PHYSICAL CONDITIONS INFLUENCING THE DISEASE. 25 



withstand the entry and devastation of a parasitic funo-us like Botrijtls, 

 at the same time favor the deveh)pnient of the funo-us itself." 



The writer thinks, as the result of observations in the tield, that 

 ExoaxcKi< deforiiianx is favored in l)oth its entrance and spread within 

 its host by the conditions which have just been considered. It is a 

 widely observed fact that leaf curl usually develops sparinj>ly in a 

 uniforndy warm and dry spring, and it is also noticed that where 

 infection has occurred a return of warm, dry wcMither, or e\-en the 

 occurrence of a hot, dry wind, will check the development of the 

 fundus within the tissues. An infected leaf mav fail to develop 

 the spores of the fungus under such circumstances. The thm, satu- 

 rated cell walls and the moist intercellular spaces thus appear to be 

 closely correlated with the active vegetation of the fungus. The 

 growth and consequent tenderness of the tissues is also important in 

 this connection. Where soil, elevation of orchard, and atmospheric 

 conditions are unfavora])le to a saturated condition of the plant paren- 

 chyma, the disease is not likely to run more than a short and feeble 

 course. Soil and elevation are here considered with atmospheric con- 

 ditions, for it is found that on the same farm a difference of elevation 

 or soil moisture may determine the degree of virulence of the disease. 

 The influence of elevation may ))e of only secondary nature — that of 

 maintaining a higher temperature — but its action on the disease is fre- 

 quently well marked. Of 92 orchardists who expressed their views 

 as to whether trees are affected by curl most on high or on low land, 

 48 say that trees suffer most on low land, 14 on high land, and 30 

 think there is no difference. While the majority claiming that trees 

 on low land are most affected is not as large as some of the majorities 

 obtained in replies to other questions, it represents over one-half 

 the replies received to the question under consideration and is more 

 than three times as great as the number who believe trees to be most 

 affected on high land, hence is sufficient to establish conffdence in its 

 reliability, even if it were not indorsed by many published statements 

 to the same effect. 



Mr. Thomas A. Sharpe, superintendent of the experiment farm at 

 Agassiz, British Columbia, has made several comparative reports on 

 the action of peach leaf curl on trees planted in the valley and upon 

 the more elevated bench lands of the farm. A few brief statements 

 from these reports should be of value in connection with the above 

 statements.' In 1892, Mr. Sharpe says, the peach trees suffered from 

 a severe attack of leaf curl. "Only 5 varieties of those planted in 

 the valley escaped" the disease. "The trees planted on the bench 

 lands did not suffer so much, and appeared to recover much more 

 rapidly than those in the valley" (1. c, p. 278). In 1893, it is said. 



1 See reports of experimental farms, Ottawa, Canada, for the years indicated. 



