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:
1.
2.
3.
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.
Environment:
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 .