I don't even think you need a hostile actor, you could just have an incompetent actor who flips the sign on a value, and instead of designing a biological agent to maximize wheat production, designs one that minimizes wheat production, and ends up exterminating all global wheat.
I'd be more worried about someone developing a plastic-recycling microbe that works too well. There's already lots of viruses that attack plants, but the plants have immune systems for that.
