Thursday, July 9, 2009

Blossom End Rot in Tomatoes

The weather has been perfect here in the Central Valley for tomatoes…and blossom end rot. Tomato plants with blossom end rot show small, light brown spots at the blossom end of immature fruit. The affected area gradually expands into a sunken, leathery, brown or black lesion as the fruit ripens. Hard, brown areas may develop inside the fruit, either with or without external symptoms.

According to the tomato heads at UC Davis, the problem occurs when tomato plants have grown rapidly during the early part of the season and are then subjected to hot dry weather when the fruits are in an early stage of development. You know, sort of like the weather in mid-May (104 degrees on May 17) and late June (+100 for most of the last week of the month).

Although the weather sets the table for blossom end rot, many other factors are major contributors. 

Mainly, you. 

Yes, blossom end rot is related to a deficiency of calcium in the tomato fruit, but that occurs for several reasons that can be classified as “operator error”. Among them:

• Too much water.

• Not enough water.
• Irregular soil moisture, brought on by, um, irregular watering.
• Too much nitrogen fertilizer.
• Planting in soil whose pH is not conducive to calcium uptake by the plant (below 5.5 or above 8).
• Planting in poorly drained soil.
• Planting in too sandy of a soil.
• Improper planting (spreading out the roots when planting helps the plant adapt better)
• Excessive levels of potassium.
• Excessive pruning.
• Lack of an organic mulch. (organic mulch helps moderate soil temperature and moisture fluctuations)
• Using a plastic mulch which might raise the soil temperature too high.
• Planting certain tomato varieties that are prone to blossom end rot, especially narrow paste tomatoes.

The most critical mistake gardeners contribute to blossom end rot: not monitoring the soil moisture at root level.

Although the surface of the soil may appear dry, the moisture level a few inches down may be correct. If more water is added at that time, then the soil becomes so moist that oxygen is unavailable for root growth and calcium will not be absorbed. Why? Excess soil moisture, combined with a lack of soil oxygen, speeds the formation of Casparian strips, deposits on the young root tips that have become suberized, waxy substances through which water and nutrients cannot move.

If the soil in the root zone is too dry, then the calcium will not move to the roots. Dry soil and hot, dry, windy days create a water and calcium deficiency in the plant. Even a brief soil water deficit can disrupt water and nutrient flow in the plant. If this occurs while fruits are developing, blossom-end rot will likely develop.

And when it comes to garden problems, many folks think the answer is, “buy something and put it on the plant”. Buying stuff won’t necessarily end blossom end rot.

Among the “store-bought remedies” that are frequently suggested that have been proven to be of little or no value to ending blossom end rot:
• Applying a foliar calcium spray to the tomato leaves. In tests done at the University of Nebraska, their studies showed that calcium does not move from leaves to the fruits. Thus, foliar sprays of calcium won't correct blossom end rot. Tomato fruits do not have openings in the epidermis (skin) through which calcium can be absorbed. Contrary to past belief, the direct application of calcium as a spray is ineffective.

• Adding a calcium supplement, such as gypsum, limestone, or eggshells. Perhaps. But it depends on your soil.
• Limestone can raise the pH in soil to a range more favorable to tomatoes and calcium uptake, around 7.0. But if your soil is already in that range, adding limestone may raise the pH to the point where calcium uptake is again, slowed.
• Adding crushed eggshells to the soil well before transplanting time may help overcome any calcium deficiency already in the soil. But it ain’t gonna help your tomatoes if they are already in the ground.

• And gypsum (calcium sulfate)? Dr. Linda Chalker Scott of Washington State University's Horticulture Department, and author of the award winning book, “The Informed Gardener”, says home gardeners are wasting their money. “Most urban soils are not improved by adding gypsum,” she states in her on-line newsletter, “Horticultural Myths”. “Adding gypsum to sandy or non-salty soils is a waste of money, natural resources, and can have negative impacts on on plant, soil and ecosystem health.” However, she points out, gypsum can improve the structure and fertility of heavy clay soils; but consider another undesireable result to adding gypsum: Gypsum can have negative effects on mycorrhizal inoculation of roots.

Maintaining the proper balance of potassium, phosphorus and other soil nutrients and avoiding excessive growth due to over-fertilization with nitrogen is recommended. Excess levels of ammonium (NH4--N), magnesium, potassium and sodium have been reported to reduce the availability of calcium. That University of Nebraska  study reported that the use of nitrate nitrogen (NO3) stimulates Calcium uptake while ammonium nitrate (NH4) reduces the uptake of Calcium.

Their best advice: A soil test should be conducted to help determine what needs to be added and what should not be added to your garden soil.


And, use a moisture meter.



  1. After 25 years in the nursery business I finally bought a combo moisture and pH meter. What a difference it makes. I realized that in the past I was watering my tomatoes too much. The soil looks dry on top, but a few inches down it's moist. So now I water them every three days, and they are doing great.

    Here in the foothills the Foli-Cal Calcium spray does work. I tell people to drench the soil, not the leaves. We have a calcium deficiency up here, and most everyone that uses it tell me it has corrected their problem. Better yet is to apply Oystershell Lime at planting time. It has Calcium, as well as raising the pH, which in our acid soils is usually a must.

  2. thank you farmer fred this was very informative. dave c virginia

  3. thank you fred! I will work on it :)

  4. Hi Fred, I get BER every year. I bought a moisture ph meter. I hope I won't be getting it this year. Thanks, this is very informative.

  5. So basically it's hopeless in my situation. My pH is fine, my soil is well drained, I use Tomato-tone in the recommended amounts... no pruning. It's a rented plot that I do not get access to off season at all & we get moved around every year. No prepping the soil in advance at all, ever. We've got nothing but rain so far this year & I can't control that.

    So no calcium additives will help at all? Should I just give it all up & go back to trying to buy decent tomatoes?

  6. OK so Mich State, U of Georgia, & some others claim calcium sprays can help. But only on fresh, new leaves at the top. What is a tomato lover like myself to do? The rains appear to have stopped for now so there is hope for my heirlooms.

    This is my third year on these rented plots. First year was drought but I did very well. Last year it was rainy & fungus was a huge problem. Fungus is under control so far but now its blossom end rot.

    Thanks in advance for any ideas.