Types of Pathogens: Viruses, Bacteria, Fungi, Oomycetes and Nematodes

The specificity of plant pathogens can vary widely. Often the host range is restricted to only one species. This applies, in particular, to biotrophic pathogens. However, there are also examples of pathogens infecting multiple hosts. Several necrotrophic fungi (e.g. Botrytis species) cause disease in hundreds of plant species.

Viruses basically consist of a protein coat enclosing nucleic acids (DNA or RNA) that carry the information for the synthesis of only a handful of proteins. For their replication they have to force a host cell to produce these proteins, which then assemble and give rise to new virus particles. Unlike viruses infecting bacterial or animal cells, plant viruses do not have to destroy their host cell in order to release newly formed virus particles, which then attack neighbouring cells. Instead, they move through plasmodesmata from cell to cell and spread systemically via the phloem. Initial infection of a host plant mostly occurs through inoculation of the phloem via virus-carrying phloemfeeding insects such as aphids. Typical symptoms of virus-infected plants are yellowing or a mosaic-like colouration of leaves, stunted growth and sometimes the death of a plant.

Microbial pathogens typically infect only specific tissues or organs. Phytopathogenic bacteria, for example, can cause root rot, leaf chlorosis or stem dieback, depending on the pathogen and its infection strategy. Bacterial pathogens usually enter a plant through wounds or the stomata and multiply in the apoplast. Only a minute fraction of bacterial species have evolved the ability to infect plants. No more than a few hundred species have been described as pathogens of crop plants. Most disease-causing bacteria are gram positive and belong to a limited number of clades (e.g. Xanthomonas, Pseudomonas, Erwinia and Dickeya).

Economically the most important groups of plant pathogens are the oomycetes and fungi. Oomycetes were for a long time regarded as fungi, but phylogenetically they belong to the Chromalveolata and are thus very distant from fungi. Arguably the most famous plant pathogen is an oomycete. Phytophthora infestans is the causal agent of potato blight and was responsible for the Irish famine in the mid-nineteenth century, which left millions of people dead and triggered a mass emigration from Ireland to North America. Downy mildews are oomycetes too.

Among the possibly millions of fungal species living in association with plants, only a small fraction cause disease (Crous et al. 2015). Nonetheless, no other group of pathogens accounts for more yield loss than fungi. Important genera with phytopathogenic species include Botrytis, Alternaria, Colletotrichum, Fusarium and Puccinia.

A few taxa of nematodes are plant pathogens. Presumably because they almost exclusively attack the root system, they are less well studied. Phytopathogenic nematodes are obligate bio- trophs—that is, they are absolutely dependent on a plant host for existence. Endo- and ectoparasites are differentiated according to whether they penetrate root tissue or not, respectively. Most of the considerable nematode damage to crop plants is due to infection by cyst and root-knot nematodes.