Some plants are more susceptible to diseases than others, but all plants are susceptible to pathogens or exposure because of less-than-ideal conditions (poor soil, moisture and nutrients). Damage caused by storms, accidents, animals and other events also put a plant at risk, as it uses a lot of its resources to repair the injury.
There are more than 8,000 species of fungi and fungal-like organisms that cause diseases in plants. Fungi bring about preharvest and post-harvest diseases and produce toxic, carcinogenic and hallucinogenic chemicals. These chemicals affect not only the plants, but also affect humans and animals.
There are four categories of fungi and fungal-like organisms. Facultative saprophytes (Phytophthora and Botrytis) live on dead and decaying plant matter, and survive as a parasite. Facultative parasites survive as saprophytes, but they can become parasites, which cause disease. Obligate parasites only live and grow on living hosts. Downy mildew, powdery mildew and rusts are obligate parasites. Obligate saprophytes are always saprophytes, never parasites. The organisms only live and grow on dead and decaying plant matter.
Bacterial, single-celled prokaryotic organisms do not have a defined nucleus, and they reproduce asexually, splitting one cell into two. Pathogenic bacteria (the "bad" bacteria) cause severe--and sometimes fatal--diseases in plants, animals and humans. According to Ohio State University, anthrax was discovered in cattle and sheep in 1876, and shortly after, fireblight of the pear and apple, which is caused by Erwinia amylovora, was discovered by T.J. Burrill (University of Illinois) in 1877.
Fastidious vascular bacteria grow in phloem tissues of the plant, and they block water and nutrients from reaching portions of the plant, eventually killing it. Bacteria are spread by several ways, including by leafhoppers and psyllids.
Viruses infect living organisms, including plants. Martinus W. Beijerinck discovered the first virus--the tobacco mosaic virus--in 1898. Viruses are transmitted via mechanical or insect vector. The most common way that a virus is spread is through the grafting of plants and rooting cuttings. The virus moves with the existing plant material to new plant material. Insects such as aphids, leafhoppers and whiteflies (those that have piercing mouthparts) suck the sap from infected plants. When the pest moves to a noninfected plant, it transmits the virus to the healthy plant with its infected mouthparts.