WEED CONTROL: APPLIED BOTANY'

AMONG THE agricultural practices that have benefited by mechanization and chemicalization, weed control has been one of the last to undergo extensive development. Although much fundamental work had been done prior to 1944, introduction of 2,4-D during that year brought about an abrupt increase in activity, and within the last twelve years, weed control has become a multimillion-dollar business. Compared with plant breeding, use of fertilizers, development of insecticides, and mechanization of culture and harvest of crops, however, chemical weed control is still in its infancy. When we look into the future, attempting to chart the course of agricultural progress, we realize that the bringing in of new arable land, the altering of climate, the improvement of fertilizers, and even the continued breeding of -superior crop varieties are all laborious and c4ostly procedures. One of the most promising means for increasing production would seem to be to eliminate competition and plant depredation by the use of modern pesticides. Among these, the herbicides seem most promising. The growing of plants has always been a struggle. Anyone who has grown up on a farm can remember the perennial job of hoeing weeds in the corn or pulling them from the beets or garden crops. And many a boy has wondered, why grow some plants and destroy others? WThy not use the weeds? But agricultural experience has proved the usefulness of crops and the detrimental effects of weed competition. Insects and plant diseases likewise prey on the farmer's crops. Only within a relatively few years have means become available for handling a few of these enemies. Out of the some quarter of a million plants in the world only a few are useful as crops and likewise only a few are weeds. But during the development of agriculture, by various means of sifting and screening as superior plants have been found and improved, superior weeds seem to have been carried along to compete with them. Furthermore, insects and diseases have sought out the farmer's choice crops, and until recent times, seem to have placed a definite ceiling on production. It seemed for ages that the evolution of pests was only one step behind the improvement of crops; and man's destiny seemed to hang in the balance between them. The story of this struggle as exemplified by the activities of the U. S. Department of Agriculture has been aptly described by Quisenberry (1948). I'he story of the strides recently made in insect and disease control I will leave for others to tell. I would like to recall a few of the interesting episodes in the introduction and spread of weeds and in the battle now being waged to bring them under control. SOME SERIOUS WEEDS.-As examples of weeds that have seriously affected American agriculture, I have selected Canada thistle (Cirsium. arvense), Russian thistle (Salsola.kali var. tenuijolia), St. Johnswort (Hypericum perforatum), and Halogeton (Halogeton glomeratus). CANADA THISTLE is a perennial weed prevalent throughout Central Europe including the British Isles. It was brought into the American colonies with seeds and feedstuffs and was reported in Canada early in the seventeenth century, and in New York by 1777 (Dewey, 1901). It spread rapidly during the western migration and was found in California by 1879 (Robbins, 1940). A noxious weed that spreads both vegetatively and by seed, Canada thistle early proved itself a serious menace to agriculture. It has been named as a noxious weed in the seed laws of 37 states and it is still spreading. Not only has it invaded cultivated fields, but in the Northeast it is also penetrating the more open areas in forest, park, and rangelands where control is impossible. Its windborne seeds are carried into every possible site for germination; its dioecious habit is apparently no deterrent to its rapid spread; its vigorous, competitive character enables it to dominate the flora in many open areas that otherwise would produce good forage. The only factors limiting its range seem to be climatic, and it apparently stays in the northernmost tier of states. One hundred years of agronomic research have failed to discover a practical method for eliminating Canada thistle; ten years after the discovery of 2,4-D we are still seeking a satisfactory chemical method for its control. RUSSIAN THISTLE is an annual of the tumlbleweed habit; that is, at maturity it breaks off at the ground line and is blown by winds so that its myriads of seeds are effectively spread. It grows best under semi-arid conditions and tolerates saline soils; it is a weed found in small grains, as well as on fallow lands, abandoned fields, and non-tilled areas. It makes its major growth during summer and remains succulent well into autumn on most soils. Russian thistle was introduced into the United States from Russia around 1874 and was first reported from South Dakota (Robbins et al., 1952). By 1895 it had been reported in 16 states including California. During World War I many acres of marginal land were plowed and planted to wheat; after the war these lands were abandoned and Russian thistle took over literally thousands of acres. During this war sugar-beet growing developed rapidly in California and the industry held great promise.Then suddenly, about 1925, the curly-top virus carried by the beet leafhopper Eutettix tenellus broke forth in unprecedented violence and threatened to destroy the industry. Why did this occur? Before the war curly-top had been a disease of