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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