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Cultivar Mixtures

Effect of cultivar mixtures on the evolution of pathogen races or pathotypes

The effect of the cultivar mixtures on pathogen evolution can be analyzed based on two key questions:

i.) Will a given resistance be more durable when deployed in a mixture than deployed in a monoculture?
The lower level of exposure to the pathogen population of the resistance gene in a mixture compared with monoculture will reduce selection pressure on the pathogen population and therefore increase the gene’s durability (Mundt, 2002).

ii) Considering a given number of resistance genes, will they be more durable if deployed in a mixture than deployed sequentially in monoculture or combined into a single host genotype?
Mixtures support more diverse pathogen populations than do pure stands, and that diversity is positively related to the degree of disease control provided by the mixture (Mundt, 2002). The mixtures lead, generally, to the evolution of complex pathogen races potentially capable of overcoming many resistance genes and therefore reduce the efficacy of the crop mixture as a measure to reduce disease. However, the appearance of complex races does not mean that those races will be selected for within the mixtures. There are several factors that can prevent the prevalence of complex races. The relative efficacy and durability of the resistance genes deployed in crop mixtures compared with the other two options, depends on the rate of progress of those races in the mixture, which can be affected by several factors:

- Diversity within pathotypes. Increasing diversity reduces the rate of increase of complex pathotypes in crop mixtures (Lannou, 2001)

- Fitness cost associated with virulence. The ability of complex races to attack multiple host genotypes is countered by a reduction in fitness associated with the lack of avirulence genes (Mundt and Lannou, 1997; Lannou, 2001; Mundt, 2002)

- Differential adaptation. Complex races can infect different host genotypes but have reduced infection efficiency compared with the specific simple races corresponding to each component of the mixture, reducing therefore their rate of progress (Lannou, 2001; Mundt, 2002).

- Density dependence. It is the decrease in pathogen multiplication rate associated with an increase in lesion density. It could affect simple and complex races differentially during an epidemic and reduce selection for complex races (Lannou and Mundt, 1997).

The progress towards complex pathogen races may be relatively slow. Single races or pathotypes capable of overcoming a single resistance gene should progress faster than complex races capable of overcoming multiple resistance genes, precluding the former of becoming dominant within the pathogen population.

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Contents

Introduction

What is a cultivar mixture?

Mechanisms by which cultivar mixtures suppress disease

Effect of cultivar mixtures on epidemic development

Effect of Cultivar mixtures on the evolution of pathogen races or pathotypes

Crops and diseases suited to cultivar mixtures

Use of cultivar mixtures to manage multiple diseases

How many cultivars make a good mixture?

Reported successes with cultivar mixtures

Agronomic considerations

References