Test-Tube Terror by Robert Standish.
I read it in an SF anthology e-book of dubious legality so I won't link the details here. Apart from that the story seems to have been published only in anthologies related to the Saturday Evening Post.
The protagonist is Mark Harrowby and he has two scientist friends Giselle (the love interest) and her brother Pierre. Pierre contacts Mark one day and asks:
What would you say, Mark,” Pierre asked abruptly, “if I told you that it lies in my power now—today—to destroy every blade of grass in the world?”
The destructive agent is the green goo that they test on Kangaroo Island
“We first walked right round the island to make sure nobody was living there. Then, at six o’clock one morning, just before sunrise, we took some of It—the weed killer—to what we judged was the center of the island. We simply poured the contents of a small jar onto the grass and waited. Well, there wasn’t long to wait. Because of the hot sun, I suppose, all the processes were speeded up, just as the processes of decay are accelerated in a warm climate. All I need tell you now is that before noon the entire island, which at dawn had been clothed from end to end with coarse grass, was black—just as black as if it had been burned."
Giselle and Pierre are hunted for their knowledge of the destructive agent. In the end:
They destroy all their notes and commit suicide so that all knowledge of the green goo will be lost.