disease triangle

disease triangle

1. pathogen

   • virulence

2. environment

   • conducive

3. host

   • suscepitable

4. stages in plant disease development and disease cycle

   • 1st stage: Inoculation
     • Initial contact of a pathogen with a site of plant where infectcion is possible.
     • inoculum
       • any part of teh pathogen that can initiate infection
       • types of inoculum
         • primary inoculum
         • secondary inoculum
         • fungi
           • spores,sclerotia or hyphae
         • bacteria,mollicutes,protozoa,viruses or viroids
         • nematodes
   • 2nd stage: Pre-penetration
     • recognition
       • recognition between host and pathogen
     • spore attachment and germination
     • hatching nematode eggs
   • 3rd stage: Penetration
     • direct penetration
       • direct penetration through intact plant surfaces
       • fungi
       • nematode
     • through wounds
       • fungi
       • bacteria
       • mollicutes
       • viruses and viroid
     • through natural opening
       • stoma
       • hydathode
         • open pores at margins and tips of leaves
       • nectarthode&lenticels
         • opening on fruits, stems,tubers ----less efficient
         • fungi and bacteria
   • 4th stage: Invasion
     • fungi
       • produce intra-&intercellualr mycelium or haustoria
       • some invade xylem vessels
     • bacteria
       • invade inter-&intracellular host tissues
         • xylem vessels
   • 5th stage:colonization
     • growth and reproduction of the pathogen in or on infected tissues successful colonization results in the appearence of symptoms
     • symptom
       • all visible and detectable changes in the infected plants
       • slow
         • 2-3 years after inoculation
           • diseases caused by viruses and mollicutes
       • fast
         • 2-4days after inoculation
           • localized viral diseases
   • Infection Process in Fungi
     • pre-penetration
       • 1. Inoculation
       • 2. Attachment
         • spore adherence to plant surface requires mucilage substances
       • 3. Spore germination
         • requires stimulation germ tube formation & extension
       • 5.Appressorium formatio
         • contains lipids,polysaccharides and proteins some contain melanin (dark brown pigment) high turgor pressure due to accumulation of glycerol function:to puncture plant cuticle using penetration peg
       • 4. Growth of germ tube
     • penetration
       • 6.Penetration of host cells
     • post-penetration
       • 8.Diseases symptom appearence
       • 9. Dissemination of inoculum
       • 10. Pathogen death
       • 7.Colonization
         • IM
         • ITE
         • haustoria