I wondered the same thing about how this bacteria gets into the pool and survives when there's FC.

At one point, someone said that this is a soilborne bacteria. I do have a mesh safety cover, think the bacteria could get into the pool after closing before the ice comes, when the Cl level has drifted down?

I also would have thought that the winter temperatures here in Michigan would kill this bacteria.

I hoped to be ahead of this for this year, the ice had only melted less than a week before I checked the water, and already zero CYA, zero Cl.