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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.

Sue Saddest Farewell TGO GL
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Graham Chiu
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  • 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 '16 at 12:47
  • I'll give that a go, but in the first instance I've removed all the larger fruit – Graham Chiu Mar 28 '16 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! – Sue Saddest Farewell TGO GL Oct 07 '17 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! – Graham Chiu Oct 07 '17 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 :( – Graham Chiu Jan 27 '18 at 08:00
  • "Nearing the end of the season" may be a big clue. The plants aren't getting the strong light they need to stay healthy. They are susceptible to just about anything this time of year. The diseases can be a symptom, while fewer hours of weaker light and possibly cooler temperatures may be the actual cause. I no longer treat end of season mildew on my squashes. It was an uphill battle with low payoff. The mildew now just says to me "summer's about over, sorry." – InColorado May 23 '18 at 15:44
  • @GrahamChiu Isn't leaving the zucchinis to grow "to the size of marrows" kinda the point? :) – CoolKoon Dec 29 '18 at 17:58
  • @InColorado I was walking past my neighbour's yesterday and noticed her zucs already have powdery mildew, and it's only the end of the first month of summer here. – Graham Chiu Jan 05 '19 at 05:40
  • @GrahamChiu Yeah, powdery mildew grows from late spring to late autumn (its conidiums germinate at 5-30°C), so it's no surprise that your neighbor has them. Soon you'll have them as well and then you'll get to test your milky concoction again. – CoolKoon Jan 05 '19 at 22:31
  • And as for the calcium content, most of milk's calcium content comes from either bound within the casein or as calcium carbonate, none of which are water-soluble. Thus these forms can't be absorbed by the plant's leaves. Only the chelated forms can be absorbed, but their content in quite low in milk. Using calcium nitrate as a foliar spray would be a much better idea... – CoolKoon Jan 05 '19 at 22:34

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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).

CoolKoon
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  • https://www.researchgate.net/publication/237377262_Mode_of_action_of_milk_and_whey_in_the_control_of_grapevine_powdery_mildew electron spin resonance studies, not old wives data, show free radical production from the milk under sunlight which attacks fungi – Graham Chiu Jan 05 '19 at 05:37
  • 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 05 '19 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. https://opencommons.uconn.edu/cgi/viewcontent.cgi?referer=&httpsredir=1&article=1146&context=gs_theses I might give it another go this year though I normally don't drink milk. – Graham Chiu Jan 06 '19 at 07:10
  • 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 07 '19 at 09:45
  • Whole milk and full cream milk, both pasteurised, are the same thing. And that is what we buy in our supermarkets. http://therealmilkco.nz/full-cream-milk/ – Graham Chiu Jan 07 '19 at 19:10
  • Not necessarily. One or the other can be even "resonstituted" milk. But either way that's irrelevant because the paper above indicates that store-bought whole milk works as well. But to get back to the original question: it's not gonna help with blossom end rot at all. – CoolKoon Jan 08 '19 at 08:52