This book describes the scientific principles of plant sciences and diseases caused by pathogens (Infectious agents), health sciences, biological sciences and environmental conditions. Organisms that cause infectious disease include fungi, bacteria, oomycetes, bacteria, viruses, viroid‘s, protozoa, nematodes and many other parasitic organisms. Plant pathology involves the study of pathogens identification, disease etiology, disease cycles, economic impact, plant disease epidemiology, plant disease resistance, how plant diseases affect humans and animals, pathosystem genetics and management of plant diseases.