Both the inoculation of screening populations, and the grid screening techniques, are improved by using the same field for each season of screening. This will lead to a build up of soil-borne parasites, and a reduction of the patchy distribution problems. However, the build-up of parasites, and a possible increase in the parasitic ability of facultative parasites (see 6.6.3), can obscure gains in horizontal resistance. Obviously, this effect can be very misleading, and it can also be very demoralising. However, it can be easily revealed by the use of a susceptible cultivar that is used as a standard, and which will demonstrate a steadily increasing level of parasitism during the course of the breeding program.
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