EPWS 310 - Plant Pathology

Lectures - Fall 2002


Lecture 5


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:




*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.


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