456 Forms of the Hop Resistant to Mildew 



It is a matter, therefore, of economic as well as of scientific interest, 

 to find that certain forms of the hop-plant, Humulus Lwpulus L., 

 are markedly resistant to S. Humuli. 



It may be pointed out here that while the morphological species 

 S. Humuli is found attacking a very large number of host-species 

 belonging to many diverse genera (5, 6), it has been proved by experiments 

 that a speciaUsation of parasitism has taken place and that the form 

 on Humulus constitutes a "biologic form" with power of infecting only, 

 so far as is known (4), species of the genus Humulus. 



The phenomenon of resistance to hop-mildew was first observed during 

 the summer of 1914. For the purpose of carrying out trials with certain 

 fungicides, several hundreds of one- and two-year old seedhng hops 

 were grown in pots in a glass-house under conditions which ensured 

 their being virulently attacked by the mildew (lO). The conditions 

 provided, viz. alternations of a dry and damp atmosphere and the 

 occurrence of constant draughts, enabled the mildew to attack con- 

 tinuously the leaves and stems of the seedUng hops to the maximum 

 extent, and, generally speaking, it was only necessary to place a 

 healthy hop-plant among the infected plants in this glass-house to find 

 it at the end of a week or fortnight smothered with mildew. 



Under what were clearly ideal conditions for the growth and dis- 

 semination of the hop-mildew, two individual seedlings proved immune 

 throughout the growing season. On these plants alone among those 

 in the glass-house there was not a spot of mildew on leaf or stem. The 

 two plants were moved from time to time, and placed among batches 

 of the most virulently infected plants, from which conidia must have 

 been blown repeatedly on to their leaves and stems. Direct inoculations 

 were also made either by placing conidia on the damped surface of a 

 leaf, or by first spraying the plant with water (using an "atomiser") 

 and shaking over it affected plants bearing numerous "powdery" 

 conidial patches, so that myriads of conidia fell in a cloud on to the 

 inoculated plant. The two plants remained persistently immune. 

 These plants were seedlings raised from seed of the "wild hop" obtained 

 from Italy. Other seedUngs, to the number of about seventy, of the 

 same origin proved very susceptible to the mildew. These susceptible 

 seedlings were of the same age as the immune plants, and were raised 

 in the glass-house under the same conditions as regards soil and all 

 other cultural matters. 



During 1916 the same phenomenon was observed in the case of seven 

 other seedlings of the same origin. They were of the same age as the 



