There's no question about it, some intelligent beings made those designs. Now, where in the vicinity of this planet can we find intelligent beings? Intelligent life seems to be a very rare phenomenon in the universe, and these designs are proof that it is somewhere near the Earth. But where? Oh that's right, on its surface.

It just seems so much more plausible that these are human-created designs. Radiation is not proof of alien visitation; many natural and human-created phenomena produce radiation. And why would aliens spend huge amounts of effort and huge amounts of time to cross the huge distances between the stars just to draw graffiti on our vegetation? Now I don't know about you, but if I came upon a planet with a strange new form of intelligent life, and I wanted to send them messages, I would find a better way to express them than through mysterious shapes on their crops. I might, say, show myself and try to communicate with them or something. I'd at least leave some conclusive proof of my existence. You know, leave the ship's spare anti-gravity engine at the site or something.