This phenomenon would baffle scientists until graduate student Stephen Lindow of the University of Wisconsin–Madison with D.C. Arny and C. Upper found a bacterium in the dried leaf powder in the early 1970s. Lindow, now a plant pathologist at the University of California-Berkeley, found that when this particular bacterium was introduced to plants where it is originally absent, the plants became very vulnerable to frost damage. He would go on to identify the bacterium as P. syringae, investigate P. syringae's role in ice nucleation and in 1977, discover the mutant ice-minus strain.