A couple of autumns back, I frequently went wild mushroom forraging, initially with my Basque father-in-law, who I think had been mushrooming pretty much all his life. He explained and showed how mushrooms would appear with the full moon. They would be plentiful and fresh for a few days, then gradually diminish. By the following waxing moon, any mushrooms found would be old and/or weak. There would be virtually none to find until the next full moon, when they'd suddenly be everywhere again. Is there a scientific explanation for this?

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    Same claim in Italy. – Sklivvz Jan 8 '12 at 12:59
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    Any source for this to back it up? I never heard of it. What about Fungus-Farms, there is no moon inside those halls. The main part of a mushrooms is underground, truffels are underground completely. – Baarn Jan 8 '12 at 13:11
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    Does it need the moon to be seen, does it depend on the moonlight? Or do cloudy moon phases work too? – user unknown Jan 8 '12 at 15:08
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    @userunknown the version I know is very simple: you go mushrooming with the full moon because there are more mushrooms. – Sklivvz Jan 9 '12 at 0:09
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    I have not encountered it. A rational hypothesis would be that it's partially true: the mushrooms do appear much more plentifully, but it's because the mushroom heads reflect moonlight and so are much more likely to be spotted. – MetaEd Jan 9 '12 at 16:27

Chris, I too would go mushroom picking with my father in Sweden, and heard the same claim from him.

That said, this is more than likely a case of confirmation bias. Numerous questions have been asked here about the moon and its effect on things (I'll get to those afterwards). The biggest argument against this is would be to look at the biology of mushrooms. Since they are fungi, mushrooms do not need light for their lifecycle, since they don't contain chlorophyll.

Fungi are distinct from plants because they do not possess chlorophyll, the green pigment that allows plants to manufacture sugar from the sun's energy; they need to absorb their food from the environment in which they live.

As a matter of fact, there is a listing of mushroom species for cultivation in this PDF slideshow. In many cases, they explicitly state that no light is needed at all. Although, they may take phototropic cues to help in directionality of growth.

Now, the thing that will allow confirmation bias to creep into when you see mushrooms is that they have very rapid and cyclical life cycles. This paper for a specific white mushroom shows a 50-55 day cycle. Other mushrooms, such as the ones listed in the cultivation slideshow, show numerous time frames from 10 to 60 days.

In reading more about mushrooms, the key takeaway that I found in numerous pages is that biologists report (For instance, University of Wisconsin):

Very specific conditions of nutrition, humidity, carbon dioxide levels and temperature must be met for primordia to form.

Mushroom Life Cycle

Image from University of Northern Illinois Biology Department

Another trend in confirmation bias is that people may pick specific times to pick mushrooms based on those conditions of nutrition, humidity, carbon dioxide levels and temperature, such as after a rainy period, during specific months, etc. Add to those time phased events the natural human condition of being very susceptible to any type of confirmation bias, and there you have the claim.

As I said, there are oodles, and oodles of things people try to tie to moon phases, and in each case, there seems to be no actual correlation, only a perceived one.

Are more crimes committed during a full moon?

Do people with mental illnesses feel the effect more during the full moon?

Are more babies born during full moons?

Is the human menstrual cycle related to the lunar cycle?

In other words, there really doesn't seem to be a lot associated with a new or full moon.

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    "Since they are fungi, mushrooms do not react to light, since they don't contain chlorophyll." This is incorrect. Mushrooms and many other organisms without chlorophyll (including me) do respond to light. See, eg sciencedirect.com/science/article/pii/S1369527406001639 and books on mushroom cultivation by Paul Stamets – David LeBauer Jul 10 '15 at 4:16
  • @David You are correct, inelegant wording on my part. Could you suggest better wording as to why light (specifically moonlight) is not relevant to the mushroom life cycle? – Larian LeQuella Jul 11 '15 at 2:52
  • Hard to prove the absence of an effect, and unfortunately it is also hard to publish on effects that haven't been observed. I would say 'hasn't been observed / reported' instead of 'don't respond'. It should be straightforward to test for a 28 day cycle in a database of observations such as mushroomobserver.org. Still, it would be hard to say if such a signal were driven by mushroom response to the moon or by the response of mushroom hunters to the story. But it could be possible to account for this. Also note, the OP doesn't suggest the effect is caused by light - perhaps gravity. – David LeBauer Jul 11 '15 at 4:02
  • Sorry. Here is a decent study with one figure show no effect of the lunar cycle over 17 years of weekly samples, wsl.ch/dienstleistungen/inventare/pilzreservat/downloads/…. Their one figure is convincing, and they review a few other failures to find a relationship. – David LeBauer Jul 11 '15 at 4:13

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