:]0'2 On Forms of f/te Hoj) resistant to Mlldeiv 



The seedling Z 23, of wliich there were two cuttings in pots in the 

 greenhouse, showed the nearest approach to immunity of the "semi- 

 immune"' groiij). Tlie only visible sign of infection on the two plants 

 after practically continuous inoculation by conidia for many weeks was 

 the production of very small raised "" blisters" or "humps," over the sur- 

 face of which a very weak and evanescent growth of mycelium (never 

 abundant enough to form a visible white layer) with a few conidio- 

 phores was .produced. Thus no "powdery" patches were ever produced. 



The seedling OC 6 (two plants) became more evidently affected by 

 the attacks of the conidia than Z 23 was, and its leaves reacted in a 

 very characteristic manner. The infection spots were larger, but again 

 the mildew never produced enough mycelium to be white and con- 

 spicuous, and conidiophores were never produced abundantly enough 

 to cause a "powdery" patch. On the death of the mildew, which soon 

 took place, the leaves showed a distinct injury (confined to the places 

 where the mildew^ had existed) in the form of conspicuous yellowish- 

 brown blotches (closely resembling those found on the leaves of certain 

 plants attacked by Eriophyes (Leaf "Blister-mites")). 



With Z 15 (two plants) inoculation produced on the leaves a dis- 

 tinctly white and more or less "powdery" conidial patch, w^hich, how- 

 ever, soon died away, exposing to view a conspicuous brown spot, 

 composed of a group of aflected epidermal cells. 



The seedlings OA 33 (two plants) and OD 17 (two plants) showed 

 also very similar infection-phenomena, but with less injury caused to 

 the epidermal cells. These two seedlings may perhaps best be put in 

 the susceptible class, although they show a tendency towards semi- 

 immunity. 



With the seedling BB 5 inoculation was followed by the production 

 of small patches of mildew, white and powdery, but these patches 

 remained small and isolated, not increasing in size and becoming con- 

 fluent as on the leaves of plants showing full susceptibility. 



The behaviour of the above seedlings in the hop-garden during past 

 seasons is shown in the table on p. 303. 



My field-notes for the seedling OD 17 in the hop-garden were as 

 follows: "1917. Showed considerable resistance to mould, producing 

 a good crop of healthy hops, while OD 16 and OD 18 (two seedlings of 

 the same parentage and age), growing on either side and so close that 

 lateral slioots iiitcrniixod, were so infected with mould that they pro- 

 duced no liealthy hops. M»1(S. Mere trace of mould on the hops; OD 17 is 

 ()l)\'i()uslv ol (luitc dilferent 'constitution " (roin 01) H) and OD 18, wiiose 



