Crop Pathosystems

The modern crop pathosystem is very different from a wild plant pathosystem. It is also very different from a crop pathosystem of a century ago. Crop scientists, who have had total control over the production and availability of new cultivars, have dominated the modern crop pathosystem. This has been a classic example of over-control (see 2.5) and it has resulted in serious suboptimisation. There were too few cultivars, cultivated in excessively large populations, with the wrong kind of resistance to parasites. Important emergents, occurring at the higher systems levels, remained unrecognised. A brief review of the history of this suboptimisation is revealing.

The first very damaging plant disease in recorded history occurred in Europe in 1845. This was the newly introduced potato blight (Phytophthora infestans). The first serious insect pest of crops was the desert locust, recorded by the ancient Egyptians. But this was a sporadic pest, which was damaging only occasionally. The first continuously damaging insect pests were probably the cotton boll weevil, and the Colorado beetle of potato, which both became important in the nineteenth century.

The first crop fungicide was discovered in 1882. This was Bordeaux mixture, made with copper sulphate and slaked lime, although flowers of sulphur had been used against powdery mildews a little earlier. The first effective crop insecticide was discovered only in 1940, and this was DDT. Until the discovery of these crop protection chemicals, crop pathosystems were largely self-controlled and self-organising. They had to be, otherwise crop production would have stopped. And, if the crop parasites were causing losses, there was little that the farmer could do about it, other than using simple farming practices, such as crop rotation, and the burning of crop residues. However, the crop losses from parasites were rarely large enough to discourage the cultivation of the crop in question.

There were no professional plant breeders before 1900. There were no professional plant pathologists or entomologists either. There were just a few scientists who called themselves botanists and zoologists, and whose interest in agriculture was largely peripheral to their academic biology. The first plant pathologists called themselves 'applied mycologists' and they were primarily interested in the fungi that caused plant disease, rather than in the economic losses caused by those fungi. Until 1970, the leading abstracting journal was called The Review of Applied Mycology, and was known as "RAM". Its index included the names of hosts, pathogens, authors, countries, and little else. It was quite ineffectual as an information retrieval system for the control of plant diseases.

However, there was an external factor that had enormous influence on twentieth century agriculture. This was an unbalanced medical progress. The human death rate, particularly among infants, was drastically reduced by a greatly improved medical science. But the human birth rate was not correspondingly reduced in compensation. The result has been a human population explosion. Agricultural production has been increased accordingly, and this increase was a remarkable achievement. But it was made at great cost. One of the costs was the loss of pathosystem stability and balance. Another was the replacement of host resistance with crop protection chemicals.

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