This scarce form of Kryptonite, which is harmless to Superman and all other Kryptonian survivors, emits a radiation that "destroys all plant life." It was brought into being when a flock of Green Kryptonite meteors passed through a weird "space cloud" somewhere in the vast reaches of interstellar space. Supergirl uses a chunk of White Kryptonite to destroy the diabolical extraterrestrial â€œplant intelligenceâ€ Xasnu in July 1961 (Act No. 278: "The Super Powers of Perry White!").
In 1968, a near-death Superman is unintentionally cured of Virus X when a group of Bizarros throw chunks of White Kryptonite at Superman's coffin while it passes by them in outer-space, thus killing the plant-based virus within him (Act No. 366/1, Aug 1968: "Substitute Superman!").