114 



SCIENTIFIC AGRICULTURE 



March, 1921. 



and has only befonie evident since tlie advent of high- 

 pressure pumps and greater capacity nozzles. 



Another important investigation by Sanders and Kel- 

 sall bore on the question of the influence of fungicides 

 on the killing value of poison insecticides when the two 

 are mixed together. Tliej' found that with Bordeaux 

 the average poison is decreased in value by about 50 

 per cent. With soluble sulphur the value of the poison 

 is increased from 10 to 15 per cent, and with lime-sul- 

 pliur the decrease is about 20 per cent. 



Kerosene Emulsion. — A solution of Kerosene Emul- 

 sion has long been a valuable insecticide against sucking 

 insects. The standard formula (Riley-Hubbard) was 

 originated in 1884, although several modifications have 

 also appeared. When properly prepared Kerosene 

 Emulsion can be applied with safety on most plants, 

 but of late years tobacco extracts have taken its place. 



Tobacco Extracts. — Tobacco dust and extracts have 

 been used probably for more than a hundred years for 

 the control of aphis and other sucking insects, but it 

 is only within the last ten years that concentrated ex- 

 tracts of nicotine have been placed on the market. The 

 best known of these are "Black Leaf 40", being a 40 

 I)er cent solution of nicotine sulphate, a non-volatile 

 substance, and "Nicofume", a 40 per cent solution of 

 nicotine in the volatile form. Black Leaf 40 can be 

 combined with lime-sulphur, Bordeaux and arsenates of 

 lead and lime as a 3-in-l mixture, and is used at 

 strengths varying from 1 to 800 to 1 to 1600 of water. 



Fungicides. 



It is not surprising that plant growers of two or more 

 generations ago had difficulty in controlling plant dis- 

 eases, since the nature of these maladies was not un- 

 derstood. One has but to peruse some of the old works 

 on the nature and treatment of disease to find how crude 

 were the conceptions of the early plant growers and 

 bctanists, and how far the modern ])lant pathologist 

 lias travelled during the last fifty or sixty years. 



Thi-ee outstanding factors contributed to the great 

 development of our knowledge of plant diseases and 

 methods of control : 



1. — The epochal investigations of Eui-opean botanists 

 on the causal organisms and their relation to such 

 plant diseases as smut (1853), the potato-rot disease 

 (18(il), and wheat rust (1865). 

 2. — The rise of American investigators in the seventies 

 and eighties, such as Burrill, Farlow, Arthur, Bes- 

 sey, Halstead, Earle and others whose contributions 

 added substantially to our knowledge of fungi and 

 fungus and bacterial diseases. 

 3. — The work of the U. S. Department of Agriculture 

 and of the newly established Agricultural College 

 and Experiment Stations which gave a tremendous 

 impetus to the study of plant diseases and control 

 measures from 1885 onward to the present. 

 It is true that sulphur and powdered lime were often 

 dusted upon the plants and gave some relief, but it is 

 only since the discovery of Bordeaux in 1883 that the 

 control of plant diseases has been carefully investigated- 

 The plant pathologist has shown cleaidy that the fun- 

 gicide should be applied just before the rain, not after. 

 In the case of apple scab, for example, the spores are 

 carried to the leaves during rains, and if the rainy 

 spell lasts 48 hours infection will take place. To pro- 

 tect the leaves, therefore, the spray must be applied be- 

 fore the rain. 



Again, the plant pathologist has shown clearly that a 

 knowledge of the life history of the fungus is essential 

 if best results are to be secured in spraying operations. 



Lime-Sulphur. — Lime-sulphur has already been men- 

 tioned among the insecticides, but it is also one of the 

 best fungicides. For a time after the discovery of Bor- 

 deaux mixture it was discarded, but in 1906 its im- 

 portance as a fungicide was rediscovered, and ever since 

 it has been employed extensively for the control of 

 many fungous diseases of the orchard. 



In Nova Scotia, however, the fruit growers observed 

 that the calyx spray and especially the following spray 

 were responsible for a jieavy fall of fruit on account of 

 its artion on the leaves. For this reason they substituted 

 in 1918 and 1919 a modified Bordeaux for the lime- 

 sulphur. 



Ijime-sulphur is in the main a mixture of polysulphi- 

 des of calcium. The fungicidal properties of lime-sul- 

 phur lie in the free sulphur that is formed on the sur- 

 face of the leaves and fruit on the evaporation of tlie 

 water and the oxidation of the polysulphides. 



Wlien trees have been drenched with the mixture in- 

 jury is liable to occur, but under proper conditions lit- 

 tle or no injury follows, except in the case of potatoes, 

 American grapes and .some varieties of peaches. 



Bordeau.r Mixture. — From the time of its discovery 

 by Prof. Millardet, of France, in 1883 up to 1910. Bor- 

 deaux mixture was the standard fungicide for summer 

 spraying. But with the rediscovery of lime-sulphur in 

 1906 as a summer spray, Bordeaux has been relegated 

 to second place in commercial orcharding on account of 

 the russetting of the fruit and the yellowing of the 

 foliage that followed the standard application, made 

 according to the formula 4 :4 :40. 



The structure and ccmposition of Bordeaux mixture 

 has been recently ascertained. It consists of colloidal 

 membranes composed of a complex mixture of basic 

 copper sulphates, enclosing a solution of calcium hy- 

 droxide and calcium sulpliate holding lime particles in 

 susjiension. The fungicidal properties dejiend on the 

 number and size of the colloidal membranes and to the 

 lime particles. The smaller tlie membranes, the more 

 effective does the mixture cover tlie sprayed surface. 

 When evaporation of water occurs the minute mem- 

 branes dry down and attach themselves firmly to the 

 surface. Then the copper in the membrane is slowly 

 dissolved when the leaves become moist, and either kills 

 or inhibits the germinating spores which lodge on the 

 leaf. Moreover, it is ticlieved that the lime particles are 

 also fungicidal. 



It will be seen, therefore, that the (juality of the Bor- 

 deaux depends upon the method of preparation. 



The minute membranes will never be formed when 

 concentrated solutions of Copper sulphate and milk of 

 lime are brought together. On the other hand, they 

 will be formed when vigorous stirring takes place when 

 the dilute solutions are brought together. 



Reference has already been made to the peculiar con- 

 ditions in Nova Scotia, which compelled the fruit grow- 

 ers to return to a modified fin-m of Bordeaux where lime 

 is used in excess. In 1919 they used the 3:10 :40 and 

 the 2:10:40 formulae with gratifying results. 

 Dust Spraying. 

 While dust spraying was jirobably employed before 

 liquid .spraying, it was superceded by the latter when 

 Bordeaux mixture and Paris Green became the standard 

 spraying materials. However, as fruit growing was de- 

 veloped more extensively, and intensively as well, the 

 factors of economy of time and cost of etiulpment in 

 spraying operations assumed more importance, and ef- 

 forts were made by the more progressive men to meet 

 the needs of the industiy by devising more economical 

 methods of spi-aying. 



