Plant L8 – Breeding For Disease Resistance And L9 – Breeding For Disease Resistance - Different Strategies

Front 1

Explain the GENE-FOR-GENE CONCEPT?

Back 1

FOR EACH GENETIC LOCUS IN THE HOST GOVERNING RESISTANCE AND SUSCEPTIBILITY, THERE IS A SPECIFIC AND RELATED LOCUS IN THE PATHOGEN GOVERNING VIRULENCE AND AVIRULENCE.

Front 2

Explain BREEDING FOR DISEASE RESISTANCE STRATEGY 1: INCORPORATING A SINGLE VERTICAL RESISTANCE (VR) GENE

Back 2

STRATEGY 1
INCORPORATING A SINGLE VERTICAL RESISTANCE (VR) GENE
•Places selection pressure on pathogen to evolve new races.
•Virulence gene may already be present in pathogen in low frequency or may arise by mutation.
•Subject to rapid and complete breakdown, usually with substantial yield losses

Front 3

Explain STRATEGY 2
TWO OR MORE VR GENES

Back 3

•With 2 or more VR genes in a cultivar, require 2+ mutations for virulence to overcome each VR gene, then recombination into one race.
•This has a very low probability of occurrence.
•Cannot use individual VR genes separately and cannot add the VR genes as single steps.
•This strategy used for stem rust in Australia since 1950’s/1960’s - two or more Sr genes pyramided into same cultivar.

Front 4

Explain STRATEGY 3
HORIZONTAL RESISTANCE (HR)

Back 4

•Selection pressure on pathogen eliminated. HR is equally effective against all races.
•Not subject to rapid or complete breakdown.
•HR under polygenic control and is difficult to select for in segregating populations.

Front 5

Explain STRATEGY 4
MODERATE (PARTIAL) RESISTANCE

Back 5

•Genes confer moderate resistance only (therefore more damage is caused than with strong VR genes).
•Mature plants show a large number of small prickles of fungus (in stem rust).
•Limited selection pressure on pathogen as all races could build up on host.

Front 6

Explain STRATEGY 5
VERTICAL RESISTANCE & HORIZONTAL RESISTANCE TOGETHER

Back 6

•Benefits of VR. Less dramatic yield loss if VR breaks down, due to HR.
•Difficult to select for HR when VR genes present.

Front 7

Explain STRATEGY 6
ANY RESISTANCE YOU CAN FIND

Back 7

•Often have very little choice.
•Often have little knowledge of type of resistance gene, inheritance etc.
•Any form of resistance is better than none.
- e.g. - blackleg in canola in Australia
- faba beans in Australia: 2-5 sprays
required for control of chocolate spot. Resistant cultivars now available.
- Ascochyta in chickpeas.

Front 8

Explain STRATEGY 7
MULTILINE VARIETIES

Back 8

•A variety consisting of a number of phenotypically similar lines (6-15), each possessing a different VR gene.
•A race carrying virulence genes for all VR genes is not likely to occur by mutation and recombination.
•If there are 6 lines in the multiline, selection pressure for each new virulence gene to arise is one sixth that for a single VR gene in a genetically uniform cultivar.
•Any new race arising will not be able to increase sufficiently rapidly to cause serious damage.

Front 9

Explain STRATEGY 7a
CLEAN CROP MULTILINES

Back 9

•All component lines of the multiline would be resistant to all current races.
•If the resistance of one component breaks down, it is immediately replaced.
•Aims to keep crop, as far as possible, free from disease.

•Unlikely to get serious yield losses, with only one line likely to break down at any time.
•Healthy, resistant plants may be able to outcompete weakened, diseased plants and partially compensate for loss of one component.
•Defect – simultaneous exposure to the pathogen of a substantial fraction of the available resistance genes.
- e.g. CIMMYT – wheat rust.

Front 10

STRATEGY 7b dirty crop multilines

Back 10

•Each component would be attacked by only one race. The remaining lines act as spore traps and reduce the rate of disease build up.
•Would extend life of resistance genes.
•Relies on stabilizing selection to ensure that simple races predominate – OPEN TO SERIOUS QUESTION.

Front 11

STRATEGY 8
GENE DEPLOYMENT – GEOGRAPHICAL MANAGEMENT OF VR GENES

Back 11

Need to manage release of cultivars containing resistance genes in such a way as:
–to not exert pressure on pathogen to overcome resistance genes in crop
–to provide a useful reserve of resistance genes for further breeding.

Front 12

What are some SOURCES OF RESISTANCE?

Back 12

•Closely related material – local and foreign cultivars, local land races.
•Primitive cultivars and weedy relatives.
•Related species and genera.
•Induced mutation.
•Transgenic sources e.g. proteinase inhibitors etc.