This document outlines the key stages in the pathogenesis or disease cycle of pathogens: inoculation, pre-penetration, penetration, and post-penetration. Inoculation involves the pathogen coming into contact with the host via inoculum such as spores or mycelium. Pre-penetration includes spore germination, attachment, and recognition between the pathogen and plant. Penetration can occur through natural openings like stomata or directly through the plant surface. Post-penetration consists of infection, invasion, and reproduction of the pathogen within the host leading to symptoms, colonization, and survival or dissemination of the pathogen.

1. Pathogenesis
• It is a process or chain of events by which disease
development occurs.

2. Events in Disease Cycle
• Inoculation
• Pre Penetration
• Penetration
• Post Penetration

3. Inoculation
• It is the process by which the pathogen come in contact with its host
• Inoculum: any part or propagule of pathogen that can cause disease
(may consist of single unit of the pathogen or whole of the pathogen)
• Inoculum potential/ density

4. Types of Inoculum
• Primary inoculum (Sclerotia, mycelium,
oospore)
• Secondary inoculum (conidia, uredospore,
zoospores)

5. Infection structures
• Germ tube
• Appresorium
• Infection hyphae
• Haustoria
• Infection cushion
• Rhizomorphs

6. Pre Penetration
• The process by which the pathogen enter its host.
-Attachment of pathogen to host.
Tip of spore become sticky to adhere to the plant. Then the
fungal spore germinate and produce the germ tube. The tip of the
germ tube becomes bulge and its is called appressorium with the aid
of appressorium it with hold the host surface.
-Spore germination
-Recognition between plant and pathogen.
Plants have lectins (protein and glycoprotein) as recognition
factor on their surface. The pathogen compounds that acts as elicitors
of reginition by the host plant includes glucan and chitin.

7. Penetration
• Direct penetration
• Indirect penetration

8. Indirect penetration through natural opening

9. Penetration
• Penetration through natural openings - Stomata / hydathodes /
lenticles.
Eg. Fungal diseases and Pseudomonas tabaci
Erwinia amylovora
Streptomyces scabies
Phytophthora arecae
• Penetration through wounds / Insects/ nematodes
Eg. Xanthomonas campestris p.v oryzae
Cocoa pod rot
Fusarium wilt
• Direct penetration

10. Post Penetration
• Infection
The process of establishment of pathogens
Local infection
Systemic infection
“ successful infection result in the appearance of symptom”
• Symptom- visible changes in the infected plant.
• Invasion
Successful establishment / spreading of pathogens
• Reproduction and growth of pathogen (Colonization)

11. • Survival
• Dissemination
• Roll of Enzymes
• Roll of Toxins