Field measurements

In practice, the best way to measure parasite damage and, hence, horizontal resistance, is by crop yield, provided that this yield is produced (i) in the presence of parasites, and (ii) in the absence of crop protection chemicals. Indeed, this is the only practical method of field measurement of horizontal resistance, and it is based on the following considerations:

A susceptible line cannot yield well in the presence of parasites, however high its potential yield may be.

A line that has both a high level of horizontal resistance, and a high yield potential, will yield well in spite of the presence of parasites.

A highly resistant line with a low yield potential will give a false indication of susceptibility, but this is unimportant as such a line has little practical value and it can be discarded without loss.

However, these field measurements must be made under the conditions of on-site selection. That is, they must be made in (i) the area, (ii) the time of year, and (iii) according to the farming system, of future cultivation.

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