This document discusses the biotic and abiotic causes of plant diseases. The major biotic causes are fungi, bacteria, viruses, nematodes, and phytoplasmas. Fungi are the largest pathogen group and reproduce via spores. Bacteria are unicellular and reproduce by binary fission. Viruses can only be seen with an electron microscope and reproduce by taking over a host's reproductive machinery. Nematodes are parasitic worms that reproduce via eggs. Phytoplasmas are wall-less bacteria that live in the phloem and reproduce by binary fission.