Excellent work! I agree with Danimal85 both in regards to how cool your theming is and how many cards are in the spore deck - I know it's better to not run out of them, and as the game goes on more and more will make their way into the shared deck, so perhaps it's better to be safe than sorry?
The inhaled spore effects are very good! Since the rot within chooses the replaced piece, and since it happens on another player's turn, I worry it has too much potential to mess with their plans and scoring - perhaps if the person who drew the card chose what warrior they replaced it would feel less punishing and more manageable?
You've given it some really cool unique mechanics which are both evocative of other root factions and yet not too similar to them - adding to the deck makes me think of frogs, yet it's more unique and organic than what frogs are doing and gets gradually added in rather than shuffled in from the start. The choice between scoring and shuffling the discard pile into the deck feels very tactical and fun!
Power wise, I'm not sure if this faction feels too strong or not strong enough! Filling the deck with spore cards will take time, and their warrior recruitment is quite slow, which is hard as that's their method of building as well, so it really is imperative that they fill the deck with spores to fill their recruiting needs. I think it'll be a slow to start faction, but also very difficult to stop once it gets going. I look forward to seeing it played at some point, because you're exploring some interesting ideas and design spaces very elegantly and cohesively!