My old friend Ivan Buddenhagen first proposed that parasites come in three categories, according to their history. Old encounter parasites evolved with their host, and they have been in contact with it since its original domestication. This happens in the centre of origin as well as in new areas, when both the host and the parasite were taken there together.
New encounter parasites evolved on a botanical relative, but they remained out of epidemiological contact with their domesticated host. They were brought into contact with their domesticated host by people.
Re-encounter parasites evolved with their host, but they were separated from it when people took the host to another part of the world, leaving the parasite behind. At a later date, the parasite is inadvertently introduced to the new area where it re-encounters its host.
Blight, Colorado beetle, and the temperate virus diseases are new encounter parasites of potato. This may mean that a few more breeding cycles will be necessary than if these were old encounter parasites. However, it would be quite wrong to conclude that it is impossible to obtain adequate levels of horizontal resistance to new encounter parasites. High levels of horizontal resistance to blight have been clearly demonstrated. The fact that no one has achieved high levels of horizontal resistance to Colorado beetle is because such breeding is only now being attempted (see Fisher et al, Recommended Reading). And it seems that no work has been attempted with horizontal resistance to the temperate virus diseases of potato.
Most other potato parasites are re-encounter parasites. The host tends to lose resistance in the absence of a parasite. If the separation between host and parasite has been lengthy, the loss of horizontal resistance may be considerable. The classic example of this occurred with the maize of tropical Africa.
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