I'm working on that right now. Planted a mushroom, gotta wait on sprouts. Here's some info off another site though:
Talk to Abquhbah for a CS.
Then talk to Hagakoff (K-10) for a CS.
Go back to Abquhbah for another CS.
Return to Hagakoff. You should get a mention about scourgshrooms.
Now, head to Aydeewa Subterrane (G-7) entrance from Wajaom Woodlands.
Your goal here is to find "Mushroom Patches" which has the shroom. What you need to do is to get a sourgeshroom from a patch, and plant it in another patch near by. I was able to locate 3 patches:
http://www.ffxi-atlas.com/maps/aydee...ne/general/ (this is the map i used)
Map 2 - J-9 area
Map 2 - E-7 area
Map 3 - H-10 area
Once you plant, you need to wait a vanadiel day (or once the new day starts). Click on the patch you planted the shrooms in to get sprouts. Its been reported you get 0-2 sprouts. maybe more. You will have to wait a full day in order to plant new shrooms.
Once you have at least 2 (not 100% sure, I had 4), return to Abquhbah for a CS. Now I am guessing that the more sprouts you get the bigger the end bonus you receive, like many of the other rank evaluations prior to it.
Then, congrats on your promotion.
Good Luck all!