🦠 Phenomenon of Infection

Infection is a stepwise biological process in which the pathogen reaches, enters, and establishes itself in host tissue under favorable conditions.

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Phases of Infection

The process is commonly divided into:

1. Pre-entry phase.
2. Entry phase.
3. Colonization phase.

Failure at any stage can prevent disease.

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Pre-Entry Events

Before penetration, inoculum must survive on the host surface, germinate, and recognize entry sites.

Important pre-entry processes:

• Adhesion to host surface.
• Germ tube growth.
• Appressorium formation in many fungi.
• Influence of moisture, temperature, and nutrients.

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Entry Mechanisms

Pathogens enter by:

• Direct penetration through cuticle and epidermis.
• Natural openings such as stomata, hydathodes, and lenticels.
• Wounds created mechanically or by vectors.

Direct penetration often involves mechanical pressure plus enzymes such as cutinases, cellulases, and pectinases.

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Colonization and Establishment

After entry, pathogens colonize intercellular or vascular tissues.

Nutrition strategy types:

• Biotrophs: feed on living cells.
• Necrotrophs: kill tissue and feed on dead matter.
• Hemibiotrophs: initial biotrophic phase followed by necrotrophy.

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Consequences for Symptom Development

Colonization disrupts water flow, nutrient transport, and metabolism, producing wilt, chlorosis, necrosis, blight, rot, and systemic decline patterns.

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Summary Cheat Sheet

Infection Stage	Key Event
Pre-entry	Germination and host recognition
Entry	Penetration via direct, natural, or wound routes
Colonization	Tissue establishment and nutrient acquisition
Biotroph	Keeps host cells alive
Necrotroph	Kills host cells before feeding
Hemibiotroph	Starts biotrophic, later necrotrophic

References

1 source • [1]

[1]

Plant Infection Biology

Book

Used for: Pre-entry, entry, and colonization framework used in introductory plant pathology.