(1917-9) in England, have been rapidly adding to our knowledge of this group of parasites.
Among the notable outbreaks of plant diseases which have occurred are the American gooseberry mildew (Sphaerotheca mor- suvae) originally introduced from America and now fixed past all eradication in the fruit-growing countries of Europe; the terrible " wart disease " of the potato (Synchytrium endobioticum) neglected in the early days of its appearance in Great Britain and now firmly entrenched in the Midlands, in Wales, in the seed-growing districts of Scotland and sporadically elsewhere in England, reaching in 1920 to the great potato-growing county of Lincolnshire. On the Conti- nent it has appeared in Hungary, Holland, Sweden, Germany etc. Such is the virulence of this potato disease that in infested soil potato growing becomes impossible unless recourse is had to an im- mune variety. The disease has been made the subject of much inter- national legislation, affecting export trade. In 1918 it appeared in the United States in miners' gardens among the coal-fields of Penn- sylvania; a strict quarantine has been established round the infested regions, and a publicity campaign has been started in America with an organization and vitality unknown in Europe, the results of which later years will reveal. In South Africa, Australia and America the Citrus canker (Pseudomonas cilri), originally intro- duced on a plant from Japan, has swept through the citrus planta- tions with devastating results. In South Africa the canker eradica- tion campaign up to 1919 had cost over 60,000. In the United States the white pine Blister Rust (Cronartium ribicola), which was introduced into America from Europe about 1892, continued to make steady progress in the white pine regions of the eastern and north-eastern states, notwithstanding that for several years a coordinated campaign by the State and agricultural organizations had been in progress, under which " scouts " and " State eradication crews " destroyed wholesale the species of Ribes which constitute the alternate host of this heteroecious Rust. In one survey of an area of 72 sq. m. in New Hampshire, one-fourth of the pines was found to be infected. The Chestnut Bark disease (Endothia para- sitica), which appeared in 1904 in a park in New York, increased to such an extent that by 1921 losses of hundreds of millions of dollars had already been caused and it threatened to destroy every chestnut wood in North America. This disease, probably introduced from Japan, girdles and kills the chestnut tree; its spores are distributed from tree to tree by the wind, insects and birds.
Renewed warfare (reminiscent of the Massachusetts Barberry law of 1755 and the similar law in Denmark in 1903) has been declared in the United States against the barberry. In 1916 it was estimated that Black Rust (Puccinia graminis) on wheat caused a loss of nearly 200 million dollars in the United States. This Rust is heteroecious and field observations in the States have shown that the aecidiospores from the barberry start, in the spring, epidemic outbreaks of Rust on wheat.
Another group of diseases which excites alarm in the United States and also in Europe is the " Mosaic " diseases or " infectious chlorosis." A prominent American pathologist has said of this class of diseases that " it gives one the creeps," so obscure is the cause, so infectious is it and so increasingly prevalent on a rapidly increas- ing number of host plants (potato, tomato, tobacco, cucumber, spinach, bean, red clover, sugar-cane, maize, grasses). The investiga- tions of a band of workers, among whom may be mentioned Allard (1914-8), Brandes (1920), Dolittle (1920), Schultz et al. (1919), appear to show that the cause of these menacing diseases is an ultramicroscopic organism, rather than an enzyme as previously supposed. On some plants the disease is freely transmitted by cer- tain sucking insects, such as Aphides. In Europe the chief example is the disease of the potato called " Leaf curl," the subject of im- portant investigations by Appel (1911) (1915), Quanjer (1913-20), Doby (1911-5), Artschwager (1918), Neger (1919) and Murphy and Wortley.
The decade ending in 1920 was rich in investigations bearing on the scientific control of plant diseases. Stimulated by the now classic discovery made by Biffen in 1907, that the inheritance of Rust resistance follows on Mendelian lines, workers in genetics (see Genetics) in many countries, e.g. Nilsson-Ehle, Biffen, Pole- Evans, Stakman, Parker and Piemeisel, and Hayes, Parker and Kurtzweill, have been engaged in the breeding of disease-resistant plants. It has been shown that resistance and susceptibility to Rust can hardly be considered as simple characters, the F 2 results giving evidence in favour of the multiple factor hypothesis. Immunity from, or resistance to, many different types of fungous diseases has been sought for and found, either by selection or cross breeding, in many genera of economic plants. Varieties of beans and sugar-cane immune to " Mosaic disease " have been discovered (Reddick and Stewart, 1919); Earle (1919); beans immune to Colletotrichum, found by Barrus (1915) and used in cross breeding by Burkholder (1919) and McRostie (1919); asparagus resistant to Rust (Norton, 1913), resistance to citrus canker (Peltier, 1918); wilt (Fusarium) resistant cottons, tomatoes, cabbages and flax (Orton) ; potatoes, English, German and American varieties, immune to " wart dis- ease " (Malthouse, Snell et al.); Werth (1919), Marlatt (1919); potatoes immune to " blight " (Salaman, 1910); cereals immune to Erysiphe Graminis (Vavilov, 1913), Reed; hops immune to Sphaero- theca Humuli (Salmon, 1917-20).
Of great importance, economically, has been the scientific study of fungicides. The great work of Pickering (1907-12) in elucidat- ing the chemistry of " Bordeaux mixture ' profoundly affected the method of making copper-containing washes. Additional knowledge has been gained by the researches of Sicard (1914) and Vermorel and Dantony (1914) in France, Ewert (1912) and Wober (1919) in Germany, Mond and Haberlein (1919) in England, and Butler (1914- 20) in America. Gimingham and Barker (1911-14) showed that a biological rather than a chemical explanation holds good for the efficacy of copper-containing fungicides on the sprayed plant.
A notable advance, accelerated in many countries by wartime organizations, has been made in the legislative control of plant diseases by the State. In most countries the grower is now required by law to notify the outbreak of certain infectious diseases of plants. In Great Britain " wart disease " of the potato (Synchytrium endo- bioticum) has been the subject of numerous legislative orders, where- by the sale of " seed " potatoes, and their import and export, are controlled, and the grower prohibited from growing susceptible varieties of potatoes in affected districts. On the other hand, the State, by a system of inspection, guarantees the purity of stocks of immune varieties. Other recent examples of legislation against fungi in England are the " Silver leaf " order of 1919 and the Onion Smut order of 1920. The former was introduced to try to save the " Vic- toria " plum and other valuable plums from being exterminated by the " Silver leaf " disease, now definitely known, through the work of Percival and of Brooks (1911-9), to be caused by the fungus Stereum purpureum. The grower is now required by law to burn the dead tree or the dead wood of the tree on which the Stereum fructifications are formed. This order, however, is ad- ministered as an educational rather than as a punitive measure. The United States, in their efforts to stop the importation from Europe of new fungous pests, passed in 1912 a " Plant Quarantine Act," under the provisions of which the importation of all five-leaved species of Pinus from Europe and Asia was prohibited, for fear of their carrying the white pine Blister Rust; also, potatoes coming from many European countries were excluded in fear of " Wart disease." Later legislation has prohibited altogether the importa- tion of plants into the United States except under special licences. In this step South Africa has followed. In the United States, an Act to prevent fraud in the sale of fungicides and insecticides is in force, and in England, where during the World War the purity of copper sulphate for spraying was legally guaranteed, further legisla- tion to secure the purity of lime sulphur and arsenical washes has been contemplated. Legislative measures against plant diseases commonly affect international interests, and the first of what were planned as regular international Phytopathological Conferences (interrupted, however, by the World War) was held at Rome in 1914. A Convention was signed at Rome by the delegates of some 30 states pledging themselves not only to maintain an official phytopathological service for the detection and suppression of certain diseases, but also to maintain institutes for scientific re- search, so that state officers may be supplied with the best technical advice. Another branch of state activity is seen in the surveys of plant diseases which are being made in many countries. The " Report " (1918) of the American plant pathologists, compiled by Lyman and others, is a document of absorbing botanical and eco- nomic interest, and from it can be gathered a good idea of the wide development of extension work with field laboratories and of " team work ' in research now existing in the United States. Similar organization for plant disease surveys exists also in Germany (Appel, 1914). The Annual Reports of the Plant Diseases Branch of the Ministry of Agriculture in England, and the similar reports issued in France, Holland and other European countries, as also in India and pur Colonies, are forming the basis for a world-wide census of plant diseases, the necessity for which has been so ably put for- ward by Sorauer and Eriksson in Europe, by Butler in India and by the leading American plant pathologists. (E. S. Sa.)
V. Soil Sterilization. Intensification of culture leads always to an increase in the soil flora and fauna, and among the forms that assume importance are many that are directly or indirectly harmful to plants.
It has been found that a simplification of the soil population leads to increased productiveness and greater healthiness of crop. This simplification can be brought about by mild killing agents which are not too drastic in their effect which will kill living germs, but not all spores.
Steam heat is the most effective agent: it is so effective that it would always be adopted if questions of cost and convenience never arose. It not only kills animal pests, ova and larvae of eelworms, wireworms, woodlice, etc., and reduces fungi, but it also brings about a certain amount of useful soil decomposition, thus greatly facilitating the work of the food-producing organisms of the soil. Steam heat is used in two ways in the glasshouses of the Lea Valley in England: in one the soil is dug over, then covered with a large wooden tray under which steam is blown
