E. S. Salmon 459 



repeated inoculations with conidia of Erysiphe Polyqoni growing on 

 another individual of R. re/pens^. 



From the observations of Biffen(i2), Eeed(i3), Wawilow(i4) and the 

 writer (9), it is clear that with respect to another species of mildew, 

 viz. Erysiphe Graminis, there occur, among the host-species of its 

 "biologic forms," varieties or forms which are either immune or strongly 

 resistant. 



According to the following communication from Prof. Kingo Miyabe, 

 the "biologic form" of S. Humuli which attacks Humulus does not 

 occur in Japan, although the species S. Hiimuli is common in that 

 country on a number of host-species belonging to different genera (7, 8). 

 Prof. Kingo Miyabe writes: — "One thing which strikes me as most 

 peculiar is the entire absence of the powdery mildew on both the wild 

 and cultivated hop-plants in the vicinity of Sapporo and also probably 

 in the rest of our country. The cultivated hop has been introduced 

 from Europe, mostly from Germany." (It may be noted here, however, 

 that Hennings has recorded (15) S. Humuli from Japan: — "Tokyo: auf 

 Blattern von Humulus Lupulus L. (Shirai)^.") 



In 1916 I received from Prof. Kingo Miyabe examples of the male 

 plant of the wild hop of Japan {H. Lupulus var. cordifolius Maxim) ; on 

 growing them in the glass-house where, as mentioned above, all the 

 conditions were favourable to inoculation by conidia of the hop-form 

 of S. Humuli, the plants became infected. 



A cognate case is doubtless that of the "immunity" of the "Virginia 

 Creeper" {Vitis hederacea) in Europe. In the United States this plant 

 is attacked by the mildew Uncinula necator (Schwein.) Burr. This 

 mildew is very common in Europe on cultivated vines, but so far as I 

 know has never been found attacking the " Virginia Creeper," doubtless 

 because speciaUsation of parasitism has occurred in the species U. 



1 In 1907 Dr W. W. Stockberger, of the United States Department of Agriculture, 

 recorded ("Improving the Quahty of Domestic Hops"; paper read at a Meeting of the 

 American Brewing Institute, Nov. 20, 1907) the following fact: "During the past season 

 in some of the badly-moulded yards of New York State, hills were found which were 

 absolutely free from mould, although entirely surrounded by badly-moulded hiUs. These 

 hills have been marked, and cuttings will be taken from them next spring and used for 

 propagating these resistant individuals." In a recent communication to me Dr Stock- 

 berger informs me that the proposed plan of testing these varieties was never carried out; 

 further,, he now states that in his opinion too much importance as evidence of immunity 

 must not be attached to the phenomenon recorded by him in "1907. 



2 The second record of S. Humuli on Humulus in Japan given in my '' Erysiphaceae 

 of Japan," II. Amial. Mycolog., m, p. 252 (8), is an error of transUteration, the host- 

 species being Impatiens Bahamina !>., Tokyo Botanic Garden, Coll. Kusano. 



