Phytopathology

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Phytopathology or plant pathology is the science of diagnosing and managing plant diseases. It covers all infectious agents that attack plants and abiotic disorders, but does not include herbivory by insects, mammals, etc. Approximately ten percent of food production is lost to disease worldwide.

Common plant pathogens (in rough order of importance) include species from these groups:

Fungi - the most common pathogens (root rot fungi, molds, rusts, and mildews).

Water molds (Oomycetes) - such as Phytophthora infestans (potato blight).

Bacteria - prokaryotic organisms which include as many as 18 genera.

Viruses - more than 700 are plant pathogens, comprised of only protein and DNA.

Roundworms (Nematoda) - both as pathogens and vectors, the only animals normally regarded as plant pathogens.

Viroids - small, single-stranded RNA pathogens causing a dozen plus diseases.

Parasitic angiosperms - plants that parasitize other plants, mistletoe and witchweed are common examples.

Slime molds (Myxomycetes) - usually not actual pathogens but may be aesthetically unpleasant.

Phytomyxea - true pathogens represented by the genera Polymyxa and Plasmodiophora

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Fungal plant pathogens

The majority of plant pathogens are fungal, with ascomycetes the leading group. Fungal plant pathogens can be described as polycyclic if the disease is capable of making spores and reinfecting plants during a growing season, or monocyclic if it has to wait until a new season. Fungal pathogens are also defined by the types of spores they create and the methods by which they enter the plant tissue. Once the pathogen enters the plant tissue it either creates a haustorium and grows inside the living tissue (biotroph) or kills the surrounding cells and lives off the dead tissue (necrotroph). Indications of pathogen presence are divided into signs and symptoms. Signs are observational evidence of the pathogen (such as visible sporulation). Symptoms are secondary evidence that a pathogen is present (such as wilting of the leaves).

Significant abiotic disorders can be caused by:

Natural

Drought

Frost damage, and breakage by snow and hail

Flooding and poor drainage

Nutrient deficiency

Salt deposition and other soluble mineral excesses (e.g. gypsum)

Wind (windburn, and breakage by hurricanes and tornados)

Lightning and wildfire (also often man-made)

Man-made (arguably not abiotic, but usually regarded as such)

Soil compaction

Pollution of air and/or soil