Phytopathology or Plant Pathology Phytopathology (plant pathology) is the scientific study of plant diseases caused by pathogens (infectious diseases) and environmental conditons (non-infectiousness). Organisms that cause infectious disease include fungi, oomycetes, bacteria, viruses, viroids, virus-like organisms, phytoplasmas, protozoa, nematodes and parasitic plants. Not included are insects, mites, vertebrate or other pests that affect plant health. Phytopathology also involves the study of the identification, etiology, disease cycle, economic impact, epidemiology, pathosystem genetics and management of plant diseases.