Award of the Boijle Medal io George H. Pcthyhridge. 2^7 



the parasite's active season. As the result of the interaction of these organs^ 

 globular reproductive bodies — oospores — are formed. Having hard, thick, 

 protective coats, these oospores lie dormant for a time, but when suitable 

 conditions supervene they germinate and produce new parasites. 



In the case of Fhytophthora infestajis such oospores had not been discovered. 

 True, there were one or two investigators who claimed to have found them, 

 and who maintained that the parasites developing from them were responsible 

 for the infection of the crop in the succeeding year. Nevertheless, the critical 

 studies of de Bary, alluded to above, show that these claims were not 

 substantiated. 



The investigations of Dr. Pethybridge and others have been so complete 

 that it is now practically certain that no such infection by means of oospores 

 takes place. These bodies have never really been found in the tuber or in 

 any other part of the plant, in spite of most careful and prolonged search. 

 If they ever are formed there, their formation is so rare that it cannot 

 be of any importance as a source of infection of the crop. 



Seeing that re-infection of the crop is not provided for by the production 

 of oospores, it was maintained that the mycelium of the parasite grew down 

 from the leaves, reached the tubers through the aerial and underground 

 stems, became dormant there as the tubers matured, and remained in this 

 condition throughout the winter. In the following season, when the " seed " 

 potatoes began to sprout, it was believed that this mycelium, till then 

 supposedly dormant in the tubers, woke up, and, keeping pace with the 

 growing stems and leaves, entered the latter, and remained there in a 

 quiescent state, until the warm weather of the summer started it into activity 

 and destruetiveness. 



Despite the most painstaking microscopic investigations, these invading 

 and temporarily innocuous mycelia were never discovered ; and all the 

 observations and experiments directed towards solving the question showed 

 that the well-known and easily visible spots of blight on the foliage — up to 

 that time usually regarded as the earliest stages of a fresh outbreak — were 

 the result of infection by air-borne spores. 



By experiments carried out over several years, Dr. Pethybridge has 

 contributed important evidence supporting this latter view. Furthermore, 

 his experimental work in greenhouse and garden (supplemented later by that 

 of other workers under field conditions) has revealed that there is a still 

 earlier phase, rarely to be met with and easily overlooked, in which the 

 fungus (by no means in a dormant condition) does invade certain sprouts 

 which, if not all too quickly killed, may succeed in getting above ground. 

 It is now generally accepted that it is from aerial spores produced by the 



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