The evolutionary advantage of breeding for tolerance over resistance against viral plant disease

A tolerant strain of a crop permits the propagation of an entire virus population, thus exerting a much lower selection pressure in favor of a virus mutant overcoming resistance. One may suppose that a tolerant crop may build up a large virus reservoir and therefore increase the chance of emergence of a more virulent type. However, the tolerant crop exerts a much lower selective pressure for a given virus line, and therefore the new virulent type remains a very low proportion of the total virus population. Susceptible crops, into which resistance to a given virus was bred, may be immune to this virus infection, thus preventing the buildup of a large virus reservoir. However, other host crops around it, or wild weeds facilitate propagation of this virus. This peripheral buildup of virus reservoir in the vicinity of the resistant crop creates a strong selection pressure for the propagation of the mutants overcoming resistance. Therefore, the planting of monoculture of a resistant crop accelerates the emergence of virulent virus lines that overcome this resistance. There is no reason to assume that resistance introduced into transgenic plants will be different. In the long run, tolerant crops may have the advantage over resistant crops, preserving for a longer time conditions in which virus infection causes only minimal damage. The production of tolerance to virus infection is an approach not yet favored by breeders and seed producers. However, it may be regarded as a less expensive, additional route to reduce the damage to crops inflicted by viral diseases.