PLANT DISEASES DEVELOPMENT

PLANT DISEASES DEVELOPMENT

1. TERMS

   • PATHOGENICITY
     • Qualitative-all or or nothing. A species (or isolate or mutant) is pathogenic or it’s not.
   • VIRULENCE
     • Quantitative-degree of pathogenicity.by geneticists to describe new races of pathogens, which have overcome major resistance (R) genes (when they are describing pathogenicity!)
   • AGGRESSIVENESS
     • Equivalent to original use of virulence. Less ambiguous perhaps.

2. Stages in the Development of Disease: Infection Process

   • 1) Inoculation
     • Initial contact of a pathogen with a site of plant where infection is possible.
       • Any part of the pathogen that can initiate infection  1) Fungi: spores, sclerotia or hyphae  2) Bacteria, mollicutes, protozoa, viruses or viroids: whole individual  3) Nematodes: adults, juveniles or eggs
       • Primary inoculum causes primary infections
       • Secondary inoculum causes secondary infections
   • 2) Pre-penetration
     • Spore attachment & germination
     • Hatching of nematode eggs
     • Recognition between host & pathogen
   • 3) Penetration
     • Direct penetration through intact plant surfaces( fungi & nematode)
     • Penetration through wounds (Fungi, bacteria, mollicutes, viruses & viroids)
     • Penetration through natural openings
       • stoma, hydathode (open pores at margins & tips of leaves), nectarthode & lenticels (openings on fruits, stems & tubers – less efficient) Fungi & bacteria
   • 4) Invasion
     • Fungi
       • 1) Produce intra- & intercellular mycelium or haustoria. 2) Some invade xylem vessels (vascular wilt diseases)
     • Bacteria
       • 1) Invade inter- & intracellular host tissues E.g., xylem vessels
     • Nematodes
       • 1) Most do not invade host cells but feed by piercing epidermal cells with their styles. 2)Some invade inter- or intracellular tissues
     • Viruses, viroids, mollicutes, fastidious bacteria & protozoa invade host tissues
   • 5) Colonization
     • Symptom:  1) All visible & detectable changes in the infected plants  2)Fast: 2-4 days after inoculation, e.g., localized viral diseases 3) Slow: 2-3 years after inoculation (latent infection), e.g., diseases caused by viruses & mollicutes
     • Growth and reproduction of the pathogen in or on infected tissues
     • Successful colonization results in the appearance of symptoms