Download Horizontal Resistance to Plant Diseases

Horizontal Resistance
to Plant Diseases
John Navazio
Organic Seed Alliance
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
How Do Pathogens
Cause 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)
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
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”
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
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 Goode’s lectures in
Plant Pathology, Univ. of Arkansas, 1978