A total, qualitative failure of vertical resistance resulting from a matching allo-infection. Being matched, the vertical resistance stops functioning, and it is said to have broken down. In a wild pathosystem, which has genetic diversity, breakdowns occur only in individual host plants. In a crop pathosystem, which has genetic uniformity, the breakdown involves the entire cultivar, because every allo-infection, from plant to plant within that crop, is a matching infection. Because some matching always occurs, vertical resistance is temporary resistance. Because horizontal resistance operates against matching pathotypes of the parasite, it does not break down in this way; it is durable resistance. See also: discontinuous pathosystem.
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