258 Variation in Wild Hop 



then the question considered as to whether any correhition with mor- 

 phological characters exists — a point not hitherto dealt with. 



The method of testing the individual seedlings for susceptibility to 

 mildew was as follows. The seedlings when 1- or 2-years old, were ex- 

 posed to infection in the greenhouse by being constantly inoculated with 

 conidia under conditions very favourable to the growth and spread of 

 the mildew. During the following winter these seedlings were planted 

 out in the Experimental Hop-garden, where natural inoculation by the 

 mildew was relied upon — a method found to be quite satisfactory. Owing 

 to the late-flowering habit of the seedlings it was found that October 

 was the best month for examination as to the incidence of mildew in the 

 open. In the case of the $ plant, the production at the end of August 

 and during September of the female inflorescence and young developing 

 hops provided the best possible infectible material. With the (/ plant, . 

 the infectible material in the autumn consisted for the most part of the 

 young leaves of the axillary side shoots which developed from the lower 

 portion of the main stem. Mildew was prevalent generally in the Hop- 

 garden in the autumn of each season from 1916 to 1920, and particularly 

 severe outbreaks occurred in 1916, 1919 and 1920. From 1914 onwards 

 480 seedlings were kept under observation; first as seedlings in the 

 greenhouse and then in the Hop-garden where they grew on to mature 

 plants. 



From the data contained in the Table on p. 259, it will be seen that 

 different seedlings — exposed in the open to identical conditions of soil 

 and weather^ — showed very different reactions to the attacks of the 

 mildew. 



In the case of the most susceptible $ plants (indicated by the sign 

 *S'^ in the above Table) the greater proportion of the " hops " (cones) 

 were deformed by the mildew year after year, or not infrequently the 

 whole crop of " hops " was entirely destroyed, the female inflorescences 

 being attacked when quite young and their further development per- 

 manently arrested, the inflorescences becoming converted into white 

 "knob"-like growths (see PI. XXV, fig. 1). With the most susceptible cf 

 plants, the mildew sometimes invaded the branches of the inflorescence. 



On the other hand, complete or almost complete resistance to mildew 

 was shown in the open by certain individual seedlings, both cf and ? . 



Where, in the Hop-garden, such resistant seedlings grew side by side 

 with the very susceptible plants, the contrast, season after season, was 



^ The seedlings being all "trained" the same way ensured similarity of conditions as 

 regards illumination. 



