The chance for the spores to fall on either side would make it more balanced - each spore with separate chance, so you might end up with only one for you, 2 for the opponent
And as WexMajor wrote, a burnt fungus might not have spores, though that would be a too obvious counter. Like if destroyed by fire, only 50% chance to spawn spores (I have absolutely no idea how hard that would be code-wise)
Also, how about if the spores entered the field burrowed (to simulate the extreme resilience if real life spores)? It would also have the added effect of slowing down the swarm a bit. First you would need to unburrow, then to spawn? Too slow?