FORTY-FOURTH ANNUAL REPORT. 95 



THE PRINCIPLES OF SPRAYING. 



BY PAUL S. ARMSTRONG^ WASHINGTON^ D. C. 



The difference between a snccessfnl and an nnsnccessfnl nrcliardist 

 at the present time lies in the methods which he nses in the care and 

 maintenance -of his orchard. Among these modern methods there is none 

 which occnpies the important place that careful, thorough, and intelli- 

 gent spraying does as a factor of success. The market demands clean 

 healthy fmit and it is only through the best spraying practice that we 

 can supply this demand. A knowledge of the disease causing organisms 

 and the ways in which they operate often furnish valuable hints as to 

 the time and means of combating our particular trouble. 



With few exceptions all of the common fruit diseases are caused by 

 minute parasitic plants called fungi. A fungus is a plant which lives 

 within or upon another plant and derives its food from the plant upon 

 which it is growing. In the way which it obtains its food the plant 

 parasite is very similar to the animal ])arasites which are more com- 

 monly known. Briefly the effects of a fungus upon its host are two. It 

 may cause a general starvation of the plant on account of the feeding 

 of the fungus threads upon the food cells of its host. This damage shows 

 up in the small size of the fruit and the light yield. The other type of 

 injury is apparent to all because of the disfiguring or rotting of the fruit 

 as in cherry or plum rot or apple scab. 



The manner in which the fungus gains entrance to the interior of the 

 plant is the key to its prevention in either case. As a type we will fol- 

 low the progress of the disease illustrated here. A fungus has its seed 

 as does any other plant. These spores are produced in countless mil- 

 lions in many generations a year and are everywhere present in the air. 

 Dishes of absorbent cotton exposed for twenty-four hours on the roof 

 of a high building were found to have caught the spores of several com- 

 mon diseases. The seed or spores fall upon the leaf and the rain or dew 

 furnishing suflScient moisture, the spore begins to send out a fine white 

 thread like a root Avhich grows along the surface of the leaf and when 

 this thread comes to a breathing pore in the leaf it passes down through 

 it and gains entrance to the food cells of the plant. Once there it in- 

 creases its growth enormously, sending a network of threads among the 

 cells, deriving food from each that it passes by means of a little branch 

 sucker pushed into the cell. Growing this way to maturity, all the time 

 exhausting the food and vitality of the host, it soon begins to develop a 

 fruiting layer or provide spores for the spread of the disease. It is while 

 in this stage that we notice the injury particularly, the gray tufts which 

 appear on the brown rotted cherry or the black splotch from the apple 

 scab. 



The first principle which we must apply in our spraying practice is 

 to remember that spraying is essentially a preventative and not a cure. 

 Once a spore has germinated and its threads have penetrated the leaf 

 spraying is valueless as far as that leaf is concerned. There are several 

 theories regarding the way in which the Bordeaux kills the spore but 



