orton: pathology of solanum tuberosum 



183 



ing factor in potato production. Only young plants can survive 

 exposure to 90°F. for any extended priod, hence we find a tendericy 

 to plant late in the north in order that the time of tuber formation 

 may come during the cooler weather of autumn. 



It appears that most or all of our present varieties originated 

 in northern districts, from parent stocks having low temperature 

 requirements. To secure varieties capable of extending potato 

 culture southward local breeding should be practised if stocks 

 possessing the necessary physiological qualities can be found. 

 Our great need is to discover a variety of Solanum tuberosum, 



Fig. 2. Chart showing average monthly rainfall for points in southern Chile 

 as compared with Europe. 



or a species sufficiently related to hybridize with it, that is indi- 

 genous to a region of high temperature, and capable of transmitting 

 to crosses with existing cultivated races a heat enduring quality 

 derived thru better control of transpiration, or in other ways. 

 It will be evident from a consideration of the south Chilean ch- 

 mate that the heat resistant factor cannot be found there. We 

 must turn to more northern and warmer sections of South or 

 Central America, a region that has been very inadequately ex- 

 plored to date, but where Mr. Wight of this Society is now engaged 

 on this mission. 



Turning now to the consideration of potato diseases, we shall 

 find in the late blight, caused by the fungus Phytophthora infestans, 

 an excellent illustration of the extent to* which climatic environ- 



