EPWS 310 - Plant Pathology
Lectures - Fall 2002
The Disease Cycle
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).
-Colonization- by fungi- can grow
throughout the plant and then produce spores.
Bacteria- divide every 20 to 30 minutes.
Number becomes very large. Fastidious bacteria and mycoplasmas reproduce much
slower than bacteria and are usually in lower numbers in the plant.
Viruses and Viroids-reproduce in the
individual cells.
Nematodes- female lays about 300-600 eggs,
about 1/2 are females. Two to 12 generations produces per year. Each generation
increases the number of nematodes in the soil by 100 fold.
4. Dissemination - spread of
pathogen inoculum.
Dissemination is mostly passive.
Air-most of these spores do not contact a
susceptible hosts. They have a better chance in monculture. What would they
have hit in a polyculture? Rusts occur at several thousand meters above
infected fields and can be carried for miles.
Water- Important in disseminating
pathogens in three ways:
1.
2.
3.
*Water dissemination is more efficient in
that the pathogens land on an already wet surface and can move or germinate
immediately.
-Insects, mites, nematodes, and other
vectors-
Aphids and leafhoppers are primary vectors
for viruses. Leafhoppers are the main vectors for mycoplasmas and fastidious
bacteria. The Dutch elm disease also depends on the a bug. In these vectored
diseases, the pathogen is completely dependent on the vector.
*Very efficient method of transmission.
Humans-within a field, machinery,
tools, airplane.
5. Survival- When the host tissue
dies, whether an annual or perennial plant, the pathogen survives until the new
season.
a. Methods of survival
1:-fungi- perennial plants, mycelium in
infected tissues. Annual plants- mycelium in infected plant debris, as resting
spores, sclerotia, on seeds, tubers.
Soil inhabitants- able to survive
indefinitely as saprophytes.
Soil transients- are
specialized parasites that generally live in close association with their host
but may survive in the soil for relatively short periods of time.
2. bacteria-same as fungi. Many overwinter
in insect vector.
3. viruses-in living plant tissue such as
the tops and roots of perennial plants, the vegetative propagating organs, and
in the seeds of some hosts. Some viruses overwinter in their vectors. TMV
(cigarettes).
4. Nematodes-as eggs in the soil and in
plant roots .
5. Parasitic plants- survive either as
seeds or as their vegetative form on their host.