Another feature of the pathosystem concept is that the parasitism is studied in terms of populations, at the higher levels and in ecologic aspects of the system.
It also applies to wild populations and to agricultural, horticultural, and forest crops, and to tropical, subtropical, as well as both subsistence and commercial farming.
This is self-evident from the evolutionary survival of wild plant pathosystems, as systems, during periods of geological time.
[7][6] This system of locking cannot function in a crop pathosystem in which the host population has genetic uniformity.
This type of error is called sub-optimization and it results from working at too low a systems level.
Comparable biological emergents are the schooling of fish, and the flocking of birds, which cannot be observed at any systems level below that of the population.
[2] It can also be argued that the gene-for-gene relationship must function on a basis of heterogeneity in the wild pathosystem because the gross instability of the 'boom and bust'[4] of modern plant breeding would have no evolutionary survival value.
With the onset of a new growing season, all discontinuous host tissue (e.g., new leaves of a deciduous tree, newly germinated annual seedlings, or newly emerged tissue of a perennial herb) is unmatched and each host individual has a vertical resistance that is functioning.
This alternation of matching and non-matching (or unlocking and re-locking) is an essential feature of any system of locking, and it is possible only in a discontinuous pathosystem.
Crops that are derived from a continuous wild pathosystem (e.g., aroids, banana, cassava, citrus, cocoa, coconut, date palm, ginger, mango, oil palm, olive, papaya, pineapple, pyrethrum, sisal, sugarcane, sweet potato, tea, turmeric, vanilla, yams) have no gene-for-gene relationships, not withstanding a few erroneous reports to the contrary.
[2] Breeding for comprehensive horizontal resistance will require simultaneous quantitative improvements and will eventually control all the parasites that have epidemiological competence in a particular agro-ecosystem.
Of particular importance is the concept of parasite interference, first defined by Vanderplank,[8] who called it the cryptic error in field trials.
[2] This factor, which has only recently been recognised, largely explains the almost total neglect of horizontal resistance during the twentieth century.