Species redistribution

Trouble began with the relatively recent movement of plant and parasite species from one continent to another (see 3.8). The first major disaster was potato blight (Phytophthora infestans), introduced to Europe from Mexico and the United States in 1845. Next was the grape Phylloxera, introduced to Europe from the United States in the mid-nineteenth century, and which soon threatened to destroy the wine and table grape industries of Europe. This problem was solved by grafting the classic wine grapes on to rootstocks of wild, resistant, North American grapes. However, downy mildew was imported with these rootstocks and the wine industry was threatened with ruin for a second time. This second problem was solved by the discovery of Bordeaux mixture in 1882.

Sometimes, it was a new host that was introduced to an indigenous parasite. The classic example of this was the introduction of potatoes to the State of Colorado, where they newly encountered a parasite of the wild buffalo bur (Solanum rostratum). This parasite became the one of the worst crop pests ever known, and it is still famous as the Colorado beetle (Leptinotarsa decemlineata) of potatoes. Other examples include the introduction of bananas to the Caribbean, where they newly encountered Panama disease (Fusarium oxysporum f.sp. cubense) and Moko disease (Pseudomonas solanacearum).

These were all new-encounter parasites, in which the host and parasite had evolved separately in different parts of the world. Equally damaging were the re-encounter parasites, in which the host was moved to a new area, and some of its parasites were left behind. At a later date, these parasites were inadvertently introduced to the new area where the host had inevitably lost resistance to them. Some of the sugarcane and coffee diseases are the most notorious of the re-encounter parasites in the New World. Tropical rust of maize in Africa is one of the most important and is described above (see 7.2).

Finally, some of the old-encounter parasites became more damaging in new areas, or with new cultivation methods. Wheat diseases in North America are an example.

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