Strategies for Major Genes One gene at a time Gene rotation Gene “ pyramids ” Mixtures Regional 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 material can 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. SizeNumber of QTL

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 Resistance Components 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 Puccinia striiformis

Use of Top Crosses to Maintain Minor Genes for Stripe Rust Resistance While Improving Other Traits

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Effects of Interplot Interference (uredia/tiller) Isolated Adjacent t Cultivar 3x4 m 2x2 m 1/4x1 m L Sultan Volla Julia Vada Range 1000X 42X 18X

Test with Single Race or Multiple Races? No. races No. of to which lesions Cultivar resistant per cm 2 Zenith64 2 Kataktara5514 Kano Lacrosse1966 KTH 494

Vertifolia Effect

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

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 Genes One gene at a time Gene rotation Gene “ pyramids ” Mixtures Regional 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%

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