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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.
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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
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(virulence, abundance, etc.)
(susceptibility)
(conducive)
Amount of Disease
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Stages in the Development of Disease: Infection Process
1. Inoculation
2. Pre-penetration
3. Penetration
4. Invasion
5. Colonization
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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
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Stages in the Development of Disease: Infection Process (cont’)
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
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Stages in the Development of Disease: Infection Process (cont’)
2. Pre-penetration
Spore attachment & germination
Hatching of nematode eggs
Recognition between host & pathogen
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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
Stages in the Development of Disease: Infection Process (cont’)
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Stages in the Development of Disease: Infection Process (cont’)
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
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Stages in the Development of Disease: Infection Process (cont’)
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
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Stages in the Development of Disease: Infection Process (cont’)
5. Colonization
Growth and reproduction of the pathogen in or on infected tissues
Successful colonization results in the appearance of symptoms
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Stages in the Development of Disease: Infection Process (cont’)
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
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Disease Cycle
Source: Agrios (2005)
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Dissemination of Pathogens
Important agents:
Air/wind
Water
Insects, mites, nematodes & other vectors (virus dissemination)
Seeds, propagation materials & plant debris
Animals
Human
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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
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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
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Infection Process in Fungi (cont’)
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)
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Infection Process in Fungi (cont’)
A=Appressorium, PP=Penetration Peg, IM=Intracellular Mycelium
Source: Agrios (2005)
Stages 1-7
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Infection Process in Fungi (cont’)
A germinating conidium with a germ tube covered with extracellular material.
U=Uredospore, GT=Germ tube, A=Dome-like appressorium
Source: Agrios (2005)
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Infection Process in Fungi (cont’)
Colonization structures Intracellular mycelium (IM)
Intercellular mycelium (ITE)
Haustoria
Source: Agrios (2005), Webster (1988)
Haustoria
ITE
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Infection Process in Fungi (cont’)
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)
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Thank You
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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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