How does host genotype diversity affect the pathogen population?

Technically…

“Mid-component”: the mean of the mixture’s component cultivars when they are grown as pure stands

More on disease reduction mechanisms in small-grain blends

From data in Finckh & Mundt, 1992, Stripe rust, yield, and plant competition in wheat cultivar mixtures, Phytopathology 82:905-913

Club wheat mixture: components can be distinguished by head color

Good reviews of host genotype diversity literature

Characteristics that predict whether blends will reduce disease (Garrett & Mundt, 1999)

“Wheat blends stabilize yields,” The Furrow, John Deere, January 1997.

Wheat blends in the U.S. (NASS, planted acres)

Cereal blends internationally

Barley mixtures planted until political re-unification of East and West Germany when government support for the project stopped.

Other blend experiences with disease reduction

Potato late blight – mixed results:

Hemileia vastatrix – coffee rust

Figure 12. Susceptible Coffea arabica on the left; a resistant hybrid with C. canephora on the right.

Willows (and poplars) grown as “short-rotation coppice” – major energy crop in N. Ireland, UK, Sweden

Rust caused by the fungus Melampsora is the most serious disease in short rotation coppice (SRC) willow plantations for renewable energy. Willows (Salix) are grown as a major SRC crop  for energy because of their yield potential and coppicing ability. In most willows, rust only infects expanded leaves (left picture). However, in some willows, such as Salix viminalis and S. caprea, rust also attacks stems and young leaves. When severe, rust defoliates susceptible plantings prematurely (right picture) and reduces yields by as much as 40%. Severe rust also predisposes plants to infections by secondary pathogens which may lead to death of the plants.

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Didelot et al, 2007, Effects of cultivar mixtures on scab control in apple orchards, Plant Pathol. 56:1014-1022

Will host diversity select for super-races?

Chin and Wolfe (1984, Plant Pathol. 33:535-546) : planted pure stands and mixtures of 2 or 3 barley cultivars (H, W, M), each with a different gene for mildew resistance

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Summary: Chin and Wolfe

Pathogen evolution in mixtures

Mixtures of cultivars with different levels of partial resistance

Most empirical studies of mixtures of susceptible and partially resistant cultivars are of splash-dispersed necrotrophs on cereals

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A given R gene should be more durable deployed in a mixture than a monoculture