White Kryptonite was a variant of Kryptonite. It is lethal to all plant life, regardless of origin.
White K was also deadly to all pathogenic microbes, evinced on two occasions. First was when Superman was infected with a reverse-engineered strain of Virus X and launched himself into space to be cremated in Flammbron, only for the residents of Bizarro World to unwittingly save his life by blasting a collection of Red and White K shards in his funeral rocket's flight path. Later, Superman used a White K meteoroid drifting through the void to destroy the bacteria responsible for a virulent space-plague on the artificial planet of Sunworld III.