Some important influences of the origins of crops on pest and disease susceptibility were first recognised by Buddenhagen (1977).

A new encounter parasite is one which evolved away from its agricultural host, usually on a closely related wild host species. A classic example is potato blight (Phytophthora infestans) which evolved in Mexico and was brought into contact with cultivated potatoes by people. Another example is the Colorado beetle (Leptinotarsa decemlineata) which occurs wild in Colorado, USA, and became a savage pest of cultivated potatoes. Similarly, bananas in Latin America came into contact with Fusarium oxysporum f.sp. cubense which is now a major infliction called Panama disease. When the old and new hosts are closely related, a new encounter parasite can be very damaging.

An old encounter parasite is one which had been in contact with is crop host ever since that crop was domesticated. Wheat rust (Puccinia graminis) is a typical example. Old encounter parasites are generally less damaging unless there has been a severe vertifolia effect.

A re-encounter parasite is one that is left behind when the cultivated host is taken to another part of the world. At a much later date, the parasite is also taken to the new location where it is often very damaging because the host has lost resistance during its cultivation and breeding in the absence of that parasite. The classic example of this was maize in tropical Africa which was re-introduced to the Central American tropical rust (Puccinia polysora) after about four centruies of cultivation in its absence.

Reference:

Buddenhagen, I. W. (1977): Resistance and vulnerability of tropical crops in relation to their evolution and breeding, in The Genetic Basis of Epidemics in Agriculture (P.R. Day, Ed) Ann. New York Acad. Sci., 287: 309-326