots may be small or up to one-third of the fruit. They may occur on green fruit or ripe fruit, rendering it inedible. A quick internet search will reveal that Blossom End Rot is caused by a lack of calcium.
Incorrect advice abounds on how to add calcium: stir ground eggshells into the surrounding soil, water with powdered milk, bury scraps of Gypsum board (“sheetrock”) nearby, or (most amusing) water plants with a couple of dissolved antacid tablets such as Tums. Yes, Blossom End Rot is caused by a shortage of calcium in the fruit’s tissues, but it may have nothing to do with the calcium present in the surrounding soil. The problem can be found in plants growing in soil with plentiful calcium. So what gives?
Scientifically speaking, Calcium is a large molecule. It takes a lot of water to move Calcium from the soil through roots and stems to where it is needed, in the fruit. If a soil test indicates sufficient Calcium, applying consistent water will remedy the problem. Note that the solution is consistent irrigation, not necessarily more. Over-watering interferes with a plant’s ability to absorb available nutrients from the soil. Over-fertilizing forces the plant to grow faster and worsens the problem. Root damage caused by energetic cultivation also exacerbates the issue.
How to deal with the problem:
Prior to planting, perform a soil test to ensure an acceptable pH and sufficient nutrients, including Calcium. A soil that is too acidic can prevent the uptake of nutrient. High acidity is treated by the addition of lime (limestone). Do not add lime unless the soil test indicates a need,
Water consistently. One inch per week, either via rain or irrigation, is ideal. More may be needed in the hottest part of the summer.
Use a soaker hose or ground-level irrigation rather than overhead watering. Wet leaves may encourage certain diseases.
Use mulch to aid in soil moisture retention.
Don’t try to “push” tomatoes by over-fertilizing.
Remove any fruit that shows signs of Blossom End Rot. While it does not spread from fruit to fruit or from plant to plant, the affected tomato will be inedible.
Do not try to treat Blossom End Rot with insecticides or fungicides. They have no effect.
What about those spray-on calcium products? Allow me to quote directly from a Clemson Extension agent: “Sprays of any form of calcium, such as calcium chloride, do NOT reduce the problem of blossom end rot. Foliar applied calcium will indeed go into the leaves, but will not be translocated into the stems or into the forming fruit. One can treat each plant with a SOIL applied form of water soluble calcium, like gypsum, which is calcium sulfate. So, apply 1 tablespoon calcium sulfate around each tomato plant per month and water it into the soil for root uptake. This will reduce Blossom End Rot. Plants need mulch to keep soil moisture uniform, and when hot, plants may need to be watered every 1 to 2 days. When the soil gets dry, plants have a difficult time taking up the calcium they need, especially during the rapid growth in late spring/early summer. Blossom End Rot is usually only a problem in the first 2 or 3 months of the growing season, so that may be all that is needed. Do not over apply the calcium, as nutrients will compete to get into the plant, and you don’t want to affect the balance of nutrients in the soil by over-loading it with calcium. If one uses leaf compost and mixes it into the soil prior to planting, it too can add the needed calcium for plant uptake.”
About the aforementioned misinformation: Gardeners can protect themselves from well-meant but incorrect advice by a simple search restriction. For example, if you type “tomato problems” into a Google search bar, you will receive ten pages of search results, some of which are helpful and others which are designed to sell you a product. If you add the term “site:.edu” after your search term, you receive six pages of science-based results, produced by university research. University sites have web addresses that end in .edu rather than .com or .org. I never do any horticulture research without using this tool. [Search term + space + site + the colon symbol + dot + edu]. To quote the character Joe Friday in Dragnet, give me “just the facts, ma’am.”
Tomatoes with Blossom End Rot. Photo by Joey Williamson, Clemson University. Used with permission.