JOURNAL OF AGRWniAL RESEARCH 
DEPARTMENT OF AGRICULTURE 
Vol. V Washington, D. C., October ii, 1915 No. 2 
PERENNIAL MYCELIUM IN SPECIES OF PERONOSPO- 
raceae RELATED TO PHYTOPHTHORA INFES- 
TANS 
By I. E. Melhus, 
Pathologist, Cotton and Truck Disease Investigations , 
Bureau of Plant Industry 
INTRODUCTION 
Phytophthora infestans having been found to be perennial in the. Irish 
potato (Solanum tuberosum), the question naturally arose as to whether 
other species of Peronosporaceae survive the winter in the northern part 
of the United States in the mycelial stage. As shown in another 
paper (13), 1 the mycelium in the mother tuber grows up the stem to the 
surface of the soil and causes an infection of the foliage which may result 
in an epidemic of late-blight. 
Very little is known about the perennial nature of the mycelium of 
Peronosporaceae. Only two species have been reported in America: 
Plasmopara pygmaea on Hepatica acutiloba by Stewart (15) and Phytoph¬ 
thora cactorum on Panax quinquefolium by Rosenbaum (14). Six have 
been shown to be perennial in Europe: Peronospora schachtii on Beta 
vulgaris and Peronospora dipsaci on Dipsacus follonum by Kuhn (7, 8); 
Peronospora alsinearum on Stellaria media , Peronospora grisea on Veronica 
hederaefolia, Peronospora effusa on Spinacia oleracea , and A triplex hor- 
tensis by Magnus (9); and Peronospora viiicola on Vitis vinifera by 
Istvanffi (5). 
Many of the hosts of this family are annuals, but some are biennials, 
or, like the Irish potato, are perennials. Where the host lives over the 
winter, it is interesting to know whether the mycelium of the fungus may 
also live over, especially where the infection has become systemic and 
the mycelium is present in the crown of the host plant. The absence or 
sparse production of oospores in some of the species of Peronosporaceae, 
coupled with the appearance of the fungus as soon as the host puts out 
foliage in the spring, suggests that the mycelium may play an important 
1 Reference is made by number to " Literature cited,” p. 68-69. 
Journal of Agricultural Research, 
Dept, of Agriculture, Washington, D. C. 
ac 
Vol. V, No. 2 
Oct. 11,1915 
G —57 
(59) 
