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I've been using "Tomato Maker" (4-2-6) fertilizer, which supposedly has enough calcium to prevent blossom end rot, but I'm still getting blossom end rot on my super marzano tomatoes (no BER problems on any other varieties I'm growing this year).

Are there other things that can cause blossom end rot besides calcium deficiency? Is the super marzano variety (or plum tomatoes in general) particularly susceptible? Could my plants still be suffering from calcium deficiency even with the Tomato Maker fertilizer?

In case it would be helpful to know, I live in the California Central Valley, which has hot, dry summers, but spring this year was unusually cool and rainy.

I'd be interested in any advice on what, if anything, I can do now to reduce blossom end rot on this year's crop and also what I can do in the future to prevent BER from turning up at all.

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  • Thanks to everyone who answered. I selected @Mancuniensis's answer because the linked article is very helpful, but I also suggest looking at @btspierre's answer as well, because it adds additional factors to check up on.
    – eipi10
    Commented Jun 29, 2011 at 17:11

4 Answers 4

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According to notes by the Alabama A&M and Auburn Universities:

BER is caused by calcium deficiency, usually induced by fluctuations in the plant's water supply. Because calcium is not a highly mobile element in the plant, even brief fluctuations in the water supply can cause BER. Also, if plants are growing in highly acidic soil or are getting too much water from either heavy rain, over-irrigation or high relative humidity, they can develop calcium deficiency and BER.

This is a very full paper, which goes on to suggest steps that can be taken to control BER, and points out that some varieties tend to be more susceptible to the conditions that cause BER.

Blossom End Rot in Tomatoes: Causes and Prevention

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    I concur with this, I had a pretty BER on my larger intermediate plants last year due to infrequent watering coupled with a six week long dryspell in my area.
    – Jakkwylde
    Commented Jun 28, 2011 at 0:20
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Blossom End Rot is caused by calcium deficiency -- you need to figure out the mechanism that caused the deficiency. It's a physiological problem resulting from the plant's inability to move calcium where it is needed.

  • Have you done a soil test?
  • Does your soil have enough calcium?
  • Do you have too much potassium? (K interferes with Ca uptake.)
  • Is your pH in the right range?
  • Does your soil have enough organic matter? (So it can hold moisture.)
  • Mulch! (To conserve moisture.)
  • Do you irrigate to ensure that the plants get steady, even water? Various sources say to provide 1-1.5" of water a week during fruiting. (But don't overwater, that's not good either.)
  • Yes, some varieties are more susceptible -- though yours isn't listed in that table.
  • Cull fruits that show signs of BER to avoid letting diseases and fungus in.
  • As an immediate short-term solution, you can use a foliar spray of calcium nitrate (see #8).
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Dr Hessayon says of Blossom End Rot that it is a "frequent problem where growing bags are used" and, in fact, makes no mention of calcium deficiency. For prevention he advises:

Never let the soil or compost dry out, especially when the fruit is swelling

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Dr. Carolyn Male, who is possibly the world's foremost expert on tomatoes, has written up an extensive article on blossom end rot. Basically, among other things, it says that even though calcium is implicated in blossom end rot, the problem is usually not a deficiency in the soil, and adding extra calcium will probably not help in most circumstances.

Now, you can read that article to see what else she says, but I'll tell you what I have to say about it personally, at the moment: I know from personal experience that adding extra calcium will not always prevent or reduce blossom end rot. I wouldn't even be surprised if it could cause it. It is my suspicion that the following things are the major causes of blossom end rot (in no particular order):

  • Heat
  • Drought
  • Planting susceptible varieties (like Martino's Roma, Pomodoro San Marzano, many plum-shaped tomatoes, etc.) Some varieties are genetically predisposed to get it easily, I've read. Some resist it very well, I've also read.

That's interesting what bstpierre says about too much potassium inhibiting calcium uptake. I actually did give my plants a fair amount of potassium (and calcium), this year. Nevertheless, the only ones to suffer much from it were Martino's Roma, Pomodoro San Marzano and Roma x Lemon Boy (or else a mutant yellow Roma). If that's true, low nitrogen could also contribute to blossom end rot. I also wouldn't be surprised if an imbalanced nitrogen to calcium ratio could contribute to it (I didn't give my tomatoes much nitrogen, and I just thought it may have been related).

Edit: My personal hypothesis in 2022 is this: Blossom end rot is caused by an imbalance of any of the following nutrients (too much or too little of any one of them in relation to the others): nitrogen, calcium, magnesium, and potassium. Note that according to my hypothesis watering and drought probably seem to play a role because of how nitrogen is more available when the soil is wet.

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    They say that too much nitrogen can cause it by causing rapid growth, faster then calcium can be mobilized from the soil.
    – Graham Chiu
    Commented Mar 25, 2016 at 23:05

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