2 BULLETIN 64, U. S. DEPARTMENT OF AGRICULTURE. 



The more clearly this danger is made apparent to the growers and 

 the more general and concerted their efforts to combat it the greater 

 the likeLiliood that the final result will place the potato industry on a 

 liigher plane than it occupies to-day. The same system of seed 

 selection and treatment and crop rotation that will free the potato 

 fields of vnlt, leaf-roll, and curly-dwarf will at the same time not only 

 bring under control the blackleg and some other diseases, but will 

 insure the maintenance of the strains cultivated in their most vigorous 

 and productive condition and free from objectionable mixtures with 

 other varieties. 



Past experience warrants these statements. The history of potato 

 pathology is a story of the gradual recognition and differentiation of 

 previously confused diseases and the introduction of control measures 

 that brought with them more progressive cultural practices. From 

 about 1845 the late-bHght, Phytophthora infestans, occupied the cen- 

 ter of the stage and is still one of the most destructive diseases (Jones, 

 Giddings, and Lutman, 1912).^ It causes heavy losses in nearly all 

 the potato districts of the world, especially in cool and humid seasons. 

 In the United States, however, there are many sections where Phy- 

 tophthora occurs somewhat rarely. An examination of a map pre- 

 pared by Dr. Erwin F. Smith and published in the Annual Report of 

 the Department . of Agriculture for 1885 shows that the losses from 

 late-bUght and rot were even then recognized to be mainly in the 

 northern tier of States. This has been confirmed by an annual plant- 

 disease survey of the United States, which has been made during the 

 past 12 years under the direction of the writer. This survey shows 

 conclusively that Phytophthora as a common parasite of the potato 

 is limited to the Northeastern States east of the Mssissippi River, 

 with only sporadic outbreaks in southeastern trucking regions, in the 

 Puget Sound district, and occasionally elsewhere. This disease is 

 now successfully combated by spraying with Bordeaux mixture, and 

 it is to be expected soon that more disease-resistant varieties will 

 introduce a new era of late-blight control. 



To the southward it has been found that early blight, Alternaria 

 solani (E. and M.) J. and G., and tip-burn play a greater r61e than 

 Phytophthora in the injury to the potato crop. Early blight is 

 apparently not so common in the cooler and more uniform climate 

 of northern Europe. Nor does one find there that tip-burn is as 

 common as here, where high summer temperatures combine with 

 the injuries of flea beetles and other insects to cause excessive trans- 

 piration and its consequent marginal burning of the leaves. Here 

 also the logical Ime of attack seems to be the production of varieties 

 possessmg heat resistance. 



I All references to literature are indicated in the text by the name of the author and the year of publi- 

 cal ion. For full citations, see the list at the end of this bulletin. 



