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