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?
To show that my father-in-law is not keeping a pet theory, there's plenty of talk associating mushrooms and mushroom harvesting with the full moon. For example:
While mushrooms are no doubt influenced by the Moon I'm willing to bet that individual species respond in different ways some flushing at the Full Moon others at the New and some contrarian species appearing in favored Quarters. A quick perusal of my notes and an Almanac hasn't revealed any particular pattern ... yet. But I'll keep looking. [Source]
Though here, mushroom picking must only happen at the time of the full moon, just as everybody also knows that root crops do best when planted in the waning half of the moon’s cycle. I raise a quizzical eyebrow. But no, I am assured: the gravitational pull of the moon brings more moisture to the surface of the soil and creates perfect conditions for plump porcini, in much the same way as the moon affects the tides. [Source]
The moon phase and sign actually do contribute markedly to environmental growing conditions. This refers mostly to gardening when starting vegetable/herb seeds in soil, but the moons positions does regulate available moisture and density within a medium. When the moon is waxing soil is expansive, and more moisture is available. [Source]
Pseudo-scientific explanations abound, some involving gravity/moisture, others involving moonlight, etc.