:J06 On Forms of the Hop reHistant to Mildew 



infected very slightly, there being a tiny "powdery" patch on the 

 undersurface of two of its leaves. Since the eight clone-plants of OR 38 

 kept entirely in the greenhouse and the six clone-plants taken into the 

 greenhouse on June 13 (of the same age and potted up in the same soil) 

 all remained persistently immune in spite of repeated inoculations, while 

 one of the 11 clone-plants exposed in the hop-garden to natural infection 

 showed this susceptibility late in the autumn (after frosts), it appears 

 safe to draw the inference that the immunity of such seedlings as OR 38 

 is broken down, or partly broken down, by certain climatic conditions — 

 possibly low temperature. 



Certain facts, observed in the hop-garden during 1919, with regard 

 to the behaviour of Z 25 tend to support the above view. This seedling, 

 Z 25, was originally planted out in the hop-garden in 1914. Each season 

 from 1916 to 1918 it remained immune to mildew, although similar 

 seedlings (as regards origin and age) on either side of it became severely 

 mildewed (see above, p. 299). In 1918 young leaves of Z 25 (in the hop- 

 garden) were artificially inoculated in June, and proved completely 

 resistant. In 1919, for the first time in the history of Z 25, a few small 

 mildew-patches were observed on the plant in the hop-garden: this 

 occurred on Aug. 7. The mildew-spots were on the young leaves of lateral 

 shoots at about five feet from the ground. The mildew-affected leaves 

 were allowed to remain on the plant, but by September all the mildew- 

 patches had died away on these leaves, and no mildew could be found else- 

 where on this plant. In early October when the plant was examined care- 

 fully, together with some hundreds of seedlings of the same parentage — 

 most of which were severely affected with mildew — not a trace of mildew 

 could be found on the leaves or hops of Z 25 — notwithstanding the fact 

 that laterals of the adjacent plant Z 26 had twined round the bines of 

 Z 25 and produced hops covered with mildew among the immune hops 

 of Z 25. The slight susceptibility shown by Z 25 in August 1919 was 

 temporary and was caused, apparently, by some special conditions of 

 growth. It is most probable that these conditions were climatic. Just 

 previously to the susceptibility being shown, there was a ])ronounced 

 spell of cold, and wet or dull weather, abnormal for the time of year. 

 As soon as normal, warm and sunny weather prevailed, the patches of 

 mildew on Z 25 died away, without any further infections having taken 

 place, although on other seedlings of ditVeient "constitutions" the 

 mildew ])revailed and inr'i(MS(Ml up t<t tlic nii(l<ll(' of autumn'. 



' Tlie Monthly Woathci- Roporls of tho Metcorolof^ical OfHco for the nu)nths of .June 

 to September, 1910, were as follows: "June. Sunny and Warm at first, then Cool and 



