Functional Sequential Discontinuity

Functional sequential discontinuity means that an apparently continuous pathosystem behaves as a discontinuous pathosystem. This may happen because the host has a greater tolerance to adverse weather conditions than the parasite. Many evergreen, perennial herbs, for example, can survive a winter that is too harsh for their more delicate parasites.

Parlevliet & Van Ommeren (1976) observed a functional discontinuity with winter barley and barley rust (Puccinia hordei). In the autumn, the newly germinated seedlings carry rust, and this pathosystem is apparently continuous. In fact, during the winter, the rust is unable to infect the barley, but the barley continues to grow, even if only slowly. By the following spring, the rusted leaves have died, and the rust has died with them. The rusted leaves have been entirely replaced with rust-free leaves, and the pathosystem is thus discontinuous.

This kind of functional discontinuity may occur in many other annual grasses, which germinate their seeds in the fall, and, conceivably, it may occur in temperate, perennial grasses as well. Many perennial grasses in the drier tropics have discontinuously available leaf tissue because of severe dry seasons that turn the entire landscape brown, and which are often aggravated by grass fires that turn the landscape black.

Functional discontinuity can involve mechanisms that are quite complex, and it is very tempting to conclude that they evolved because of their pathosystem survival value. For example, coffee leaf rust (Hemileia vastatrix) parasitises coffee (Coffea arabica) which is an evergreen perennial tree growing in the tropics, and the pathosystem is apparently continuous. However, infection requires free water on the leaves and, consequently, infection cannot occur during the dry season. The wild host apparently sheds all its rusted leaves during the dry season, and the tree is functionally deciduous with respect to rusted leaves only. The pathosystem is thus functionally discontinuous (Robinson, 1976; see also 4.12.1).

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