Week4

• Disease is a common occurrence but a wholly satisfactory definition of the term has eluded plant pathologists for so long
• Scientists have labored to give a precise definition to a rather complex and broad phenomenon
• Definitions of disease

  • PARASITE = an organism that depends wholly or partly on another living organism for its food. Most parasites are pathogens
  • OBLIGATE PARASITE = an organism that is restricted to subsist on living organisms and attack only living tissues
  • FACULTATIVE PARASITE = an organism which has the ability to be a parasite although it is ordinarily a saprophyte
  • SAPROPHYTE = an organism that lives on dead organic and inorganic matter
  • FACULTATIVE SAPROPHYTE = an organism that has the ability to become a saprophyte but is ordinarily a parasite
  • HOST = in plant pathology, refers to the plant that is being attacked by a parasite
  • SUSCEPT = a plant that is susceptible to a disease whether or not the pathogen is parasitic
  • PATHOGENICITY = the capacity of a pathogen to cause disease
  • PATHOGENESIS = disease development in the plant
  • VIRULENCE = the quantitative amount of disease that an isolate of a given pathogen can cause in a group of plants
  • AGGRESSIVENESS = measures the rate at which virulence is expressed by a given pathogenic isolate
  • DISEASE RESISTANCE = the inherent ability of an organism to overcome in any degree the effect of the pathogen
  • SUSCEPTIBILITY = the opposite of resistance
  • TOLERANCE = exhibited by a plant which is severely affected by a pathogen without experiencing a severe reduction in yields
• Resistance and Susceptibility

  Are relative terms and can be depicted on a scale; the more resistant the plant is, the less susceptible it is and the more susceptible the plant, the less resistant it is
• Tolerance is exhibited by a plant which is severely affected by a pathogen without experiencing a severe reduction in yields
• Requisites for disease development

  • A susceptible plant
  • The Pathogen
  • Favorable Environment
• Primary Symptoms are those that are the immediate and direct result of the causal agent activities on the invaded tissues whereas Secondary Symptoms are the effect of the distant and uninvaded plant parts
• For example, in the Panama wilt disease of banana caused by Fusarium oxysporum pv. Cubense
• Primary Symptoms

  Immediate and direct result of the causal agent activities on the invaded tissues
• Secondary Symptoms

  Effect of the distant and uninvaded plant parts
• Panama wilt disease of banana caused by Fusarium oxysfurom pv. Cubense

  • Primary symptom: Rotten/browning of vascular tissue
  • Secondary symptom: Wilting of the plant
• Localized Symptoms

  • Distinct and very limited structural changes usually in the form of lesions such as canker, leaf spot, and gall
• Systemic Symptoms

  • More generalized pathological conditions such as mottling, bunchy, and wilting
• Systemic Symptoms

  • Pepper Mottling Virus Disease
  • Banana Bunchy Top Virus
  • Wilting
• Symptoms classification

  • Necrotic Symptom
  • Hypoplastic Symptom
  • Hyperplastic Symptom
• Necrotic Symptom

  Involves the death of the protoplast, cells, or tissues. Examples are spot, blight, scorch, canker, and dieback
• Necrotic Symptom

  • Scorch Disease on Trees
  • Dieback Disease on Trees
• Hypoplastic Symptom

  Appear when there is inhibition or failure in the differentiation and development of some aspect of plant growth. Examples are stunting, chlorosis, mottle, mosaic, curling, and rosetting
• Hypoplastic Symptom

  • Bunchy Top
  • Rosetting of Citrus
• Hyperplastic Symptom

  Expressed with the occurrence of excessive multiplication, enlargement, or overdevelopment of plant organs including the abnormal prolonged retention of the green color. Examples are gall formation, fasciation, scab, premature defoliation or fruit drop, and greening
• Specific symptoms

  • Dumping-off
  • Dieback
  • Etiolation
  • Fasciculation of fasciation
  • Flecks
  • Gumming or Gummosis
  • Leak
  • Mosaic
  • Mottling
  • Mummification
  • Phyllody
  • Pitting
  • Rosetting
  • Rotting
  • Russeting
  • Sarcody
  • Savoying
  • Scab
  • Shot-hole
  • Spot
  • Streak or Stripe
  • Vein clearing
  • Virscence or greening
  • Wolting
• Blast = term applied to the sudden death of young buds, inflorescence of young fruits
• Blotch = large irregular spots on leaves and fruits with necrotic injury of epidermal cells
• Callus = an overgrowth of tissue formed in response to injury in an effort of the plant to heal the wound
• Canker = an often sunken necrotic area with cracked border that may appear in leaves, fruits, stems, and branches
• Chlorosis = yellowing caused by some factor other than light, such as infection by a virus or mycoplasma
• Curling = abnormal bending or curling of leaves caused by over-growth on the other side of the leaf or localized growth in a certain portion
• Dieback = a dying backward from the tip of twigs or branches
• Etiolation = yellowing of normally growing tissues caused by inadequate light
• Fasciculation of fasciation = clustering of roots, flowers, fruits, or twigs around a common focus
• Signs of plant diseases

  • Fungal Mycella
  • Spores and Fruiting bodies
  • Bacterial ooze
  • Sclerotial Bodies
  • Nematodes at various growth stages
  • Plant parts of phanerogams (Parasitic Flowering plants)
• Examples of signs

  • Fungal Mycella
  • Spores and fruiting Bodies
  • Bacterial ooze
  • Sclerotial Bodies
  • Nematodes of various growth
  • Plant parts of phanerogams (Parasitic Flowering plants)
• Example of Bacterial Blight of Rice

  • Bacterial ooze
• Examples of Nematodes

  • Meloidegyne incognita
  • Radohpolus similis
  • Cuscuta campestris, parasitic on Glechoma hederacea. This is one of the most common species of dodder worldwide.
• Plant disease diagnosis is the identification of specific plant diseases through their characteristics symptoms and sign including other factors that may be related to the disease process. Correct plant disease diagnosis is necessary for recommending the appropriate control measures, and in plant disease surveys.
• Diagnosis based on symptoms alone is not a very reliable means of identifying a particular malady since some diseases exhibit the
• Correct plant disease diagnosis

  Is necessary for recommending the appropriate control measures, and in plant disease surveys
• Diagnosis based on symptoms alone is not a very reliable means of identifying a particular malady since some diseases exhibit the same symptoms although their pathogens may be very different
• The presence of signs of the disease increases the reliability of diagnosis but it should be borne in mind that it is quite common to find secondary invaders or saprophytic microorganisms in the invaded portions of the plant especially during advanced stages of infection
• The structures of innocuous organisms may be mistaken for the primary pathogen