Phytophthora nicotianae is an oomycete pathogen that causes black shank of tobacco and is a major threat to tobacco production worldwide. P. nicotianae has been reported on 255 plant genera. Tobacco roots and crowns are the primary areas for disease symptoms but lower canopy leaf lesions can arise following initial infection. P. nicotianae can be isolated with semi-selective media from symptomatic tissue, contaminated water, and soil samples. The objective of this diagnostic guide is to provide a collection of current descriptions and methods for the symptomology, isolation, storage, and identification of P. nicotianae.