14 
BULLETIN" 967, U. S. DEPAKTMENT OF AGRICULTURE. 
wounded. At this stage slugs ate the infected bark and prevented 
further development of the fungus. In 1916 and in 1917 7 150 
seciospore inoculations were made on leaves, on twigs of various ages, 
and on branches of P. strohus trees up to six years. No infections 
have resulted. 
Clinton and Miss McCormick (14, 15), have published details of 
successful inoculations, through the leaves, with sporidia on P. 
strohus. Inoculations were unsuccessful upon leaves of P. excelsa, 
P. Jlexilis, P. Icoraiensis, and P. cembra; also on the pitch pines P. 
resinosa, P. sylvestris, P. densiflora, and P. austriaca. Yellow spots 
Fig. 11. — Outline map of the northeastern part of the United States, showing (by black dots and cross 
hatching) the known distribution of white-pine blister rust in North America to and including 1917. 
have been secured on leaves of P. lambertiana, P. pinea, and P. 
sabiniana. 
Cross-inoculations that are known to have been successful up to 
July 1, 1920, are shown in Plate I. 
Species of Ribes That Have Been Infected Naturally. 
In Europe and North America, where extensive outbreaks of 
Cronartium ribicola have occurred, a considerable number of species 
of Bibes have been found naturally infected by the fungus. Prac- 
tically all of the cultivated species and most of the wild ones take the 
disease in every extensive outbreak area. More species have been 
found infected in Europe than in North America, because outbreaks 
have been discovered there in botanical gardens, parks, and nurseries 
