Crop vulnerability means that a crop is susceptible to a foreign parasite which, however, is absent from the region in question. When that parasite is accidentally introduced, the vulnerability is manifested, and potential parasitism becomes actual parasitism. Crop vulnerabilities can vary from the trivial to the catastrophic.
Historically, two of the worst crop vulnerabilities were the potatoes of Europe prior to the introduction of blight and Colorado beetle. Two more were the European wine grapes susceptible to Phylloxera and downy mildew, which were both new-encounter parasites accidentally imported from the New World.
Crop vulnerability is important to amateur plant breeders in two ways. First, they should make themselves aware of the crop vulnerabilities in their region, and they should be very careful not to introduce foreign parasites on imported breeding stocks. They should familiarise themselves with their country's phytosanitary regulations and take great care to obey them. This is particularly true of breeding clubs in island nations, such as the United Kingdom, Australia, Madagascar, etc.
Second, a serious crop vulnerability may be realised during the breeding activities of a club, and their new cultivars would then be susceptible to the foreign parasite. This can be very disappointing as promising new cultivars would be ruined. However, the club members should immediately make horizontal resistance to the new parasite one of their selection criteria, provided that this was legal. They may well be the first to solve this new problem, and to provide a valuable service to their region.
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