388 Report of the Mycologist of the 



They must be of the same variety, planted at the same time, in 

 the same manner, on the same kind of soil, treated with the same 

 kind and quantity of fertilizer and given the same cultivation. 

 The spraying must be properly done, the land accurately meas- 

 ured and the crop weighed. Failure to comply with any one of 

 these conditions makes the test an unfair one. 



THE PHILOSOPHY OF SPRAYING. 



It is believed that, in some unexplained way, the Bordeaux 

 mixture has a direct beneficial influence on potato foliage, in 

 addition to its value as a fungicide and repellent to insects. Its 

 chief value, however, lies in the protection which it affords the 

 leaves against the attacks of parasitic fungi and insects. The 

 leaves of the potato plant are very essential organs and it is of 

 the greatest importance that they should be perfect in order that 

 they may do their work properly. The inorganic food substances 

 which the plant absorbs from the soil through its roots are trans- 

 ferred to the leaves and by them assimilated, or in other words, 

 transformed into starch and certain other organic substances 

 which pass down the stem and are stored up in the tubers. The 

 size and quality of the tubers are^ therefore, directly dependent 

 upon the activity of the leaves. If portions of the leaves are 

 eaten away by insects or destroyed by disease their capacity for 

 assimilation is lessened and the tubers are correspondingly 

 smaller. 



The truth of this is recognized when there is great destruction 

 of foliage such as is caused by a severe attack of late blight or 

 by hordes of Colorado potato-beetles, but it seems certain that 

 the amount of damage done by leaf-eating insects and parasitic 

 fungi is greatly underestimated. This is proven by the results 

 of the spraying experiment reported in the previous pages. Ta 

 that case, spraying increased the yield on one variety by the 

 amount of 62 bushels per acre, chiefly by protecting the leaves in 

 the following three ways: (1) from the apparently slight injury 

 of the early blight fungus; (2) by affording partial protection 

 from the injury caused by flea-beetles; and (3) by preventing the 

 attacks of Colorado potato-beetles more thoroughly than could 



