EPWS 310 - Plant Pathology

Lectures - Fall 2002


Lecture 3

A.    Disease pyramid= includes component of time also

B.     Plant disease epidemics

Epidemics occur when there are additions or changes to the components of the triangle such as:




C. Genetic uniformity

            1. Many of our major crops are genetically uniform within their particular species. (Page 170-172 Agrios).


            2. Pesticide use. Use of pesticides to control plant diseases and other pests has been increasing steadily


            3. Genetic make-up of crops in the past.

            Farmers saved seed that was adapted to the area and the pests.

D. Components of an epidemic

1. Susceptible host

2. Pathogen

3. Environmental factors

4. Time

E. Examples of epidemics as a result of genetic uniformity

            a. Late blight of potato in Northern Europe (1845-6)..

            b. Southern corn leaf blight (1970)-

The pathogen: fungus Bipolaris maydis.

The host: corn produced in the U. S. contained TMS cytoplasm.


            c. Helminthosporium blight of Victoria oats- (1947)

Pathogen: fungus Bipolaris victoriae,

Host: oats.


            d. Coffee rust in Ceylon (COFFEE or TEA)-

Host: Coffee (Coffea arabica)

Pathogen: rust fungus Hemileia vastatrix,


F. Diagnosis of Plant Disease

Diagnosis of plant disease is both an art and a science. The first step in diagnosis is to determine between environmental (abiotic) and organismal (biotic) effects. The basic symptoms of pathogens and parasites are as follows:

1. Diseases caused by parasitic higher plants- Ex. dodder, mistletoe, witchweed

2. Diseases caused by nematodes-

3. Diseases caused by fungi and bacteria- When fungi or bacteria are found in the plant tissue, it must be determined whether the organism is a pathogen or secondary saprophyte.

Fungi- The morphology of the fungal hyphae, spores and fruiting structures needs to be determined.

Bacteria- Based on symptoms, the appearance of a large number of bacteria in the area, and the absence of any other pathogens.

4. Diseases caused by phytoplasmas- Diseases caused by phytoplasmas appear as stunting of plants, yellowing or reddening of leaves, proliferation of shoots and roots, abnormal flowers, and eventual decline and death of the plant. Phytoplasmas are small, wall-less bacteria that live in the phloem of the hosts, invisible. Therefore, diagnosis is based on symptoms, graft transmission, insect vectors, electron microscopy, sensitivity to certain antibiotics (Tetracycline but not penicillin), sensitivity to high temperatures, and PCR.

5. Diseases caused by viruses and viroids- ID by symptoms-mosaics, streaking, epinasty, curling, stunting, discoloration.

Identification also by

1. virus transmission test to specific host- use vectors such as insect, nematode, fungus, or mite. Vector- an animal able to transmit a pathogen.

2. serological tests.

3. electron microscopy.

4. Microscopic examination of infected cells for specific crystalline or amorphous inclusions.

5. electrophoresis/ dsRNA/ PCR

6. Hybridization of commercially available radioactive DNA complementary to a certain viroid RNA, with the viroid RNA present in plant sap and attached to a membrane filter.

G. Parasitism and disease development terminology

1. Parasite- an organism that lives on or in some other organism and obtains its food from the latter.

2. Symbiosis- both host and non-host benefit. Mycorrhizal fungi, Rhizobium bacteria (nitrogen fixers).

3. Pathogenicity- The ability of the parasite to interfere with one or more of the essential functions of the plant, with parasitism frequently playing an important, but not always the most important role.

4. Biotrophs, obligate parasites- They can grow and reproduce in nature only in living hosts. Obligate parasite doesn't kill the host directly, but redirects the nutrients to itself.  Examples?

5. Nonobligate parasites- Parasites that can live on either living or dead hosts and on various nutrient media. The non-obligate lives like a saprophyte in a living host.

6. Facultive saprophytes- Parasites that can live on dead organic matter, but live most of their life cycle on a living host.

7. Facultative parasites- Parasites that live most of their lives on dead organic matter but under certain environmental conditions, can attack living tissue and become parasitic.

8. Saprophyte- organisms that lives on dead organic tissue.

9. Virulence-the degree of pathogenicity of a given pathogen. Often a quantitative scale when comparing isolates in a population.

            (scale) Virulence (10)-------------------------Avirulence (1)

            Pathogenicity is a qualitative scale- either a pathogen or not.

10. Aggressiveness- a quantitative measure of the severity of disease over time in a pathogen population.

11. Host range - plants that a parasite can infect . 


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