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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 Jun 29 '11 at 17:11
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3 Answers

up vote 10 down vote accepted

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 Jun 28 '11 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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