1. Plant Disease Development
CHAPTER 3
Plant Disease Development

2. Factors for successful disease development
Disease triangle

3. Disease triangle Three important components of plant disease :
Susceptible host
Virulent pathogen
Favourable environment
For disease to occur all three of these must be present.

5. Properties of pathogen
Level of virulence
Adaptability
Dispersal efficiency
Survival efficiency
Reproductive fitness

6. Properties of host Susceptibility Growth stage & form
Population density & structure
General health

7. Properties of environment
Temperature
Rainfall / Dew
Leaf wetness period
Soil properties
Wind

8. Intensive questions Question 1 Define the following terms
Virulent (2 marks)
Susceptible (2 marks)
Question 2
Describe the main characteristics of pathogen for successful disease development. (5 marks)
Give the other two factors that are required for successful disease development. (2 marks)
With the aid of a diagram describe disease triangle. (8 marks)

9. Stages in disease development
Inoculation
Penetration
Infection
Growth and Reproduction
Dissemination of Pathogen

10. Inoculation
Inoculation = The arrival of pathogen on the host Inoculum = The pathogen (s) that land on the host or Any part of the pathogen that can initiate infection.

11. Types of inoculum
Primary inoculum = An inoculum that survives the dormant period and caused the first infections Primary infections = infection caused by primary inoculum Secondary inoculum = An inoculum produced from primary infections Secondary infections = infection caused by secondary inoculum

12. Inoculation process Landing or arrival of inoculum
Germination of spores and seeds ( fungus)
Hatching of nematode eggs
Attachment of Pathogen to Host (virus and bacteria)
Recognition Between Host and Pathogen (Pathogen recognize the host)

13. Penetration The initial invasion of a host by a pathogen
Direct Penetration
Direct with haustoria
Direct, subcuticular only
Direct with appressorium  penetration peg  intracellular mycelium

14. Direct with haustoria

15. Direct, intercellular mycelium

16. Direct with appressorium

17. Penetration Through Natural Openings
a. Through stoma
b. Through lenticels
c. Through hydathodes
Penetration Through Wounds
a. Through wound
b. Through natural cracks between main and lateral roots.
c. Openings made by fungus

18. Infection
Establishment of pathogen in the host and take the nutrients from the host Successful infection will produce symptoms

19. Growth and Reproduction of
The pathogen will grow and multiply within the infected host.

20. Dissemination Dissemination by air Dissemination by water
Dissemination by Insects, Nematodes & other Vectors
Dissemination by Seed and Transplanting process
Dissemination by human