Answer: Your cosmos have not developed a disease but have become home to a growing population of Spittle Bugs. These tiny insects find a suitable host and then produce a blob of foamy stuff over their bodies to hide themselves from predators. You can control them by hosing them off with a strong stream of water from the hose. Since the insects can fly they may hop back onto the plants, but as long as they are manufacturing more spittle to hide in, they won't be feeding. Keep after them every few days and they'll eventually give up.
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