HIBERNATION OF PHYTOPHTHORA INFESTANS 
IN THE IRISH POTATO 
By I. E. Melhus, 
Pathologist, Cotton and Truck Disease Investigations, 
Bureau of Plant Industry 
INTRODUCTION 
How Phytophthora infestans perpetuates itself from year to year has 
been studied ever since Unger in 1847 (34) 1 finally proved that the fungus 
causing the disease is a species of Peronospora. No sooner had this fact 
been established than students began searching for resting organs like 
those so common in other species of Peronosporaceae. As is well known, 
progress was slow, and the question as to whether P. infestans does or 
does not have oospores ended in a controversy between W. G. Smith (30) 
and De Bary (4) in the early seventies of the last century. The outcome 
is too well known to need repetition; suffice it to say that De Bary's 
negative evidence has been generally accepted. 
Recently the oospore question has been taken up anew and bodies 
resembling oospores have been found by Jones (15, 16, 17), Clinton (9), 
and Pethybridge and Murphy (27) in pure cultures of the fungus. 
Although no direct claims that similar bodies exist in the potato plant 
(Solanum tuberosum) have been made, these recent investigations have 
at least weakened the perennial-mycelium theory, which probably was 
first advanced by Berkeley in 1846 (5). Like many of the botanists dur¬ 
ing the first half of the last century, Berkeley unfortunately submitted 
no experimental evidence to support his contention. The credit of first 
submitting such evidence belongs to De Bary, who in 1861 in an inter¬ 
esting paper (1) showed that the conidia can not live over winter; that 
no relation exists between the mycelium of P. infestans and of the sapro¬ 
phytes that occur on diseased tubers; that it is impossible to infect 
potatoes with any of the Peronosporaceae that occur on plants common 
about potato fields; and that the potato fungus is able to spread from 
diseased seed tubers up into the shoots, sporulate, and renew infection 
on the foliage. 
About 10 years later, Scholtz, Bretschneider, Peters, and Reess took 
up for the “Central Commission fur das Agrikulturchemische Ver- 
suchswesen” the problem how P, infestans perpetuates itself. They 
were unable to confirm De Bary (1), and Pringsheim (29), who sum- 
1 Reference is made by number to “ Literature cited,” p. 100-102. 
Journal of Agricultural Research, 
Dept, of Agriculture, Washington, D. C. 
ad 
Vol. V, No. 2 
Oct. 11,1915 
G—58 
