PHYSICAL CONDITIONS INFLUENCING THE DISEASE. 27 



bodies of water .sprino- foj^s coiniiioiily oecur, and thesc^, lead to the 

 iiu'r(>ase of the atiuospherie humidity at a time when the foliao-e is 

 tender and growing- rapidly, thus stimuhiting a development of eurl 

 almost annually and over wide stretches of country. IndependiMit of 

 fogs, the atmosphcu-e about large bodies of water is also much more 

 humid than in an inland region. Instances of the inllucnct^ of large 

 bodies of water on the general prevalence and frequent occurrence of 

 curl in a region are found in western New York, near the shore of Lake 

 Ontario; in Ontario, Canada, near Lakes Erie and Ontario; in Michi- 

 gan along the shore of Lake Michigan; in California about the bay of 

 San Francisco and at other points along the Pacific coast; in Washing- 

 ton and British Columbia about Puget Sound; and in many similar 

 situations in all portions of the world where the peach is grown. The 

 writer believes, however, that the influence of large bodies of water 

 upon the development of curl depends in part upon the noiiual spring 

 temperature of the region, and likewise upon the source of the prevail- 

 ing winds. Where the prevailing spring winds are from a dry, inland 

 region instead of from the water, the atmospheric does not feel the 

 influences of the latter. Moreover, where thti spring temp(M-ature is 

 high, transpiration may proceed normall}^ even in the neighborhood of 

 large bodies of water, and cuirl may not commonly prevail. 



In contrast to the influences of large . bodies of water are those of 

 neighboring dry and arid plains or desert regions. In the midst of 

 such influences peach leaf curl can rarely attain to an epiphytotic 

 development, and then only under special favoring seasonal condi- 

 tions. The atmosphere is normally too dry in such situations to exert 

 a predisposing action upon the host, and it certainly does not favor 

 the serious development of the parasite. Exemplifying these condi- 

 tions are large areas in Arizona, New Mexico, Nevada, Utah, Colorado, 

 Texas, Kansas, and California. Little or no curl is reported from the 

 more arid portions of these sections of the country, its absence being 

 due, at least in part, to the influences here considered. 



Another of the broader influences aflecting the general and perma- 

 nent prevalence of curl over extensive regions is the normal annual 

 rainfall. Comparisons of this kind must be made, however, between 

 regions of approximately similar temperature. Under such condi- 

 tions it may be said that the general annual prevalence of leaf curl 

 increases with the increase of normal annual precipitation. Compari- 

 sons of this kind can hardly be justly drawn in the Mississippi Valley 

 or "on the Atlantic coast, as the temperature conditions vary too 

 greatly in those regions from north to south. On the Pacific coast, 

 however, owing to the modifying influence of the Pacific Ocean, the 

 temperature prevailing from Lower California to British Columbia, 

 a distance of about one thousand three hundred miles, presents no 

 such great variations as are found in a like distance from south to 



