Oct. II. 191S Phytophthora infestans in Irish Potato 93 



Pringsheim's theory, it must be conceded, won some consideration at 

 the hands of practical growers. This is well illustrated in an early paper 

 by Farlow (11) and an article by Jenkins in 1874 (13). The latter dis- 

 cusses 100 reports made by potato growers on the potato fungus. It 

 is very apparent from these articles that clover or straw was thought 

 by many to be an alternate host for P. infestans. This theory, as well 

 as others equally fictitious, was not expounded after 1876, when De 

 Bary published his second paper (4) on this subject. At this time he 

 submitted further evidence supporting the perennial-mycelium theory. 



De Bary's theory was not confirmed until about 26 years later, when 

 Jensen (14) repeated De Bary's experiments and obtained infected 

 plants which later became centers of secondary infection. He, like De 

 Bary, worked only in the open, where accidental infection by conidia or 

 by oospores is always possible and where such conditions as moisture 

 and temperature are variable factors. In other words, the technique 

 used by Jensen was no more refined than that used by De Bary 26 years 

 earlier; and he, like De Bary (4), was unable sufficiently to define his 

 method so that his results might be duplicated. In view of this fact it 

 is not surprising that Jensen's researches failed to materially strengthen 

 the perennial-mycelium theory. 



During the last 25 years repeated efforts have been made by Boehm 

 (6), Smorawski (32), Hecke (12), Clinton (8), Massee (20), Pethybridge 

 (25), and Jones (17) to grow such diseased plants as were described by 

 De Bary and Jensen from infected seed tubers, both under glass and in 

 the open, but little confirmatory evidence has been obtained. This fact, 

 coupled with the very important discovery by Jones (15), Clinton (9), 

 and Pethybridge and Murphy (27) of resting spores borne by the late- 

 blight fungus in pure cultures, has made the perennial-mycelium theory 

 seem even more questionable. This feeling is liberally expressed by 

 Clinton (8). 



The fact that so many students have failed to show the relation of 

 infected seed potatoes to epidemics of the disease may well be due to 

 one or all of three factors: (i) Stage of activity of the tuber, (2) tem- 

 perature, and (3) moisture of the air and soil. 



It is well known that the tuber requires a rest period before it will 

 begin to germinate. If an infected tuber is planted in moist, warm soil 

 before this period has elapsed the tuber rots quickly, owing to the activity 

 of P. infestans and soft-rot organisms. If, on the other hand, diseased 

 tubers are held in cold, dry storage until late in the winter or early in 

 the spring and then planted, the tuber makes considerable growth before 

 it is overrun by P. infestans and soft-rot organisms. In several of our 

 northern potato-growing sections potatoes are stored at temperatures 

 ranging from 0° to 10° C. until only a short time before planting. The 

 fact that P. infestans and soft-rot organisms make little or no growth at 



