Relative measurements

In practice, horizontal resistance can be satisfactorily measured only with relative measurements. That is, the resistance of a line is described relative to other lines in the screening population, or to other commercial cultivars. A separate comparison should be made for each locally important species of parasite. These comparisons can be made in plant growth chambers, field trials, or even farmers' fields.

This is an aspect of constant ranking (see 6.1). For example, we can say that Cultivar 'A' is more resistant to a specified parasite than Cultivar 'B'. And this will be true in all seasons, and in all localities. The resistance of both cultivars may be inadequate in one season, or one locality, and excessive in another, but their ranking will not change. Or, if the ranking does change, it will be by an amount that is not statistically significant.

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