Who first demonstrated that a bacterium causes a plant disease?

Thomas J. Burrill first demonstrated that a bacterium causes a plant disease. In the 1870s, the American botanist and plant pathologist investigated fire blight, a destructive disease affecting pear and apple trees, and identified a bacterium as its causal agent.

Burrill called the organism Micrococcus amylovorus. It is now known as Erwinia amylovora, the bacterium responsible for fire blight. His work was important because it extended the developing germ theory beyond human and animal disease, showing that bacteria could also produce recognizable disease in plants.

The wording “first demonstrated” matters. Robert Koch and Louis Pasteur were central to proving microbial causes of human and animal diseases, but neither is the best answer for this plant-pathology milestone. Erwin Frink Smith later became a leading authority on bacterial plant diseases and helped develop the field, but he followed Burrill’s pioneering observation.

Burrill was also a botanist, educator, and University of Illinois administrator. His fire-blight research helped establish bacteriology as a foundation of plant pathology, a discipline that now studies how pathogens interact with crops and how plant diseases can be prevented.