The 'one pathotype technique' must usually be employed against all parasites in which a gene-for-gene relationship occurs. This will ensure that the resistance that emerges from the breeding program is horizontal and not vertical. There are a few occasions when the one-pathotype technique is not necessary. For example, if the on-site selection is being conducted in the centre of origin of the crop, any vertical resistances that may occur are likely to break down so quickly that they will not matter. Equally, now that the second mating type (A2) of Phytophthora infestans has been spread all over the northern hemisphere, there is probably no need to use the one-pathotype technique when screening for horizontal resistance to this disease. This is because functional oospores, which exhibit great genetic variability, will be the main initial inoculum in blight epidemics.
It is impossible to see, measure, or screen for horizontal resistance if vertical resistances are present and operating. For this reason, all vertical resistances must be matched, and thus inactivated, during screening. The one-pathotype technique is the only effective method of ensuring this and, fortunately, it is an easy technique to employ.
However, the one-pathotype technique is another aspect of breeding for horizontal resistance that is somewhat specialised, and amateur breeders will probably need technical assistance. The first step is to find out how many of the locally important parasites have a gene-for-gene relationship. The next step is to employ the one-pathotype technique for each one of them. This will ensure that all vertical resistances are matched during the screening process, regardless of how the vertical resistance genes may recombine during sexual reproduction in the host population.
For each species of parasite in which a gene-for-gene relationship occurs, a single vertical pathotype is chosen. This becomes the designated pathotype, and it must be cultured on its matching, designated host for the entire duration of the breeding program. It is essential that only one pathotype be designated for each species of parasite.
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