I understand you can't alter the functionality of the tower much, but I thought of a few workarounds where the issue could be addressed in another way.
1) you could have the sprouts spoil into nothing instead of into spoilage
2) instead of planting the sprouts directly, the sprouts could be processed into a seed/spore that doesn't spoil
3) the sprouts could even spoil into something else besides spoilage that is plantable, but doesn't have the same yields as a sprout. Maybe a dead sprout that can be planted that harvests for a spoilage?
It isn't the end of the world if it stays as it is since you can mostly account for it by having the fungicultural tower feed fresh sprouts back into itself and just keep it running by disposing of unneeded products so that it doesn't back up.
I'll also take this opportunity to thank you for putting together this mod. I have been having a lot of fun on Muria :)