An organism that causes disease. Many plant diseases are caused by fungi or fungus-like organisms (water-molds), but several other types of organisms can also cause plant diseases (bacteria, viruses, parasitic plants, etc.). Disease symptoms are the impact of the pathogen on its hosts (chlorotic foliage, growth reductions, contorted form, etc.). Signs of disease are the visible parts of the pathogen (fruiting bodies, hyphae, etc.).