REED: SPECIALIZATION OF PARASITIC FUNGI i ie 
The occurrence of bridging hosts has been suggested in other 
groups of parasites as well as in the rusts. Salmon (126, 127), in 
connection with his work on the powdery mildew of the bromes, has 
suggested the possibility that Bromus hordeaceus may act as a bridging 
host for the mildew on Bromus racemosus and B. commutatus. He 
found that the mildew on B. racemosus failed to infect B. commutatus 
(0/12),'° while it readily infected B. hordeaceus (34/34). Furthermore 
conidia from B. commutatus failed to infect B. racemosus (0/36), while 
infecting B. hordeaceus (40/49). Salmon supposes that B. hordeaceus 
may act as a bridge for the mildew on B. racemosus to pass over to 
B. commutatus and also the reverse. In one case, Salmon infected 
B. hordeaceus with conidia from B. racemosus. The conidia produced 
on the former were then used to infect B. commutatus. Salmon, how- 
ever, did not test the infecting capacity of the conidia thus produced 
on B. commutatus. 
Steiner (150), in his work with the mildew (Sphaerotheca Humuli 
(DC.) Burrill) of Alchemilla, reports the occurrence of bridging hosts. 
He states that conidia from Alchemilla connivens and A. pubescens 
readily infected A. pastoralis and A. impexa but gave negative results 
when sown on A. micans. On the other hand, A. micans is readily 
infected with conidia from A. pastoralis and A. impexa. Accordingly 
Steiner supposes that A. pastoralis and A. impexa may carry the mil- 
dew over to A. micans from A. connivens and A. pubescens. Some 
evidence is also given to indicate that A. impexa is a bridging host 
between A. nitida and A. fallax. Steiner’s conclusions, however, are 
based on only a few tests. 
In the Erysiphaceae the question has been raised whether the 
ascospores and conidia from a particular plant possess the same in- 
fecting capacities. Marchal (98), Salmon (124, 132), and Voglino 
(166) have, in a few cases, used ascospores for inoculation tests and 
they report that the capacity of ascospores for infection is identical 
with that of conidia from the same host. 
It is, however, somewhat surprising that the evidence that various 
races of parasites may increase their range by means of bridging hosts 
is so vety meager, if such really occurs. In those cases where the 
suggestion of bridging has been most emphasized one is not impressed 
with the data supplied. In fact, in all such cases the races of the 
parasite are not sharply limited in their host range. They may infect 
some hosts more readily and more vigorously than others, but the 
virulence of the parasite does not seem to be increased or decreased 
by developing on congenial or uncongenial hosts. 
16 The denominator of the fraction indicates the number of leaves inoculated 
and the numerator the number infected. 
