There is no plausible way for a biological system, especially one based on plants, to spread that fast.
We are talking about a malevolent AI that presumably has a fair bit of tech infrastructure. So a plane that sprinkles green goo seeds is absolutely a thing the AI can do. Or just posting the goo, and tricking someone into sprinkling it on the other end. The green goo doesn’t need decades to spread around the world. It travels by airmail. As is having green goo that grows itself into bird shapes. As is a bunch of bioweapon pandemics. (The standard long asymptomatic period, high virulence and 100% fatality rate. Oh, and a bunch of different versions to make immunization/vaccines not work) It can also design highly effective diseases targeting all human crops.
We are talking about a malevolent AI that presumably has a fair bit of tech infrastructure. So a plane that sprinkles green goo seeds is absolutely a thing the AI can do. Or just posting the goo, and tricking someone into sprinkling it on the other end. The green goo doesn’t need decades to spread around the world. It travels by airmail. As is having green goo that grows itself into bird shapes. As is a bunch of bioweapon pandemics. (The standard long asymptomatic period, high virulence and 100% fatality rate. Oh, and a bunch of different versions to make immunization/vaccines not work) It can also design highly effective diseases targeting all human crops.