EPWS 310 - Plant Pathology
Lectures - Fall 2002
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.
-Nematodes-
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.