Resistant, high-yield varieties of major food crops carefully crossbred and highly selected, are the success story of modern genetics. But the more successful a variety is, the more likely farmers and breeders are to choose its hybrid seed, and an entire society may come to depend on a few highly selected varieties. When a new or mutant form of pest or disease arrives to which the favoured crop has no inbred resistance, then entire crops may be decimated. Crop diseases often occur because crops are too narrowly based genetically to carry adequate resistance. In fact, disease losses have been shown to directly correlate with the degree of genetic homogeneity in some crops.
History has proved the dangers inherent in genetic uniformity: it was the unrecognized cause of the Irish potato famine, which left millions dead in the mid-19th century. An informed few, mainly botanists, value traditional farmers for the rich variety of crops they produce. By cultivating numerous strains of corn, legumes, grains and other foods, they are ensuring that agricultural scientists have a vast genetic reservoir from which to breed future varieties. The genetic health of the world's potatoes, for example, now depends on Quechua Indians, who cultivate more than 50 diverse strains in the high plateau country around the Andes mountains in South America. If these natives switched to modern crops, the global potato industry would lose a crucial line of defence against the threat of insects and disease. Unfortunately, the same is not true for man other wild and domesticated varieties of crop plants - such as wheat, rice, millet, beans, yams, tomatoes, potatoes, bananas, limes and oranges - are already extinct and many more are in danger of following them.
A century ago the world's farmers grew literally hundreds of different grain and produce items. At present 30 crops provide 90% of the world's calories. Four of these (corn, wheat, rice and sorghum) supply more than half of those calories. Because of intensive selection for high performance and uniformity the genetic base of much modern food production has grown dangerously narrow. Only four varieties of wheat produce 75% of the crop grown on the Canadian prairies; and more than half the prairie wheatlands are devoted to a single variety. Similarly, 72% of potato production in the USA depends on only four varieties, and just two varieties supply US pea production. Almost every coffee tree in Brazil descends from a mere six plants from one place in Asia. A mere handful of parent wheat lines have contributed to the gene content of all the Australian wheats. A very substantial percentage of the intermediate wheat grass acreage in the USA is planted with strains that trace to a single introduction. These and other crops in a similar position are extremely vulnerable to outbreaks of pests and diseases and to sudden unfavourable changes in growing conditions.
The International Board for Plant Genetic Resources of the FAO recently issued warnings that wheat, rice, sorghum, millet and barley were crops whose genetic diversity was imperilled. The USA National Academy of Sciences has warned that the USA's major crops were impressively uniform genetically and are therefore vulnerable to widespread attack by blight or pests.
Attempts to combat disease with resistant varieties have resulted in an increase in the incidence of disease. This occurs because the pathogen is at least as variable as the host crop. The newly released resistant variety can actually select a strain of the pathogen virulent on it, thus removing itself from usefulness and necessitating a replacement. For example, pure-line varieties of oats have an average useful life expectancy of only about 5 years. The entire acreage of the grass Digitarta decumbens in Central America and the West Indies originated from a single clone introduced in 1940, and already it is necessary to spray pastures of this grass in Central America as a protection against aphids and fungal diseases.
The situation has worsened considerably since hybridization involving single-gene resistance began, because now a single resistance gene can be quickly incorporated into crop varieties right across a continent. For example, various pure-line varieties of small grains were released in North America because they were highly resistant to prevalent races of rust fungi. Though these varieties resisted most rust races, they were susceptible to rare, virulent races which, though occurring in extremely small amounts in the rust population, could increase with sufficient rapidity in the absence of competition on a susceptible host to soon become devastating. Yet pure-line varieties of small grains continue to be released for commercial production.
An increasingly restricted genetic base appears to underlie periodic production failure in economically important crops, leading to increased yield variability and increased synchronicity of variation across large areas; for example, a 15 per cent reduction in maize harvest in 1970 in the United States was attributed to widespread cultivation of a blight-susceptible variety (WCMC 1992).
Varietal improvement programmes have so reduced the stabilizing genetic diversity of small grains that these crops are a ready substrate for one or more variants of the pathogen population.
The ideal way of controlling all diseases would be by means of resistant varieties. The more man can get crop plants themselves to resist diseases, the less work and worry he will have in controlling them. Moreover there are now no feasible methods of controlling some of the most destructive diseases except by means of resistant varieties.