[faull] presidential ADDRESS 9 



are so intricate, their range of hosts so extensive, or their taxonomic 

 relations so complicated that intensive studies on them are indis- 

 pensable. Conversely much is to be said for host specialization, for 

 after all unless the normal structure of a host, its physiology and 

 ecological relationship are intimately known, progress in a knowledge 

 of its abnormal physiology remains uncertain. 



In dealing with life histories it is pertinent to point out that in 

 spite of all that has been learned of the more important parasites 

 there is scarcely one the life history of which is known in complete 

 detail, and very often the gaps in our knowledge are the very ones to 

 be filled in before most effective control measures can be applied. 

 Thus we are still puzzling over the ways in which the stem rust of 

 wheat may be carried over from one season to the next, exact inform- 

 ation on which is essential in the campaign against that most destruc- 

 tive species of the cereal fungi. We are still uncertain as to the exact 

 conditions under which apple scab infection takes place, a feature that 

 is directly related to spraying operations. We are still confronted 

 with the unsolved problems of biological strains or species. That this 

 phenomenon exists in certain rusts and powdery mildews has been 

 absolutely demonstrated, but a beginning only has been made and 

 many important fungi in which this phenomenon probably occurs 

 have yet to be scrutinized from this angle. Here are problems of vital 

 significance in breeding for resistance, since experience seems to show 

 that resistance is to be spoken of in terms of the relation of the host 

 to biological strains and not to morphological species. These examples 

 will suffice to show that investigators may turn again to the best and 

 longest known parasites in the expectation of uncovering new facts 

 of value; they, too, have the advantage of new points of view and an 

 improved modern technique. 



(2) But a complete knowledge of the parasite is only one step in 

 understanding the phenomenon of parasitology. Experience teaches 

 us that the factors of the environment are to be considered along with 

 the parasite among the conditions that are responsible for disease. 

 Some pathologists even give the place of first importance to environ- 

 mental factors such as temperature, moisture and soil content, and 

 they are probably right in the case of many diseases. It is a well 

 known fact that where these factors are more or less controllable, as 

 in commercial greenhouses, attention to heating, watering and lighting 

 is all that is necessary to avoid losses from such pests as gray mold of 

 lettuce, geraniums, etc., or the sooty mold of tomatoes. That climatic 

 and soil conditions, singly or combined, regulate epidemics or the 

 prevalence of diseases seems to be true beyond question. It is stated 



