Blood Artist is necessary to get additional damage in when saprolings are sacrificed or die. You enerally play it when you are ready to combo off.
Craterhoof Behemoth is a win-con that can help you swing for lethal with your little fun-guys. There is enough ramp and token generation that the hoof should be an auto include.
Deathspore Thallid creates saprolings and provides you with a sac outlet. Giving a creature -1-1 until end of turn tends to be useful when you need to remove a troublesome creature from the board.
Elvish Farmer helps to create saprolings and provides you with a sac outlet to gain life.
Eternal Witness helps to get back lots of useful cards from your graveyard. Generally an auto include in my decks running green.
Jade Mage provides us with a mana sink throughout the game. You will generally find her useful late game but if you have a turn where you've got nothing to do, this can at least create a saproling.
Korozda Guildmage is a card I am not sure will stay forever. You can pump a creature but the real reason it is included is for the second ability. The problem is that your creatures aren't always very big so I don't know useful this card will be. Time will tell.
Mazirek, Kraul Death Priest receives a bonus for being present when the saprolings are sacrificed. There are enough sac outlets in this deck that I am keeping this in the 99.
Mycoloth synergizes with our commander very well. You can have it eat any number of saprolings you would like to get double that number of +1+1 counters. You have to think about this card though. It is not fun when you eat all of your saprolings to have it die before your next upkeep. This is one of my favorite cards in the deck.
Nemata, Grove Guardian is a Jade Mage and a Thallid Germinator combined but with better stats. Great for making tokens and provides us with another sac outlet.
Psychotrope Thallid is fantastic. We can create saprolings and we have a sac/ramp outlet. This little guy can help us push out our more expensive spells.
razaketh, the foulblooded
is a tutor for all intents and purposes. Quite expensive to get out onto the field but worth it once it happens. It can help us search up whatever card you need in a pinch.
Sakura-Tribe Elder is included for ramp and chumping purposes. This guy is great.
Savage Thallidcreates saprolings and is a sac outlet that can help us save the most important fungus on the field with its regeneration ability.
Sporecrown Thallid helps boost power and toughness for all fungus and saprolings. This guy can help turn a game around quickly if someone wasn't expecting the boost. He can also help get some of your creatures out of burn spell range.
Sporemound has landfall for saprolings. Why not?!
Sporesower Thallid helps to make saprolings even faster. Line him up with other fungus that are getting spore counters to create an army in less time.
Sporesower Thallid helps to reduce the number of spore counters needed to be removed from all fungus. Combined with Sporesower Thallid, you can get a token per fungus on each of your turns.
Sporesower Thallid creates saprolings and provides a sac outlet while boosting power and toughness
Thallid Omnivore is another sac outlet but helps us to gain additional life in the process. We will almost always be sacrificing a saproling.
Thallid Soothsayer is a sac outlet and card draw. Drawing cards is one of the most powerful things you can do in a game of magic. This guy is a keeper!
Tendershoot Dryad is a powerhouse in this deck. Getting a saproling at EVERY upkeep is invaluable. You will generally Ascend almost immediately and if your opponents cannot remove it, you will have significant advantage.
Thelon of Havenwood pumps his friends with spore counters and can help create additional spore counters late game by removing fungus from your own graveyard.
Thelonite Hermit is another lord that provides the extra benefit of morphing to get 4 saprolings. Auto include.
Utopia Mycon creates saprolings, is a sac outlet, and can help ramp. Auto include!
Verdeloth the Ancient is our final lord that can create a large amount of saprolings depending on how late in the game you play it.
Verdant Force is great because it pumps out saprolings at each upkeep. In 4 player games this will likely draw removal. If it sticks around you will get the upper hand. Plus the 7/7 body is great.
Whisper, Blood Liturgist is a card I included to make sure I always have a sacrifice outlet. I am not entirely sure that it belongs in the 99 but I haven't seen any downside at this point.
Zulaport Cutthroat is added redundancy to hit our opponents when saprolings are sacrificed or die. The fact that each opponent loses life here is great.