EPWS 310 - Plant Pathology
Lectures - Fall 2002
The best means of disease control, particularly against biotrophic pathogens is resistant varieties. Varieties that already have resistance to the pathogen(s) are least expensive, most effective, and environmentally safest.
A. Genes and Disease
Some plant genes code for the resistant mechanisms we have discussed.
The pathogen has genes that code for virulence mechanism. The occurrence and interaction of specific genes for virulence in the pathogen and of specific genes for susceptibility in the host that determine the initiation and development of disease.
The gene or genes for virulence in the pathogen are usually specific for one or a few related kinds of host plants. Also, the genes that make a host plant susceptible to a particular pathogen are present only in that one host and possibly a few related kinds of host plants.
2. Compatible (disease resulting) interaction
Why are some pathogens virulent to several hosts?
Examples- Phymatotrichum omnivorum-- attacks 2000 different species
Puccinia graminis var. tritici --attacks wheat only
Fusarium oxysporum f. sp. lycopersici--attacks tomato only
3. Resistance is the rule, susceptibility the exception
4. Introducing a new resistance gene
A resistance gene confers resistance to the different races of a pathogen. However, it has been observed that once a new resistance gene is in a plant population, new virulent races often occur.
*How did this new population of pathogens acquire the new gene for virulence?
B. Stages of Variations in Pathogens
Species - the entire population of an organism on earth, for example a fungal pathogen, has certain morphological characteristics in common and makes up the species of the pathogen.
Some of the individuals within this species attack only wheat or barley or oats. These groups are called varieties or special forms (formae specialist).
Within these varieties or f. sp. there are races that attack only certain host plant varieties.
An offspring of a race that can suddenly attack a new variety or can cause severe symptoms on a variety that it could barely infect before - called a variant or Biotype. Each race may consist of several biotypes
C. Types of Plant Resistance to Pathogens
Each kind of plant is a non-host to the vast majority of known plant pathogens.
Non-host are immune (non-host resistance).
1. True resistance - Disease resistance that is genetically controlled by the presence of one, a few, or many genes. The host and pathogen are incompatible with one another due to chemical recognition or other defense mechanisms.
2 kinds of true resistance:
1. Horizontal resistance-
Many genes (multigene resistance) control horizontal resistance.
2. Vertical resistance- very resistant to some races and very susceptible to others.
*usually controlled by one or a few genes. (monogenic or oligogenic)
2. Apparent resistance- susceptible plants that do not become infected.
a. Disease escape-lack all parts of disease triangle.
B. Tolerance to disease is the ability o f plants to produce a good crop even when they are infected with a pathogen.
The Gene- for-gene concept - for each gene that confers resistance in the host there is a corresponding gene in the pathogen that confers virulence to the pathogen, and vice versa.