EPWS 310 - Plant Pathology

Lectures - Fall 2002


Lecture 4

Readings Chapter 2

A. Stages in the development of disease

The disease cycle- involves the changes in the plant and the plant's symptoms as well as those in the pathogen and spans periods within a growing season and from one growing season to the next. It refers primarily to the appearance, development, and perpetuation of the disease as the pathogen relates to it rather than to the pathogen itself.

1. Inoculation

2. Penetration


3. Establishment of infection


4. Colonization (invasion)


5. growth and reproduction of the pathogen


6. dissemination of the pathogen


7. survival of the pathogen in the absence of the host (overwintering or oversummering of the pathogen).



1. Inoculation- when the pathogen comes in contact with the host. Inoculum- the pathogen (s) that come into contact with the host.


            Propagule = one unit of inoculum. Colony forming units.


A.    Types of inoculum- Primary inoculum - inoculum that survives the overwintering or oversummering. The infections it causes are called primary infections.

B.     Secondary inoculum- inoculum produced from primary infections. The secondary inoculum in turn induces primary infections.


            B. Sources of inoculum?



            C. Landing or Arrival of Inoculum?



2. Penetration-

            A. Germination of Spores and seeds-

            -requires certain environmental conditions


            *spore germination is often stimulated by plants

            *Fungistasis- Due to toxic metabolites in the soil or competition, the spore is not able to germinate or the hyphae lyses soon after germination.

What is hyphae?

What is germ tube? 

            -What affects direction to penetration sites??

Zoospores- chemical stimuli, some of elongation of roots, physical simuli, nutrient gradient.

            -Seeds penetrate by producing a radicle that penetrates or produces a haustoria.



B. Attachment of Pathogen to host

How do they stick? Viruses, phytoplasmas, protozoa, and fastidious bacteria are placed directly into cells of plants by their vectors. Fungi, bacteria, and parasitic higher plants must first become attached. They have on their surface polysaccharides, glycoproteins, polymers of hexosamines, and fibrillar materials, when moistened becomes sticky and helps the plant to adhere. LECTINS. Germ tubes also have this characteristic. The muscilaginous material contains degradative enzymes that chew away the outer cell wall.


C. Recognition between host and pathogen

Not known for sure how it works. 

D. Penetration-the act.

            1. Natural openings-stomata, lenticels, hydathodes, nectarthodes.

            2. Wounds-

            3. Direct penetration- enzymatic or pressure.


Direct penetration- Explain about appressorium, penetration peg, intercellular hyphae, intracellular hyphae, haustoria. Penetration in parasitic higher plants is similar to fungi. Penetration by nematodes is accomplished by repeated back and forth thrusts of their stylets.


3. Infection- the process by which pathogens establish contact with the susceptible cells or tissues of the host and procure nutrients form them. During infection pathogens grow or multiply, or both, within the plant tissues and invade and colonize the plant to a lesser or greater extent.

            -one result of infections is symptoms. An infection that does not appear right away is called a latent infection. The latency is due to environmental conditions or maturity level of the host. Symptoms can occur at 2 days or 2 years (phytoplasmas) depending on the pathogens, environment and host. IN most cases, symptoms occur within two weeks of initial invasion.

            -Incubation period- the time between inoculation and the appearance of disease symptoms.

            -Invasion- 1. Subcuticular,

2. surface of the plant but send haustoria into the epidermis,

3.     intracellular or intercellular,

4.     xylem tissue.

5.     Bacteria invade intercellularly until the cell wall breaks down and then the bacteria grows intracellularly.

6.     Viruses, viroids, and phytoplasmas invade intracellularly,

7.     nematodes invade intercellularly in most cases.

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