It's nearing the end of the season for zucchinis for me, and some of my zucchinis are showing signs of blossom end rot. However, other fruit on the same plant are fine. Now BER is said to be caused by either an actual or functional deficiency in calcium. So, since some fruit (the largest) are not affected, but only some of the smallest are, is there a critical period for this to happen, and can I prevent it by removing larger fruit so that more calcium is available to the smaller ones?

The plants are also affected by powdery mildew and I've been spraying with a 10% milk solution which hasn't helped so far, and as a foliar spray of calcium, it doesn't seem to have helped either.

  • I have had success spraying the young fruit themselves w a lime spray. Calcium mobility is limited in plants, so the youngest fruit would normally be the ones affected most by ber.
    – gorav
    Mar 28, 2016 at 12:47
  • I'll give that a go, but in the first instance I've removed all the larger fruit Mar 28, 2016 at 23:47
  • Hi Graham! I'm looking through the "unanswered questions" on the site, and found this interesting one, which is probably pertinent to a lot of people. I'm wondering if you've found an answer you could write up yourself. If not, do you want to edit it to bring it forward for new eyes to see it, and possibly receive an answer? If you'd prefer to just leave it be, I'm sorry to have bothered you! Oct 7, 2017 at 19:55
  • @Sue no, I don't think I solved the problem, but I can try again this year. However, my zucchini seeds are not sprouting yet! Oct 7, 2017 at 20:39
  • @Sue this year I'm not really seeing much BER but I've been away a lot and the zucchinis have been left to grow to the size of marrows :( Jan 27, 2018 at 8:00

1 Answer 1


I know that this question is almost 3 years old, but spraying against powdery mildew with a 10% milk solution?! Jesus Christ, please, no, don't do that! Elemental sulfur (during shady hours) and milder copper solutions (e.g. copper hydrochloride) are the ONLY remedies to this problem, not old wives' tales!

That being said BER is a problem caused by multiple factors. The problem is not with a "critical period" or a specific stage of the produce, but the environmental factors. Some cultivars are more susceptible to this problem than others (so you might want to pick your seeds/seedlings accordingly this year), hot days with no (or little) water might increase its likelihood and so does RELATIVE calcium deficiency (usually not the lack of calcium in the soil, but the plants' inability to absorb it). Other than that the problem might manifest at any stage of the produce all the way until it's fully ripe (in my case one of my jalapeno chilies has started to rot while being unripe while the others that came after this one has been cut off were unaffected). I'd say that if you know that a certain plant in your environment is susceptible to BER (because of your soil, your climate, the length of your dry spells etc.) you might want to give a foliar spray of calcium nitrate to your plants every 2-3 weeks to prevent this problem from happening (once the symptoms appear, there's nothing you can do).

  • researchgate.net/publication/… electron spin resonance studies, not old wives data, show free radical production from the milk under sunlight which attacks fungi Jan 5, 2019 at 5:37
  • Actually that paper looks really impressive indeed. However they've used "dilutions of full cream pasteurised bovine milk" i.e. definitely not the kind that you can buy in stores (a BIG difference). Then they admitted themselves that the results were much more spectacular when the treated leaves have received sunlight (that aided free radical formation) as well. But anyway, sulfur and copper have been proven to work against powdery mildew out in the fields too while milk didn't work even in your own experiment. Feel free to keep using it though.
    – CoolKoon
    Jan 5, 2019 at 22:24
  • Huh? Full cream pasteurised milk is the de facto standard milk. It only works with sunlight exposure so I think it's a waste of time to spray under the leaves, and it works best as prophylaxis according to studies. I used it in an attempt to treat. Big difference. opencommons.uconn.edu/cgi/… I might give it another go this year though I normally don't drink milk. Jan 6, 2019 at 7:10
  • Full cream pasteurized milk is definitely not what you can get in supermarkets. Whole milk is the closest you can get and skim milk is almost completely devoid of all the fat. The paper you've linked has used whole milk so that might work. But either way it definitely wouldn't affect blossom end rot at all.
    – CoolKoon
    Jan 7, 2019 at 9:45
  • Whole milk and full cream milk, both pasteurised, are the same thing. And that is what we buy in our supermarkets. therealmilkco.nz/full-cream-milk Jan 7, 2019 at 19:10

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