2 BULLETIN 81, I T . S. DEPARTMENT OF AGRICULTURE. 



REVIEW OF THE POTATO-DISEASE SITUATION. 



TYL.cn seeking' protection from new plant diseases we must be 

 guided by past experience and by our knowledge of the general 

 principles controlling the occurrence and spread of plant parasites. 

 It is evident that agriculture in general bears a burden that increases 

 from year to year as new diseases or insect enemies appear. In 

 colonial limes and up to 1840 the potato seems to have been free 

 from many serious pests that have come in since. We now list IS 

 or 20 diseases, not including insects, that attack potatoes in some 

 part of the United States. The yearly loss from them is difficult to 

 estimate, but the injury from tuber rots and related troubles was 

 recently placed by the Department of Agriculture at over $30,000,000 

 annually, and diseases which attack the crop in the field probably 

 reduce the value of the harvest by another $30,000,000 per annum. 



Not only do new parasites appear at frequent intervals, but they 

 can rarely, if ever, be exterminated. A plant disease, once estab- 

 lished here, is likely to be with us forever. Under these circumstances 

 it is to the credit of American farmers that they havs, during the 

 last generation, by the adoption of scientific methods of fertilization 

 and culture and by spraying and seed treatment for diseases, main- 

 tained the average yield per acre of the country and in the more 

 progressive sections considerably increased it. On the other hand, 

 the average yield is still only about half what it might be, as judged 

 by European standards, and the cost of spraying, increased ferti- 

 lizers, etc, constitutes a heavy annual tax on the grower. 



INTRODUCED PARASITES THE MORE DANGEROUS. 



Plant parasites may be divided into two classes, those endemic or 

 native to the country and those introduced from other countries. 

 It is a general principle, fully established by experience, that parasites 

 introduced from other continents or distant parts of the same 

 continent are more injurious than the native parasites of the same 

 crop and more virulent and destructive in their new habitat than 

 they had been at home. 



The United States has had many costly examples of this fact, 

 among which may be cited the gipsy moth, the brown-tail moth, the 

 codling moth, the asparagus rust, the hollyhock rust, and that recent 

 immigrant from the Orient, the chestnut bark disease, which is 

 threatening to destroy our chestnut forests. 



Several potato diseases are of foreign origin. The examples men- 

 tioned below are of special interest. 



LATE-BLIGHT. 



I]i the period from 1830 to 1842 there was introduced into both 

 Europe and America a new potato disease which causes a blighting 

 of the foliage, followed by decay of the tubers. This disease, called 



