[4] As a specialist in botany, he held various positions in experiment stations and colleges until 1901, when he was appointed physiologist in the Bureau of Plant Industry, United States Department of Agriculture.

In 1912 he became a research professor of plant physiology at Washington University, working with the Missouri Botanical Garden in St. Louis.

[4] Duggar published many articles and books on a wide range of topics including mycology, mushroom growing, and plant physiology and pathology.

One project was with Lederle Laboratories, part of American Cyanamid, looking for a treatment for malaria based on a species of Rhododendron which he found at the New York Botanical Gardens.

As a result of his work on chlortetracycline (Aureomycin), the first of the tetracycline antibiotics, he personally met with both the Pope and the Mikado of Japan.