A Critical Analysis of Durable Resistance

Abstract

Durable resistance to a disease is resistance that remains effective during its prolonged and widespread use in an environment favorable to the disease (22, 23). The test for durable resistance must include two elements, time (long) and area (large). Resistance in a cultivar cannot justifiably be described as durable if the cultivar has been grown only in small-scale experiments, even if such tests are repeated at many locations and over several or many years. Experience shows that resistance that survives such tests may nevertheless not survive widespread use under agricultural conditions (see below). Nor can resistance be described as durable if it has only been used briefly, even though on a large area. Based on this definition, it is not correct to argue that the resistance of a cultivar can be made more durable by growing it less, as in its use in cultivar mixtures or multilines. Discussions on extending the usefulness of resistance by using cultivar mixtures or multilines, or by gene deployment, are based on exploitation of the whole system, one purpose of whlch is to avoid elCposing the individual cultivars to a powerful test of the durability of their resistance. This is a distinct topic that has been widely discussed (3, 65) and will not be considered here. This review is therefore restricted to aspects of disease resistance and its genetic control in relation to durable resistance in individual cultivars.