All mechanisms of protection against crop pests and diseases can be classified into one of two categories. Stable mechanisms do not break down to new strains of the crop parasite. They provide durable protection. Unstable mechanisms do break down in this way, and they provide only a temporary protection.
Vertical resistance involves single genes taken from a gene-for-gene relationship. Any combination of these genes operates against some strains of the crop parasite but not others. Its function is to reduce the frequency of matching allo-infections. Vertical resistance consequently fails to function on the appearance of a new, matching strain of the parasite. It provides an unstable protection.
Horizontal resistance, on the other hand, is the resistance that invariably remains after vertical resistance has been matched. Its function is to protect the host against matching allo-infections, and it operates equally against all matching strains of the parasite. It provides a stable protection.
Most natural insecticides provide a stable protection. In the Far East, rotenones (extracted from derris roots) have been used for centuries against body lice without any resistant strains appearing. Similarly, pyrethrins (extracted from the flowers of pyrethrum) have also been used for centuries to control fleas and bed bugs in Dalmatia without any resistance developing. Oils on water provide a stable protection of mosquitoes, and on stored beans a stable protection against weevils.
Most synthetic insecticides are unstable. DDT-resistant houseflies are the best known example.
The famous Bordeaux mixture is a stable fungicide as well over a century of use have demonstrated. So too are other copper formulations and the bisdithiocarbamate fungicides. But fungicides such as metalaxyl are unstable.
It seems that all antibiotics are unstable, as our medical colleagues know to their cost.