An Ecologist Examining a Plant with Yellow, Sick-Looking Leaves Wonders

An ecologist examining a plant with yellow, sick-looking leaves wonders if the fungus found on its roots is the pathogenic organism. Which of the following steps would be necessary in order to confirm that the fungus is responsible for the disease?

A) Establish that all sick plants have the fungus on their roots.
B) Cause healthy plants to become sick if the fungus is applied to their roots.
C) Be able to isolate the same fungus from newly infected hosts.
D) Both a and b
E) All of the above