Hops, which are now used almost exclusively for brewing beer. The plant is a perennial vine which dies back to ground level each fall. The above-ground parts thus have a discontinuous pathosystem and they have vertical resistance. It is a long-day plant and it is dioecious. It is propagated vegetatively, and only about eight clones dominated world production until quite recently. These include 'Fuggle' and 'Golding' in Britain, 'Hallertaur' in Bavaria, and 'Saaz' in Czechoslovakia. These clones are mostly ancient, and they demonstrate the utility and durability of horizontal resistance. A breeding program started in Germany in 1922 accumulated polygenic resistance to downy mildew by breeding within the European population. This was one of the earliest examples of horizontal resistance being chosen over vertical resistance. This is not a crop for amateur breeders.
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