Oct. ir, 1915 
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 (n) 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: (1) 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 o° to io° C. until only a short time before planting. The 
fact that P. infestans and soft-rot organisms make little or no growth at 
