Strategies for Major GenesOne gene at a timeGene rotationGene “pyramids”MixturesRegional deployment

Working With Minor Genes

Minor Gene Resistance

• Often multiple gene inheritance• Often recessive• Usually gives incomplete

resistance• Can be more complicated to

work with• Not highly specific• Tends to be stable over time

Minor Gene Resistance

• Often more available than we think

• Simply purging the most susceptible materialcan be very useful

• Inheritance is much less complex than once thought

Estimating Number of Genes and Hertitability

• Statistical, quantitative genetic approaches

• QTL approaches

• Both give same answer

Minor Gene Resistance

Commonly only 2-4 genes identified

Often 1 or 2 that contribute majority of effect

Heritability usually moderate to high

Estimating Number of Genes and Hertitability

• Statistical, quantitative genetic approaches

• QTL approaches

• Both underestimate gene number• Both depend on population size

Barley Stripe Rust

Pop. Size Number of QTL50 3 100 4 150 5 200 5.5 300 6 409 8

Barley Stripe Rust• Number of QTL increased with population size

• ID QTL with biggest effect with small population size

• ID additional QTL with smaller effects with larger population size

Components of Resistance

Length of latent period

Infection efficiency

Lesion size

Sporulation rate

Length of infectious period

Selection Based on Components of

ResistanceComponents tend to be highly

correlated

More recent evidence suggests that components are pleiotropically controlled

Example of barley stripe rust

Can be relatively easy to

maintain minor gene resistance once you

have it

VIDAL L., INRA

Stripe Rust Stripe Rust Puccinia striiformisPuccinia striiformis

Use of Top Crosses to Maintain Minor Genes

for Stripe Rust Resistance While Improving Other

Traits

Stuff Happens

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Effects of Interplot Interference (uredia/tiller) Isolated Adjacent t

Cultivar 3x4 m 2x2 m 1/4x1 mL98 1000 500 2300Sultan 750 250 1700Volla 110 40 700Julia 17 12 450Vada 1 15 130Range 1000X 42X 18X

Test with Single Race or Multiple Races?

No. races No. of to which lesions Cultivar resistant per cm2 Zenith 64 2Kataktara 55 14Kano 51 40 39Lacrosse 19 66KTH 4 94

Vertifolia Effect

Percent of Willamette Valley Winter Wheat Acreage Planted to ‘Foote’

Year Percent2001 222002 502003 502004 512005 322006 4

Lessons:

The Vertifolia Effect can be real

“Perfect Storms” do happen

QTL Analysis and MAS

Source: CAST

QTL Analysis and MAS

Molecular Breeding

• Biology and engineering are very different

• High-quality phenotyping is still the most important ingredient of a resistance breeding program, and this likely will always be so

Strategies for Major GenesOne gene at a timeGene rotationGene “pyramids”MixturesRegional deployment

Working With Minor Genes

Single Gene at a Time“Use It Till You Lose it”

• May be needed as a stop-gap measure

• In general, don’t go there- Puts growers at risk- Disruptive to breeding

programs

Rotating Genes

• Likely a disaster

• Virulence unlikely to decline to previous levels

Pyramids or Combinations of Major

Genes• Useful in increasing durability of major genes• More complex than using single genes• Takes time to determine which combinations work best

Mixtures or Blends of Major Genes

• Can be highly effective

• Should be considered more often

• Results variable, empirical testing critical

Regional Deployment

• Likely very useful if done

• Is it feasible?

Minor Gene Resistance• It’s hard to go wrong with this approach

• May take a bit more effort up front

• Likely to eliminate a lot of headaches in the longer term

Tolerance

Don’t Forget About IPM

Potato late blight severity:

Susceptible, no fungicide = 100%Weekly fungicide = 56%Minor gene resistance = 46%Resistance + fungicide = 9%

Stuff Happens

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