E. S. Salmon 149 



1914 onwards 480 seedlings have been kept under observation, first as 

 1- or 2-year-old seedling plants in the greenhouse, and secondly as older, 

 flowering plants planted out in the Experimental Hop-garden at Wye 

 College, where they have been cultivated (trained, "cut," etc.) and 

 manured according to the usual practice in a commercial hop-garden^. In 

 the greenhouse the plants were subjected to the severest test by being 

 inoculated naturally almost continuously with conidia from surrounding 

 heavily-infected plants, as well as being occasionally artificially inocu- 

 lated, with the result that, except for the seedlings noted below, they all 

 became severely infected with mildew. In the Hop-garden natural 

 inoculation was rehed upon (except in one experiment) to test the degree 

 of susceptibility of the mature plant. Under the conditions prevaiUng, 

 this method was found to be satisfactory. Mildew was present generally 

 in the hop-garden (at the time when observations were made on the 

 seedhngs) in each season from 1916 to 1920, and particularly severe 

 outbreaks of mildew occurred in 1916, 1919 and 1920. Owing to the 

 late-flowering habit of the wild hop, and consequent prolonged period 

 of growth, it was found that October was the best month for examination 

 as to the incidence of mildew. In the case of the $ plant, the production 

 at the end of August and during September of the female inflorescence 

 ("burr") and young developing hops provided the best possible infectible 

 material. With the most susceptible seedhngs the attacks were so severe 

 that season after season the greater proportion of the "hops" (cones) 

 were deformed by the mildew, or not infrequently the crop of "hops" 

 was entirely destroyed, the female inflorescences being permanently 

 arrested in development and changed into white, hypertrophied, knob- 

 like growths. With the o plant, the infectible material consisted for the 

 most part of the leaves of the axillary side-shoots which developed 

 during late-summer from the lower portion of the main stem ("bine"); 

 where vigorous side-shoots were produced, these provided excellent in- 

 fectible material, but in those cases where they were absent, as happened 

 with a few plants in certain seasons when they "ripened off" early, the 

 susceptibihty could not be ascertained. With very susceptible seedhngs, 

 branches of the male inflorescence sometimes became mildewed. 



In view of the impossibility of recording in detail the behaviour of 

 all the 480 seedlings, a selection has been made of 52 seedlings, whose 

 records are given in Table IV. The explanations of the signs used are as 



^ Owing to exigencies of space, the seedlings could not be planted all together, but 

 had to be placed, usually in large blocks, but sometimes singly, throughout the Experi- 

 mental Hop-garden of 2| acres. 



