Bridging hosts

For success in breeding disease-resistant varieties of plants it is of fundamental importance to know whether pathogenes and their biologic races can acquire the ability to parasitise newly developed immune and resistant hosts. If it is a rule that a highly resistant plant may occasionally become mildly infected by some pathogene, and if it is possible that this parasite, as a result of its sojourn in the host, can acquire increased virulence, then the outlook for successfully growing resistant varieties is not very hopeful. Some examples, gathered from experience and from the literature on plant-parasitic nematodes seem to prove that adaptation to new hosts, as well as weaning nematodes away from hosts to which they are accustomed, are very common phenomena.