28 



ments have conclusively shown that this disease can be prevented by- 

 incorporating vegetable matter in the soil and applying potash fertili- 

 zers. This so increases the vigour of the plant that the facultative para- 

 sites causing the disease are unable to gain a foothold. On the other 

 band, the injury to pear trees from blight can be much lessened by pre- 

 venting a too vigorous growth and securing the early ripening of the wood. 

 This can best be secured by witholding cultivation and nitrogenous 

 manures. In this case the disease germs only flourish in the soft rapidly 

 growing cambium and the hardening of the wood stops the spread of the 

 disease. 



Topical applications may be made to the seed before planting, to the 

 growing plant in the form of fungicidal sprays, or in some cases to the 

 soil. Treatment of the seed is useful only in those cases where the source 

 of contagion is from spores that adhere to the seeds and are planted with 

 them. Thus in harvesting and threshing oats the spores from smutted 

 heads become dusted over the sound grains. It is almost or quite impossi- 

 ble to find seed for planting that is not more or less infested in this manner. 

 If such seed is soaked in hot water of the rigbt temperature or in certain 

 fungicidal solutions, as formalin or copper sulphate, the smut spoies will 

 be killed without injuring the vitality of the grain; and the crop from this 

 treated seed will be practically free from smut. Potato scab is a disease that 

 is usually disseminated by the planting of diseased tubers for seed. Where 

 once introduced in the soil it lives from year to year, so that seed treat- 

 ment is not always effective; but, if planted on clean land, even badly 

 scabbed seed potatoes will yield a clean crop if soaked in a weak solution 

 of corrosive sublimate. 



The discovery that certain diseases can be prevented by sprinkling 

 plants with a solution of copper sulphate mixed with milk of lime marked 

 an important epoch in the treatment of plant diseases. This mixture, 

 known as Bordeaux mixture from the town in France uear which its use 

 was accidentally discovered, is now the standard remedy for a large class 

 of diseases. In the case of many orchard and garden crops, spraying with 

 Bordeaux mixture is as much a recognized part of proper culture as is the 

 tilling of the soil. As first used the mixture was simply spattered over 

 tbe leaves by means of a whisk broom. This method was unsatisfactory, 

 as it was slow and did not secure a sufficiently even distiibution of the 

 liquid. Thanks to American ingenuity and particularly to the efforts of 

 the late (J. V. Kiley, then chief entomologist, and of B. F. Galloway, now 

 chief of the Bureau of Plant Industry of the United States Department of 

 Agriculture, suitable pumps and spraying nozzles were devised, by means of 

 which plants can be quickly and evenly covered with this or other liquids 

 in the form of a fine mist-like spray. Other compounds of copper have 

 also been found to have strong fungicidal properties, but none are so gener- 

 ally useful as the Bordeaux mixture. When properly made and applied, it 

 does not injure the foliage except of a few particularly delicate plants 

 and as it is not easily washed off by rains its effects are more lasting than 

 •with other fungicides. It is now the standard remedy for potato blij;ht 

 grape rot and mildew, apple scab, peach leaf curl, and a long list of simi- 

 lar diseases. It should always be remembered however that, except in the 

 case of a few external parasites, spraying is a preventive measure and not 

 a cure. Sprays cannot reach internal parasites when once established, but 

 by coating the surface they prevent the germination of spoies that find a 

 lodgement there and thus prevent infection. The importance of early 

 fiprayitig before a disease makes its appearance, and of thorough work in 

 jeacbing all exposed parts of the plant will be apparent from these facts. 



