I would just like to point out that there is published material for Rego evaporating large quantities of water. TME, p.119 has "Alchemical Separation of Brine" which transforms a large quantity of salt water into two piles, salt and other (everything else).
I feel as if that spell is operating under the "command" aspect of Rego, rather than "speed up natural process".
Well it does use the ReAq Base 3, which which is a Change rather than Control.
By the way, reading what Pliny says about mushrooms it looks like you don't want dead wood for your Mythic Earth mushroom spontaneous generation spells:
"The generative principle of the mushroom is in the slime and the fermenting juices of the damp earth, or of the roots of most of the glandiferous trees. It appears at first in the shape of a sort of viscous foam, and then assumes a more substantial but membranous form, after which, as already stated, the young mushroom appears."
Since ArM tends to follow Pliny for how Mythic Europe works you'll want slimy damp earth, particularly around the roots of trees that drop nuts.
This is great! Thanks!
Importantly, we can see this is stated as a generative process from other things like we have with worms rather than growth from seeds or mating like we see with trees, animals, etc.
Hmm, I went and looked up the rest of that passage and Pliny's description of how mushrooms become poisonous might be useful for deciding what ability to use for getting a particular sort of mushroom:
In general, these plants are of a pernicious nature, and the use of them should be altogether rejected; for if by chance they should happen to grow near a hob-nail,7 a piece of rusty iron, or a bit of rotten cloth, they will immediately imbibe all these foreign emanations and flavours, and transform them into poison. Who, in fact, is able to distinguish them, except those who dwell in the country, or the persons8 that are in the habit of gathering them? There are other circumstances, too, which render them noxious; if they grow near the hole of a serpent,9 for instance, or if they should happen to have been breathed upon by one when just beginning to open; being all the more disposed to imbibe the venom from their natural affinity to poisonous substances.
So I could see Profession: Apothecary, Philosophiae, and Area Lore all being appropriate for determining what sort of mushrooms will grow from a given environment. Area Lore and potentially Apothecary could cover typical growth, unusual local factors, and recognizing which mushrooms are poisonous once you've produced them. Philosophiae could cover more obscure stuff, deliberately applying principles of natural magic like contagion to know which contaminants to look for if they start turning out poisonous (or if you want to grow poison mushrooms, I guess).
It is a nice way to avoid the omni-present Finesse and gives room for other skills to shine.
Talk about a shock due to differing cosmology- the magus creates some nice safe mushrooms using a lore skill and finesse to select a safe species, only to have them become poisonous because he didn't make an apothecary roll.
Or because his snake familiar breathed on them wrong.