An Ecologist Examining the Leaves of a Diseased Plant Wonders

An ecologist examining the leaves of a diseased plant wonders if the fungus found on its roots is the pathogenic organism.Which step would be least necessary in order for the scientist to confirm that the fungus is responsible for the disease?

A) Establishing that all sick plants have the fungus on their roots
B) Causing healthy plants to become sick by applying the fungus to their roots
C) Isolating the same fungus from newly infected hosts
D) Reinfecting more plants with the isolated fungus
E) Showing that the fungus arrives in fields on the feet of migrating birds