Talk to the weapon shop owner, Hagakoff at (K-10), for another cutscene.
Go back to Abquhbah for another cutscene.
Go back to Hagakoff for yet another cutscene.
Head to Aydeewa Subterrane and find a scourshroom. Plant it in a mushroom patch nearby to get a young young scourshroom the next game day AFTER zoning.
Depending on the time of day and your luck, you can harvest more than one scourshroom.
You need two to complete the quest, and five for the full reward. 'You must zone in order to harvest the scourshroom.'
There are three confirmed pairs of mushroom patches in Aydeewa Subterrane.
You can complete this quest for the maximum reward by planting a scourshroom in each of them, and harvesting five young Scourshrooms the next morning from 4:00 to 6:00.
Once you harvest all three patches (six sprouts total), you will receive the message "However, you cannot carry any more sprouts. You pick a total of 5 sprouts!"
You can choose to plant in only one or two of the three patches, but this will require multiple trips back to Abquhbah to reap the maximum reward. A patch will only yield a harvest once unless you speak to Abquhbah again.
The first pair of patches is located at the (I-9)/(J-8) intersection on the Second Map. Check one of the patches to receive a mature scourshroom, and check the mushroom patch just a little to the south of it to plant the mature Scourshroom. To get there fastest, enter Aydeewa Subterrane from (I-10) in Wajaom Woodlands.
The second pair of patches is located at the northeastern part of (E-7) on the second map. The eastern patch contains the mature scourshroom, and the mushroom patch to plant it in is just a little to the west of it. To get to this spot, enter Aydeewa Subterrane from (G-7) in Wajaom Woodlands, or simply walk from the first pair of patches.
The third pair of patches is located near the center of (G-10) on the Third Map. To get there, take the ramp at E-9 on the second map. Cross the room, staying left, and go through the passage at (E-9) on the Fifth Map, (with the label "E". You're now on the Third Map, and can follow the passages south to the third patch. Be careful in this area. Cave Tigers and Fossorial Fleas roam here.
Once you have harvested 2-5 scourshrooms, take them back to Abquhbah.
Each cleaning will require one young scourshroom. You must clean the rust at least twice to finish the quest.
After two cleanings, Abquhbah should say that the rust has completely disappeared. You can then choose to finish the quest, though you can also choose to do more cleaning if you have more young scourshrooms on you, or are planning on going back to Aydeewa to harvest some more.
After the rust is gone and you've chosen to stop cleaning, you will receive your promotion and a bonus:
If you cleaned it two to four times, you will receive one Imperial Mythril Piece.
If you cleaned it five times, you will receive an added bonus of one Imperial Gold Piece.
If you do not have inventory space before starting the final cutscene, you will be required to repeat the cutscene to complete the quest.
No sickles or other items are required to harvest these. Just check the mushroom patch to plant/harvest the scourshrooms.
You can travel to both spots from within map two without the need to zone. However you will still need to zone to harvest the young scourshroom on the next day.
There will be sound aggro mobs around the (I-8) patches, and occasionally an Air Elemental will spawn. There's no aggro near the (E-7) patch.
For a higher yield, pick during the morning hours. You will typically get two young scourshrooms when picking between 4:00 and 6:00.
The third patch at (G-10) has sound, sight, and HP aggro mobs.
Although it is the same key item, mature scourshrooms will not work for this quest. You will have to zone and wait a game day after you plant it to get a young scourshroom.
Note: If you only planted one or two patches, you will need to return to Abquhbah before you can go back to plant another round of mature scourshrooms from those same patches.