"Seeds of the Dusk", a novelette by Raymond Z. Gallun; first published in Astounding Science-Fiction, June 1938, available at the Internet Archive. There is an online review by Robert Gibson at EzineArticles.
This story was also the (unaccepted) answer to the question Identify a story about a plant that becomes conscious and takes over a planet and the accepted answer to the question What is the earliest work of sci-fi that features crows as an advanced/technologically evolved species?
It was written in the 70s or 80s.
Actually 1938, but it has been reprinted a number of times. You may have read it in one of these anthologies.
A plant floats down into future Earth
It was a spore, microscopic in size. Its hard shell—resistant to the utter dryness of interplanetary space—harbored a tiny bit of plant protoplasm. That protoplasm, chilled almost to absolute zero, possessed no vital pulsation now—only a grim potentiality, a savage capacity for revival, that was a challenge to Fate itself.
For years the spore had been drifting and bobbing erratically between the paths of Earth and Mars, along with billions of other spores of the same kind. Now the gravity of the sun drew it a few million miles closer to Earth's orbit, now powerful magnetic radiations from solar vortices forced it back, toward the world of its origin.
It seemed entirely a plaything of chance. And, of course, up to a point it was. But back of its erratic, unconscious wanderings, there was intelligence that had done its best to take advantage of the law of averages.
The desire for rebirth and survival was the dominant urge of this intelligence. For this was during the latter days, when Earth itself was showing definite signs of senility, and Mars was near as dead as the Moon.
where a race calling themselves Itooloo(?) are dominant.
Men. Or rather, the cold, cruel, cunning little beings who were the children
of men. The Itorloo, they called themselves. The invader could not have known their form as yet, or the name of the creatures from which they were descended. But it could guess something of their powers from the flying
machine they had built. Inherited memory must have played a part in giving the queer thing from across the void this dim comprehension. On other worlds its ancestors had encountered animal folk possessing a similar science. And the spore plant was surely aware that here on Earth the builders of this
speeding craft were its most deadly enemies.
The plant eventually takes over Earth.
The subterranean cities of the former rulers of Earth were inhabited only by corpses and by intruding ants, who, like the other fauna of this planet, were immune to the plague, which had been directed and designed for the Itorloo alone. The last race of men was now one with the reptiles of the Mesozoic. But all was peace.
The protagonist utters the phrase Fight Itooloo Fight after being infected by the plant with a disease which the plant has made to exterminate the Itooloo.
"Fight!" he shouted again, as if addressing a limitless multitude. "Fight, Itorloo! Study! Learn! Work! It is our only hope! Keep power flowing in the purification generators if you can. The old records of the exploration of Mars—those plants! Their approach to problems is different from our own. No metals. No machines as we know them. But in hidden compartments in their tissues it was easy for them to create the bacteria of death! They invented those bacteria, and grew them, breaking them away from their own substance. Some way, when I was a captive, I was infected. The thorns on the tendrils that held me! I was the carrier! Find an antitoxin to fight the plague, Itorloo! Work—"