In my opinion the decay rate is too high. You loose your stacks in seconds when you cant find enemies. And im not sure about the decay rate only being changeable with duration.
Im constantly having issues keeping my spores alive without loosing all my stacks. Now that they dont spread by themselves anymore you have to babysit the affected enemies even more than before. And guess what, if you cant find that last enemy with your spores and he dies, your stacks start decaying super fast and you have to run up to an enemy again to cast the spores very fast. This change basically made saryn a caster frame.
You could make the enemies that are affected by the spores visible through walls so that the player can spot them easier to spread the spores. Everyone is looking for the corrosive damage ticks anyway, to see where an affected enemy is.