Expression of coat protein genes have become increasingly important in developing new varieties of resistant crops; since 1986 there have been more than 75 reports of transgenic plants with virus resistance as a result of the expression of coat protein genes in crops as varied as potato, tomato, squash, rice, corn, sugar beets, and lettuce (Fitchen and Beachy, 1993). In 1995, the first commercial variety of such virus-resistant plants was introduced—crooked-neck squash resistant to cucumber mosaic virus and zucchini yellow mosaic virus. More recently, genes for resistance to a number of different diseases were isolated from tomato, tobacco, Arabidopsis, flax, and rice plants, to name a few. Whitham et al. (1994) isolated a gene for resistance to TMV; Johal and Briggs (1992) isolated a gene for resistance to corn blight. In both cases, resistance was achieved when the resistance gene was reintroduced into a previously sensitive plant variety.

SYNTHETIC ORGANIC PESTICIDES

Between World Wars I and II, large-scale production practices coupled with major developments in synthetic chemistry revolutionized the pesticide industry. Tetraethylpyrophosphate (TEPP), the first organophosphate insecticide, was synthesized in 1938; DDT followed in 1939. An analog of the plant hormone indole acetic acid was synthesized in 1939 as 2,4-D and introduced in 1942 as the first synthetic selective herbicide. The first dithiocarbamate fungicide, zineb, was introduced in 1943; and in 1945, chlordane, the first of the persistent, chlorinated insecticides, and propham, the first carbamate herbicide, were introduced. In 1947, toxaphene, which became the most heavily used insecticide in the United States, was introduced, followed by aldrin and dieldrin in 1948 and malathion in 1950.

In the 1940s and 1950s, soil fumigants, including methyl bromide, ethylene dibromide, dichloropropenes, and dibromochloropropane (DBCP), were found to effectively suppress soil-borne, parasitic nematodes and fungi. New insecticides, herbicides, fungicides, and nematicides were introduced almost annually through the 1950s and 1960s. As remarkable as the pace of discovery and development was the rate of adoption of the new pesticides by growers (Osteen and Szmedra, 1989), who found that the new chemicals (1) were highly effective and predictable at reducing pest populations, (2) produced rapid and easily observed mortality of the pest, (3) were flexible enough to meet diverse agronomic and ecological conditions, and (4) were inexpensive treatments compared to the crop damage that would be otherwise sustained (Metcalf, 1982; National Research Council, 1975). Synthetic pesticides quickly became the favored means of crop protection and dramatically eclipsed other approaches to pest management.

The ready acceptance of pesticides, coupled with comparable increases in the use of commercial fertilizers and mechanization, revolutionized agricultural



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