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Plant Disease Development And Disease Cycle

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Parasitism and Pathogenicity

Parasitism

The removal of food by a parasite from its host.

In some cases, both the plant & the microbe benefit from the association called symbiosis.

Pathogenicity

The ability of the parasite to interfere with one or more of the essential functions of the host, causing disease.

Parasitism plays an important role, but not the most important role in pathogenicity.


Development of Disease in Plants

The amount of disease developed depends on 3 factors

Pathogen (virulence, abundance, etc.)

Host (susceptibility)

Environment (conducive)

The Disease Triangle


(virulence, abundance, etc.)

(susceptibility)

(conducive)

Amount of Disease


Stages in the Development of Disease: Infection Process

1. Inoculation

2. Pre-penetration

3. Penetration

4. Invasion

5. Colonization


Stages in the Development of Disease: Infection Process (cont’)

1. Inoculation Initial contact of a pathogen with a site of

plant where infection is possible.

Inoculum Any part of the pathogen that can initiate

infection Fungi: spores, sclerotia or hyphae Bacteria, mollicutes, protozoa, viruses or

viroids: whole individual Nematodes: adults, juveniles or eggs



Types of inoculum1. Primary inoculum causes primary infections2. Secondary inoculum causes secondary

infections

Sources of inoculum Branches, trunks, roots of plants Plant debris, soil in the field Seeds, transplants, tubers, other propagative

organs Sources outside the field (nearby plants or

fields) Perennial weeds, alternate hosts



2. Pre-penetration

Spore attachment & germination

Hatching of nematode eggs

Recognition between host & pathogen


3. Penetrationi. Direct penetration through intact plant surfaces

Fungi & nematodes

ii. Penetration through wounds

Fungi, bacteria, mollicutes, viruses & viroids

iii. Penetration through natural openings

E.g.: stoma, hydathode (open pores at margins & tips of leaves), nectarthode & lenticels (openings on fruits, stems & tubers – less efficient)

Fungi & bacteria




4. Invasion

Fungi:

Produce intra- & intercellular mycelium or haustoria

Some invade xylem vessels (vascular wilt diseases)

Bacteria:

Invade inter- & intracellular host tissues

E.g., xylem vessels



4. Invasion (cont’)

Nematodes:

Most do not invade host cells but feed by piercing epidermal cells with their stylets

Some invade inter- or intracellular tissues

Viruses, viroids, mollicutes, fastidious bacteria & protozoa invade host tissues



5. Colonization

Growth and reproduction of the pathogen in or on infected tissues

Successful colonization results in the appearance of symptoms



5. Colonization (cont’)

Symptom:

All visible & detectable changes in the infected plants

Fast: 2-4 days after inoculation, e.g., localized viral diseases

Slow: 2-3 years after inoculation (latent infection), e.g., diseases caused by viruses & mollicutes


Disease Cycle

Source: Agrios (2005)


Dissemination of Pathogens

Important agents:

Air/wind

Water

Insects, mites, nematodes & other vectors (virus dissemination)

Seeds, propagation materials & plant debris

Animals

Human


Infection Process in Fungi

No Stage Phase

1 Inoculation

2 Attachment

3 Spore germination Pre-penetration

4 Growth of germ tube

5 Appressorium formation

6 Penetration of host cells Penetration

7 Colonization

8 Disease symptom appearance Post-penetration

9 Dissemination of inoculum

10 Pathogen death


Infection Process in Fungi (cont’)

Attachment

Spore adherence to plant surface

Requires mucilage (sticky) substances

Spore germination

Requires stimulation, e.g., contact with host surface, hydration, host-derived molecules

Germ tube formation & extension



Appressorium formation

Contains lipids, polysaccharides & proteins

Some contain melanin (dark brown pigment)

High turgor pressure (40 times > a car tire) due to accumulation of glycerol

Function: to puncture plant cuticle using penetration peg (physical force)



A=Appressorium, PP=Penetration Peg, IM=Intracellular Mycelium


Stages 1-7



A germinating conidium with a germ tube covered with extracellular material.

U=Uredospore, GT=Germ tube, A=Dome-like appressorium




Colonization structures Intracellular mycelium (IM)

Intercellular mycelium (ITE)

Haustoria

Source: Agrios (2005), Webster (1988)

Haustoria

ITE



Stages 1-8: Incubation period

(Inoculation to disease symptom appearance)

Stages 1-9: Generation period

(Inoculation to dissemination of inoculum)

Stages 9-10: Infection period

(Dissemination of inoculum to pathogen death)


Thank You


Quiz

1. Define parasitism and pathogenicity

2. Name 3 components of Disease Triangle

3. Name 5 stages in the development of disease

4. Name two important dissemination agents of pathogens

5. What is the function of appresorium?


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