White Pine Blister Rust

There are two problems concerning the alternating rust (Cronartium ribicola) which was apparently imported to North America from Eurasia at the beginning of the twentieth century. This rust parasitises white pines (i.e. five-needled pines, Pinus spp.) as its winter host, and Ribes spp., as its summer host. It has destroyed most of the five-needle pines of North America, and it has severely restricted the cultivation of domesticated Ribes.

The first problem is that several species of both the winter and the summer host are present in North America. And there is a fully functional gene-for-gene relationship in both the winter and the summer subsystems of North America. Gene-for-gene relationships almost certainly require geological time (see 10.5) for their evolution. It is most unlikely that they can evolve in the few decades of historical time that elapsed between the appearance of this rust in America, and the first demonstration of a gene-for-gene relationship.

The most likely explanation of these gene-for-gene relationships is that there were two entirely separate rust populations, located in North America and in Eurasia, respectively. These two populations had been out of breeding and epidemiological contact since the development of the Atlantic Ocean some sixty five million years ago. These two populations may be described as being different allopatric (i.e., different country) pathodemes and pathotypes. The rust was presumably present in North America all the time, but it was in such good pathosystems balance that it caused too little damage to be observed. (A search of pre-1900 herbarium specimens in the

Pacific Northwest by F. E. Williams and myself failed to reveal any symptoms of this rust, but it seems that taxonomic botanists preserve only good-looking specimens for their herbaria, and this negative evidence means little).

In complete contrast, it is thought that the allopatric pathotype, imported from Eurasia, was very damaging in North America because of an inadequate horizontal resistance. The interesting implication is that the gene-for-gene relationship, which apparently functions with both allopatric pathotypes, is older than the separation of the two continents. A further implication is that the two vertical subsystems remained the same, but that the horizontal subsystems became markedly different.

The pines were devastated because they had too little horizontal resistance to the allopatric rust. The bark lesions continued to expand until ring-barking of major branches and trunks led to the death of the tree. These bark lesions can also persist for years, and this converts the winter subsystem into a continuous pathosystem. It is to be presumed that, in the balanced, wild pathosystem, the pine lesions die out each summer, and must be renewed each fall. At the very least, the lesions would not seriously harm the pine host in a wild pathosystem. Such mild symptoms are unlikely to have been observed, let alone recorded, by North American foresters in the later years of the nineteenth century.

The second problem is why the wild Ribes populations were apparently unharmed, while the pine populations were devastated. It is thought that the wild Ribes populations may have been devastated in the early epidemics, and that they accumulated horizontal resistance relatively quickly (c.f. maize in tropical Africa, see 7.2). The devastated pine populations will presumably do the same, but this is likely to require a much longer period.

It is known that the Ribes seeds can remain dormant in the forest duff (i.e., natural mulch) for many decades. A useful experiment might be to isolate Ribes seeds from near the bottom of the forest duff, in order to grow them out and measure their susceptibility.

Chapter Nine

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