White foam sounds like spittle bugs. Don't really harm the plant. You can hose them off. You might be looking at something like fusarium wilt on the Siam Queen. If it is there isn't any cure, have to rip it out and throw it away.

The spittle bugs are a problem they carry pathogens like
Fusarium wilt etc.. by going from a infected plant to
healthy ones they can spred diseases quickly.I go out
and squish the bugs as soon as the sun is up.
The small bug is inside the spittle.
They have killed some of my salvias,rosemary other
lamiaceae too(This includes Basil).Leaves turning yellow
can be overwatering which can kill the root system the
plant wilts and dies. Pull up your dead plant see if you still have white roots or brown decaying ones. Hope all
your other Basils are fine