304 On Forms of the Hop resistant to Mildew 



In my last communication ((3), p. 253) mention was made of a group 

 of seedlings, five in number, which for several seasons had remained 

 immune both in the greenhouse and hop-garden. During 1919, while 

 clone-plants of all these five seedlings remained persistently immune in 

 the greenhouse, two of the seedlings, viz. OD 1 9 and Z 25 showed in the 

 open a very slight susceptibility. Of the remaining three seedlings, two 

 (Z 2, OA 49) still proved immune in the hop-garden, while in the case of 

 the other, Z 42, no young growth was available in the autumn to test its 

 susceptibility. 



The circumstances under which those seedlings which are immune 

 in the greenhouse show in the open, in some seasons, an approach towards 

 susceptibility are worthy of study. The susceptibility shown has been 

 noticed in past years only late in the season, in September or early 

 October, when the plant is commonly exposed to adverse conditions of 

 growth such as low temj^eratures at night, or cold rains or mists. In 

 order to try to obtain some evidence as to which among the various 

 cultural conditions are the determining ones with regard to the breaking 

 down of immunity to mildew when the plant is grown in the open, 

 cuttings of certain seedlings (all potted up in the same kind of soil) were 

 treated in three different ways. Those in Group I were placed in the green- 

 house in the autumn of 1918 and kept there throughout the season of 

 1919; those in Group II were kept in the open through the winter and 

 spring until June 2 (by which time the plants had produced shoots 2 to 

 3 ft. long) when they were placed in the greenhouse and exposed to 

 infection; in Group III the plants stood in the open with those of 

 Group II until June 2, when they were placed in the hop-garden (with 

 the pots standing in saucers so that no food could be absorbed from the 

 soil). The seedlings which were used may be divided into three classes: 

 a, those which remain persistently immune when grown in the greenhouse ; 

 6, those which are "semi-immune" in the greenhouse; c, those which are 

 decidedly susceptible (all S^ or S^) when grown in the hop-garden. The 

 number of clone-plants used is given in brackets. 



Grou]) 1. — Greenhouse throughout. 



Clanii a. OR 38 (8); OR 39 (7); Z 25 (3); V 91 (1); V 92 (1); V 93 (1); Z 42 (2); 



Z2 (2); Z14 (2); Z 17 (1); Z31 (1); OD 19 (2); Z 22 (2); HH 44 (2); OA49 (2); 



OB 34 (10). "(Joldcn Hop" (from Messrs Hidr ((i), from Messrs Bmiyard (2)). 

 Class h. Z 1.1 (2); 0(' (i (2); OA .33 (2); Z 23 (2); OD 17 (2). 

 Glass c. OA36(l); Z 39 (1); OA .35 (1); OA 26 (1); OA 34 (1); II 31 (1); 013 48 (1); 



OD 16(1); Z24 (1); Z2(5 (1). 



