Horizontal Resistance to Plant Diseases

1
Horizontal Resistanceto Plant Diseases

• John Navazio
• Organic Seed Alliance

2
Plant Disease Basics

• Pathogen disease causing agent
• Disease - the resultant effects of parasitism by
  a pathogen
• Resistance any inherited characteristic of a
  host plant which lessens the effects of
  parasitism
• Tolerance parasitism is not impeded, but the
  host suffers only marginal loss of yield and/or
  quality

3
How Do PathogensCause Disease?

• All elements of the Disease Triangle are present
  Pathogen, Host, Environment
• Pathogen must be present and reach the surface of
  the host
• Pathogen must grow when environmental conditions
  are favorable (establishment)
• Pathogen must colonize (colonization)
• Pathogen must reproduce (reproduction)

4
Vertical Resistance to Disease

• Term coined by Vanderplank in 1950s
• Vertical resistance is AKA qualitative
  resistance or race specific resistance
• Almost always conferred by a single gene
• Each resistance gene usually confers resistance
  to one race of the pathogen
• Hypersensitive Reaction is dramatic
• Easy to recognize and to screen for by breeders
• These single genes almost always overcome by
  new races of the pathogen

5
Horizontal Resistance to Disease

• Term coined by Vanderplank in 1950s
• Horizontal resistance is AKA quantitative
  resistance or durable resistance
• Always conferred by multiple genes
• Confers a level of resistance to all races of the
  pathogen also new contact races
• It is a rate reducing process to the
• establishment
• colonization
• reproduction
• It is equivalent to a strong constitution

6
Horizontal Resistance to Disease

• Horizontal resistance (HR) is not complete
• The pathogen is able to survive thereby it is
  possible to have a stable ecological balance
  between the pest and crop
• By allowing a number of races to survive, some
  more virulent, some less virulent, then when they
  intermate/genetic change there will be a wide
  range of virulence in the population of the
  pathogen

7
Goode Thoughts

• HR requires high management by the breeder of
  both the pathogen and the host, but requires
  little by the grower
• VR breeders and pathologists have been patching
  their mistakes and bragging about how big their
  patches are!
• Quotes from Dr. Jack Goodes lectures in Plant
  Pathology, Univ. of Arkansas, 1978