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Every known plant species gets attacked by many pathogens which induce several diseases. We will present some of the most popular diseases on some of the important crops.

Infection Process in Plants

Agrios pp. 44-62

Host status - non-host or compatible host.

Host range - host range of a specific pathogen may be narrow - infects 1-3 hosts; a wide host range refers to a pathogen capable of infecting many host species.

Inoculum - any structure of pathogen that can resume growth and development .

Inoculation - pathogen and host come in physical contact.

Infection - the process by which pathogens establish contact with host cells or tissue and procure nutrients from them for growth and reproduction. Pathogens types (fungi, bacteria etc.) may differ in mode of infection.

Incubation period - time interval between inoculation and the appearance of symptoms or signs.

Pathogenesis - the process of disease induction from penetration to colonization to symptom expression to pathogen reproduction. Pathogenesis is governed by a complex biochemical pathways associated with host and pathogen.

Disease - successful interaction between a pathogen, susceptible host and favorable environmental factors (abiotic and biotic) and usually result in symptom and sign expression. Degrees of disease severity, and host disruption host vary.

Events during the infection process

Attachment - the infection process begins when a pathogen and host come in contact.

Infection court- physical location where inoculum and host come in contact.

Host and pathogen recognition - chemical signals between plant and pathogen determine whether infection process proceeds.

Penetration - entrance of pathogen into host directly, natural openings, wounds or vectors.

Invasion of plant tissues - pathogen attempts to establish "organic union" with plant.

Colonization - host responds or does not respond to pathogen to resist colonization.

Growth and reproduction of pathogen - pathogen successfully colonizes the host and establishes an organic union.

Symptom and sign development- change in plant phenotype as a result of interactions among host, pathogen and environment.

Dissemination- pathogen movement to new infection courts.

Production of over-seasoning stages - pathogen may be inactive as dormant propagules or static vegetative forms; or active as parasites in vectors or alternate hosts or active on host surface or in decomposing plant residue.

Koch's Postulates - Proof of Pathogenicity

Many organisms are associated with symptomatic plant tissue. Are they capable of causing disease? Are they pathogenic? What classes of organisms are present? Could the symptoms be caused by an abiotic factor? Agrios pp. 39-40 and Lab Manual.

Frequent Association with Symptomatic Tissue

The (suspected) pathogen must be (consistently) found associated with the disease (symptoms) in all plants examined.

Isolation from Symptomatic Tissue

The (organism) pathogen must be isolated and grown in pure culture on nutrient media and its characteristics described (nonobligate parasites), or it must be grown on a susceptible host plant (obligate parasites), and its appearance and effects recorded.

Inoculate Host with Isolated Organism

The pathogen (organism) from pure culture must be inoculated on healthy plants of the same species or variety on which the disease (symptoms) appears, and it must produce the same disease (symptoms) on the inoculated plants.

Symptoms on Inoculated Host

Symptoms should develop on the inoculated plants that are identical to those observed on the original plants.

Reisolation from Inoculated and Symptomatic Tissue

The pathogen must be isolated in pure culture again, and its characteristics must be exactly like those observed in step 2.

Terms Associated with Pathogen x Plant Interactions

Pathogen Terms	Host Terms
Non-pathogenic: lack ability to invade, infect, cause symptoms and reproduce. No disease	Non-host reaction: no interaction with pathogen.
Pathogenic: ability to penetrate, infect, cause symptoms and reproduce resulting in symptom expression (disease).	Host reaction: plant species is susceptible; allows infection, expresses symptoms and supports pathogen reproduction.
Virulence: expressed as ability of pathogen to cause disease and reproduce.	Genotypes of host are susceptible to virulent forms of pathogen.
Avirulence: expressed as inability of pathogen to cause disease and reproduce.	Genotypes of host are resistant to avirulent forms of pathogen.
Pathogen race: genotypes (races, strains) of pathogen that are virulent or avirulent to different genotypes of host.	Genotypes of host express differential resistance or susceptibility to different races (genotypes) of pathogen.
Aggressiveness: Virulent forms of pathogen that cause differing degrees of symptom severity.	Different forms of pathogen cause varying degrees of symptom severity on susceptible host genotypes. Susceptible host genotypes have similar interaction phenotypes to all forms of pathogen.

Types of Host Interaction Phenotypes

Susceptible: phenotypic expression related to extensive symptom development and/or pathogen reproduction and accomplished by uninhibited invasion of host by pathogen.

Resistant: phenotypic expression related to complete or partial suppression of symptom severity and/or pathogen reproduction and accomplished by arrested or slowed invasion of host by pathogen.

Partial susceptibility or resistance: expression of symptoms, but less than full susceptibility or greater than complete resistance.

Diseases of Field Crops:

Stem rust of wheat and barley

Stinking smut (common bunt) of wheat

Southern blight

Ergot of rye

Take-all root rot of small grains and turfgrass

Barley yellow dwarf

Tobacco mosaic

Lesion nematode

Soybean cyst nematode

Diseases of Vegetables: