E. S. Salmon 257 



"cut"' of each was taken in the winter of 1917-18, and grown in the 

 greenhouse during 1918. These two plants never became really infected 

 throughout the season. On inoculation infection stopped short, after 

 the formation of small blisters or "humps" on the leaves (which I 

 have already described (3) as being frequently the first sign of infection); 

 on the surface of these "humps" a meagre, scarcely white growth of 

 mycelium with conidiophores took place which soon died away, leaving 

 minilte, brown patches of dead, epidermal cells at the place where the 

 mycelium had been. In the hop-garden, Z 15 was recorded as having 

 mildew to the second degree in 1917 and 1918; while OC 6 was recorded 

 as having no mildew in either year. 



The seedling Z 23 (S), of which three "cuts" were tested in the 

 greenhouse during 1918, was also "semi-immune" in a very similar 

 manner. Inoculation of the leaves was followed by the formation of 

 yellowish, translucent "humps" on the leaf, on which a very weak 

 growth of mycelium took place, the mildew scarcely ever presenting a 

 white patch and the conidiophores soon dying away. In the hop-garden 

 Z 23 was recorded as (S^) in 1917 and (I) in 1918. 



The seedling OA33 (c?), of which two "cuts" were tested in the 

 greenhouse in 1918, was a little more susceptible, the patches of mildew 

 produced after inoculation being larger and white and "powdery" with 

 conidia ; unlike the growth of mildew found on really susceptible plants, 

 however, these patches did not remain white and enlarge, but died away, 

 leaving behind yellowish, more or less translucent spots where the 

 mycelium had been. In the hop-garden OA 33 was recorded as having 

 no mildew on it during 1917 and 1918. 



In all the cases recorded above, the immunity to mildew shown in 

 the greenhouse by the various seedlings of the wild hop from Italy has 

 been constant for the particular plant year after year; in no single case 

 has a seedling which was immune one season in the greenhouse shown 

 susceptibility in other seasons in the greenhouse, although it may 

 have done so in the hop-garden. The case has been very different with 

 . a seedling of American ancestry. This seedling was raised at Wye of 

 the parentage (Oregon Cluster x English male hop) x English male 

 hop^. In 1916 as a 2-year-old plant it proved throughout the season in 

 the greenhouse persistently immune, in spite of the most numerous 



^ The "Oregon Cluster" is a commercial variety cultivated in the United States. In 

 our opinion (see Journal of Botany, May 1915) it belongs to the species Hamulus americanus, 

 and not to H. Lupulus, from which the varieties cultivated in Europe derive their origin. 

 The seedling now mentioned must therefore be considered of hybi'id origin. 



