The mushrooms are the new biological weapon that the elves have deployed. They took advantage of the absurd rate of plant growth by spreading the spores everywhere.
So the mushroom grows very quickly and grows everywhere on the battlefield. With the armor of soldiers scrapped, their force field can't protect them from the hallucinogenic effect of the spores.
If inhaled too much, the spores can lead to poisoning of the soldiers. So the mushrooms have to be eliminated.
Unfortunately, increasing the reward for destroying them can't eliminate them. They are practically everywhere and are constantly growing. Plus, finding them is difficult.
Only people like Kelvin, who have an advantage in identifying them and destroying them en masse, have been able to put a significant dent in the supply of the mushrooms.
This and the rarity of spiked trees is why he is contemplating going mushroom hunting instead of trying to bring down giant worms.
