154 Forms of the Hop resistant to Mildew 



mildew — S^. In 1920 there was no mildew on the plant on August 31; 

 in October a trace of mildew was found on one hop only. 



Ref. no. OE 14. A trace of milde-\v occurred on the hops in 1920. 



Ref. no. OR 38. This seedUng appears to be the most susceptible in 

 the open of all the seedlings which show immunity in the greenhouse. 

 As a seedling plant in 1914 it showed persistent immunity in the green- 

 house; it was planted out in the hop-garden in 1915. In 1916 its main 

 crop of "hops" was free from mildew, but a late, young stem, which had 

 run up the old stems to nearly the top, bore several leaves and one 

 " hop " with patches of mildew on them. In 1917 there was a fair amount 

 of mildew on the hops (both in the conidial and in the perithecial stages), 

 chiefly on the peduncles but also on the bracts and bracteoles. In 1918, 

 on September 7, there was a fair amount of mildew on the young leaves, 

 and in October, when the whole plant was minutely examined, the hops 

 generally were found to be severely attacked by mildew, a large per- 

 centage being undeveloped or deformed through its attacks. In 1919 

 there was only a trace of mildew on the hops. In 1920 there was a minute 

 trace of mildew on the leaves and an attack of medium intensity on the 

 hops. In the greenhouse OR 38 has been tested on a considerable scale 

 by means of "cuttings" taken in the four winters 1916-1919 from the 

 parent-plant in the hop-garden. In all, 42 "clone-plants" have thus 

 been exposed to constant inoculation in the greenhouse, and with the 

 exceptions noted below, all have proved persistently immune. In 1917 

 a phenomenon occurred which had not been observed before and which 

 has not been observed since. On two cuttings of OR 38 in the greenhouse, 

 a minute patch of mildew appeared, on one leaf only of each plant — -in 

 one case following artificial inoculation, in the other case, without ; these 

 mildew-patches appeared in May and soon died away without the mildew 

 spreading on the plant. The two plants concerned, together with three 

 other cuttings of OR 38 stood in the greenhouse for the rest of the season 

 of 1917 and in spite of constant inoculation with conidia, all the plants 

 remained persistently immune. It was clear that the phenomenon con- 

 sisted of the sudden appearance of strictly local and temporary sus- 

 ceptibility (induced by, at present, unknown causes) and was not a 

 general breaking down of the immunity of the plant^. 



Three cuttings of OR 38 which had remained persistently immune 

 in the greenhouse in 1917, were planted out in the hop-garden in the 

 winter of 1917-18. These three clone-plants of OR 38 have behaved 



^ No other "immune" seedling has ever shown in the greenhouse any signs of breaking 

 down. 



