166 FUNGOUS DISEASES OF PLANTS 



and it has been the cause of great disaster in many potato-growing 

 regions before methods for its control were well known. All who 

 are familiar with the history of potato growing doubtless know of 

 the potato famine of 1845. The serious famine in Ireland was very 



largely due to this failure of the 

 potato crop, a failure due to the 

 prevalence and unusual destruc- 

 tiveness of the Phytophthora. 



Distribution. At one time it 

 was the current opinion that this 

 fungus is very widely distributed 

 throughout the United States, 

 but a more careful study of po- 

 tato diseases has shown that it is 

 very largely limited to New Eng- 

 land and New York, extending 

 also into the potato-growing re- 

 gions of Canada. It is not en- 

 tirely absent from regions much 

 farther south and west, but in 

 such districts it seldom assumes 

 any importance. In Europe it 

 is widely distributed and may 

 be disastrous throughout Great 

 FIG. 60. PHYTOPHTHORA ON POTATO Britain as we n as ea st and west 

 LEAVES. (Photograph by F. C. Stewart) , 



from Russia to trance, extend- 

 ing even as far south as Italy. The distribution of this fungus and 

 its importance as a disease organism are entirely dependent upon 

 climatic conditions. It has been shown that it becomes of seri- 

 ous importance only when favored by warm, moist weather. As 

 a rule, the fungus does not appear in the northeastern United 

 States prior to the last days of July, and it is most abundant 

 during August and early September. A few days of rainy 

 weather suffice to give it a start and to bring to fruiting the 

 conidial stage on the leaves. The distribution of the fungus 

 is then accomplished with alarming rapidity, and whole fields 

 may be devastated within a period covering only a few days 

 of such weather. While it is generally stated that warm weather 



