In de Bary’s time, potato late blight had caused sweeping crop devastation and economic loss. He studied the pathogen "Phytophthora infestans" (formerly "Peronospora infestans") and elucidated its life cycle. The origin of plant diseases was not known at that time. Much as Miles Joseph Berkeley (1803–1889) had insisted in 1841 that the oomycete found in potato blight was the cause of the disease, de Bary declared that the rust and smut fungi were the causes of the pathological changes in diseased plants. He concluded that Uredinales and Ustilaginales were parasites.