Alternating Pathosystems

An alternating pathosystem has two subsystems which, in the present work, are called the winter subsystem and the summer subsystem. It was necessary to find English terms that are (i) equally acceptable to entomologists and mycologists, (ii) that are technically accurate, and (iii) that can be used in an adjectival sense to categorise the many components of alternating pathosystems. Inevitably, it proved impossible to find a system of names that was entirely free from defects.

It was concluded that 'season' names had the least objectionable flaws. These flaws are confined to relatively unimportant problems associated with the comparison between tropical and temperate contexts. They are unimportant mainly because alternating pathosystems are rather rare in the tropics. In the present work, temperate season names are used throughout, and it is assumed that 'winter' also means a tropical dry season, 'summer' also means a tropical wet season, 'spring' also means the start of the tropical rainy season, and that 'autumn' also means the end of the tropical rainy season.

Figure 8.1 The alternating pathosystem.

The life cycles of alternating (i.e., heteroecious) aphids and rusts are so similar that they can be represented by a single diagram, shown here, in which technical terms are replaced by plain English (see Table 8.1). The parasites are shown in red, and the hosts in green.

Sexual recombination followed by dormancy, or else dormancy followed by sexual recombination, occur during the winter, and the winter host is parasitised in order to produce only spring alternators that that cannot auto-infect, and that must migrate to the summer host. This migration necessitates allo-infection of the summer host, and vertical resistances can function. (Continued).

When a matching allo-infection occurs on a summer host, the parasite starts reproducing asexually and produces a clone. So long as the members of this clone (the summer colonisers) are auto-infecting their host, the vertical resistance will be matched, and the only host defence will be horizontal resistance. Other members of this clone (the summer migrants) may migrate to other hosts but this will involve allo-infection. These new hosts are likely to have a different vertical resistance, which will remain unmatched and functioning. The summer host has a discontinuous pathosystem involving seasonal tissue that disappears entirely during the winter.

In the following spring, all these newly emerged seasonal tissues could have vertical resistances that are unmatched and functioning.

In the fall, the autumn alternators must migrate to the winter host, and this too will involve allo-infection, implying that vertical resistances can function in the winter host also. The spring alternators cannot infect the winter host (but see 8.15), and parasitism of the winter host ceases during the summer. Consequently, all the winter host vertical resistances are unmatched and functioning again in the fall.

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