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 injestans perpetuates itself from year to year has 

 been studied ever since Unger in 1847 (34) ^ 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. injestans 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 tuberosiim) 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 (i) showed that the conidia can not live over winter; that 

 no relation exists between the mycelium of P. injestans 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 fiir das Agrikulturchemische Ver- 

 suchswesen" the problem how P. injestans perpetuates itself. They 

 were unable to confirm De Bary (i), and Pringsheim (29), who sum- 



' Reference is made by number to " Literature cited," p. 100-102. 



Journal of Agricultural Research, Vol. V, No. 2 



Dept. of Agriculture, Washington, D. C. Oct. 11, 1915 



ad G — s8 



(71) 



