Warning! Spoilers to Superman and the Authority #4!
In order to defeat one of his oldest enemies, Superman brings back the long-lost white kryptonite and its deadly powers to DC Comics. While some of these fragments from Kal-El's homeworld are dangerous or lethal, white kryptonite hasn't been seen since Superman Family #189 back in 1978. It is Superman's battle with the Ultra-Humanite in Superman and the Authority #4 that this particular kryptonite makes its comeback, reminding readers of its lethality to all plant life regardless of the planet, time, or universe.
In Adventure Comics #279 Otto Binder and Curt Swann, Superboy finds himself in the 50th century, a time threatened by quick-spreading alien vegetation originating from a crashed alien ship. Superman remembers a mysterious yet familiar mineral he saw on display and borrows it, acting on a hunch that pays off. The mineral is actually "white kryptonite", something not lethal to humans or Kryptonians but deadly to plants, killing the otherworldly vegetation and saving the future. It would be later seen in Action Comics #365 when Superman attempts to launch his disease-ridden body into the hottest sun in the universe. When his condition randomly heals, he owes this recovery to citizens of Bizarro World who threw fragments of red and white kryptonite at his ship, unknowingly destroying the plant-like bacteria that was killing him. It was finally seen in Superman Family #189 where Superman used white kryptonite to heal the people of Sunworld III from a rampant space virus that he unintentionally brought to their world.
In Superman and the Authority #4 by Grant Morrison and Mike Janin, the future, older Superman has his hands full while his newly assembled Authority battles a group of superhumans under the same codename, Knocked unconscious by the latest incarnation of the Ultra-Humanite, Superman wakes up in the Bottle City of Kandor on the operating table. Despite not being as strong as he once was, Superman stops the villain from swapping their brains and that he's even prepared for his current status in Solomon Grundy's body. The Ultra-Humanite gets blasted by a weapon armed by Superman's partner and wife, Lois Lane who tells him " Say hello to White Kryptonite, deadly space defoliant."
Writer Grant Morrison has shown an appreciation for Superman's history and mythology and that continues in this series which has been littered with creative Easter Eggs and nods to famous Superman stories. In battling this villain who was his first superhuman adversary, he fights an unstoppable, seemingly immortal zombie. But considering Grundy's connection to plants, Superman somehow had enough time to put together a trap where he had one of his allies use a weapon that would literally be like green kryptonite to this foe. Lois had apparently been spending time in Kandor doing research which not only makes her Earth's expert on Kryptonians but also allowed her to learn about white kryptonite, what it does, and how to reintroduce it to the DC universe again.
With the Ultra-Humanite weakened, this allows Superman to step in and behead him, allowing Grundy to regenerate naturally while scanning the villain's brain to see how this was all possible. While this sets up Superman's next confrontation with Brainiac, the reemergence of white kryptonite was a clever nod to the Man of Steel's history but also a reminder that kryptonite is not solely dangerous to just him but can be a threat to others as well.