Plant disease - Definition

Phytopathology or Plant Pathology is the science of diagnosing and managing plant diseases. Its covers all infectious agents that attack plants and abiotic disorders, but does not include herbivory by insects, mammals, etc.

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

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

Insects

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

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

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

Nematodes (Roundworms) - 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.

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

Labyrinthulomycetes (Plasmodial slime molds) - biologically more complex protists than the Myxomycetes, true pathogens represented by two genera, Polymyxa and Plasmodiophora

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

Salt from winter road salt application

Herbicide over-application

Poor education and training of people working with plants (e.g. lawnmower damage to trees)

Vandalism

Example Usage of disease