The system of locks and keys of the gene-for-gene relationship and the vertical subsystem requires a discontinuous pathosystem in order to function. In practice, this means that vertical resistance will occur only in seasonal tissues. That is, in all tissues of an annual plant, and in the leaf and fruit tissues of deciduous trees and shrubs. Crops that are derived from continuous pathosystems (e.g., sugarcane, sweet potato, cassava, olives) will not have any vertical resistances. However some crops have perennial tissue that is functionally seasonal, as with coffee leaf rust. Secale cereale
Rye, which differs from all the other temperate cereals in being open-pollinated (maize is technically a tropical cereal). For this reason, rye responds to selection pressures during cultivation and it generally has good levels of horizontal resistance to all locally important parasite.
Rye is the least important of the temperate cereals and it is used mainly in the manufacture of rye bread and rye whisky. Historically, it was important in those areas of Europe that could not grow wheat and which suffered periodically from ergot poisoning. These areas, such as Ireland, eastern Germany, Poland, and western Russia later replaced rye with potatoes, and they suffered more than most from potato blight. See also: Rimpau.
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