Understanding Spatial Patterns for Using Genetic Diversity in the Landscape: Within-Field.

Though randomly selected genetic diversity may sometimes provide practical benefits and protection against unknown risks, functional diversity is the actual goal of agricultural diversification. In natural ecosystems, functional diversity is selected via the process of evolution.  In agriculture, functional diversity is attained through purposeful manipulation based on knowledge of mechanism, via routine testing, or a combination of the two.  The use of multiline cultivars and cultivar mixtures to control rust and mildews of small grain species has been studied extensively for nearly 60 years.  The effectiveness of such mixtures varies depending upon epidemiological conditions and experimental procedure. There are now examples of both low and high levels of disease control via diversification for a range of pathogens, including those that are soil-borne, vectored by insects, splash-dispersed, and those with low host specificity, as well as both annual and perennial crops and plants of widely varying size. Diversification must be considered within the entire suite of crop production goals, including yield, yield stability, crop quality, and reaction to abiotic stresses. Within-field diversity should be considered as a potential tool within all plant breeding programs, but should neither be glorified as a simple solution to all problems nor rejected out-of-hand as an impractical approach.