HISTORY AND SCOPE OF PLANT PATHOLOGY 161 



danger of injury to foliage, and made it the most important agent 

 known to-day for the direct treatment of plant diseases. Many 

 experiments have been tried with a wide range of substances, but 

 none has been found to supplant it, although other preparations of 

 copper, like the ammoniacal solution, are used for special cases 

 when the staining of the foliage due to Bordeaux Mixture is objection- 

 able. The methods of application have been refined and extended, 

 early replacing the coarse whisk-broom first employed, until now 

 a great variety of machinery is in the market, and choice may be 

 had of many kinds of nozzles, and of knapsack, barrel, or power 

 pumps of varied designs. 



The general introduction of spraying was hastened by the advent 

 of the Colorado potato beetle, which marched in armies across the 

 country from the western plains to the Atlantic coast in a period 

 of about ten years, leaving the potato-fields a waste of withering 

 stalks. Something had to be done to hold in check these voracious 

 insects, that threatened to do for America what the potato fungus 

 had done for Ireland in 1845. As a result of these strenuous condi- 

 tions, the arsenite insecticides were brought into use. At first they 

 were applied in powder, but when the Bordeaux Mixture proved 

 serviceable for fungi, it was found that both insecticide and fungi- 

 cide could often be applied in one operation, and since then the 

 practical work of the entomologist has to a considerable extent 

 run parallel with that of the pathologist. 



Success in spraying naturally stimulated inquiry into remedial 

 and protective agents for fungous diseases not amenable to spraying 

 methods. Attention was first directed to a method of protecting 

 wheat, oats, and similar grains from smut, which would be more 

 efficient and less hazardous than the very old one of steeping the 

 seed in a solution of blue stone. In 1887 J. L. Jenson of Denmark 

 brought out a method of applying hot water to the seed, that proved 

 very effective, and was widely adopted. Ten years later H. L. Bolley 

 of North Dakota introduced formalin for the same purpose, which 

 is now extensively employed. It has also been found effective and 

 is easily applied in the prevention of potato-scab, and has recently 

 been used for flax where the presence of a fungus threatened to put 

 an end to the industry. 



In this connection a very ingenious method of ascertaining what 

 kinds of fungous spores and how many are attached to the surface 

 of seeds was devised not long ago by Professor Bolley. The seeds 

 to be examined are washed with pure water, which is then revolved 

 at high speed in a centrifuge. The resulting drop of sediment is 

 placed under the microscope, and all the germs, large and small, 

 that were present on the seeds are now clearly visible, providing 

 unmistakable evidence of the fungous parasites infesting the field 



