Harries (1978) has suggested that the domesticated coconut is the dwarf type, with nuts that are close to the ground and easy to open. This was the ancient equivalent of a modern drinks dispenser, and the domestication is probably very old. The wild type of coconut is the tall palm with large nuts that are well protected with fibre, and that are capable of surviving a sixty foot fall, and for many months when floating in sea water. The domesticated nuts do not survive sea water immersion.

The centre of origin is in south-east Asia, probably in the Malay peninsular or the Philippines. The palms were spread by natural dispersal on sea water to India, East Africa, and the many islands of the Indian Ocean and the western Pacific. These were the limits of the natural dispersal, and coconuts could not penetrate the Atlantic, or reach the western shores of South America. The

Portuguese took East African coconuts to West Africa, and then to the Caribbean. The Spanish took western Pacific coconuts to the west coast of the Americas. The west and east coasts of Central America thus have coconuts of different provenance and, it appears, of different disease resistance.

Chiarappa (1979) has suggested that there are many crop vulnerabilities in coconuts because both the natural and the human dispersal of coconuts left various parasites behind in the centre of origin. The East African palms probably represent the oldest separation from the centre of origin, and are presumably the most susceptible. These are the parents of the palms of West Africa and the Caribbean, and the latter proved to be highly susceptible to the phytoplasma that causes Lethal Yellowing disease. It seems that this parasite could not cross the Indian Ocean from its centre of origin, where the local palms are highly resistant. The palms of the West Coast of Central America are also resistant, having been imported from an area that was in epidemiological contact with the centre of origin.

Flax (Linum usitatissimum) was typically a cash crop of the early settlers of each newly settled region in North America. It was well known that flax thrived best in virgin soils and that, after a few years of flax cultivation, the soil was spoiled, and that it was no longer suitable for flax cultivation. The flax was in demand both for its fibre, to produce linen, and its linseed oil, to make paint. Flax mills would be set up close to the newly settled farms, but these mills were famous among insurance companies for they way in which they mysteriously burned down when the local soils would no longer carry flax. Both the flax cultivation, and the flax mills, would then move west with the next wave of settlers.

The cause of the flax decline was Fusarium oxysporum f.sp., lini. Kommedahl et al (1970) have reviewed the history of this disease in North America. They comment that there has been a steady increase in the level of resistance to flax wilt during a period of many decades, and that the disease is no longer a problem. It appears that this crop accumulated horizontal resistance during cultivation, but that it did so only slowly, when compared with the maizes of tropical Africa. This progress was slow because flax is self-pollinated and, although the crops of the nineteenth century were genetically mixed landraces rather than pure lines, the amount of cross-pollination and segregation was severely limited. The exposure to the disease was also limited by moving the cultivation to new soil. Nevertheless, in spite of these hindrances, modern flax cultivars are resistant to wilt. However, this crop has now declined in importance, as its fibre and paint products have been almost entirely replaced by plastics.

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