300 On Forma of the Hop resiafant to Mildew 



autumn of 1919 a trace of mildew on their leaves; the other two seedlings 

 were not able to be tested as to their susceptibility. 



The seedlings V9J, V 92, V 93 all completely resisted infection in 

 the greenhouse during the seasons 1916 and 1917; the seedlings were 

 planted out in the hop-garden in the winter of 1917-18, and the next 

 season (Sept. 1918) each plant showed a trace of mildew on its leaves. 

 Cuttings taken from each plant in the hop-garden in 1918-19 proved 

 completely resistant in the greenhouse during the season of 1919, while 

 one of the seedlings (V 93) in the hop-garden again showed a trace of 

 mildew in Sept. 1919. 



The seedling Z 22 was susceptible to the second degree in the hop- 

 garden in both 1918 and 1919, while cuttings taken in 1918-19 proved 

 to be immune in the greenhouse in 1919. 



The seedling Z 25 is persistently immune in the greenhouse, and is 

 usually immune in the hop-garden, although the two seedlings (Z 24, 

 Z 26) on either side of it become severely mildewed each season. In 

 June 1918 inoculations with conidia were made on the young leaves of 

 Z24, Z25, Z26, and infection resulted only on Z 24 and Z 26^. The 

 circumstances attending the infection of Z 25 in the hop-garden in 1919 

 are discussed below (p. 306). 



In 1918 one bine of OD 19 — a strong lateral shoot arising about 5 ft. 

 from the ground — twined round the bines of OD 18 and produced 

 healthy hops intermingled with the excessively mildewed ones of 

 OD 18. In 1919 a minute trace of mildew occurred in September on 

 OD 19. 



The seedlings OE 38 and OR 39 are immune in the greenhouse^; in 

 the hop-garden susceptibility is shown, OR 38 being sometimes infected 

 severely^. 



The seedling HH 44 has shown a trace of mould in the hop-garden 

 each season in 1917, 1918 and 1919, yet cuttings taken both in 1917-18 

 and 1918-19 proved to be immune when grown in the greenhouse the 

 next season. 



The evidence appears conclusive that (1) seedlings which are immune 

 when grown in the greenhouse may show susceptibility when grown in 

 the open; that (2) the susceptibility shown by such seedlings in the open 

 is as a rule slight; and that (3) cuttings taken fi-om such seedlings after 



^ See (3), p. 254 for further details. 

 2 See, however, (2), pp. 84, 8(>. 



' In 1918 two plants of OR 38, iu difforcnt \n\rts of tlH> liop-uanlcn, were susceptible 

 to the extent of S- and S^ (see Table 1). 



