94 TIh Rr St. stance to Fungicides 



which may be briefly described as a "powdery" patch. This stage was 

 chosen both in order to make the experiments as strictly comparable 

 as possible and on the assumption that in such a stage of development — 

 where the fungus has a well-developed mycelium furnished with very 

 numerous haustoria — the resistance to the fungicidal properties of the 

 chemical would be at its highest. 



During experiments this season (1916), however, facts have been 

 observed which show that the mildew when in an earlier stage of develop- 

 ment on the host-plant is more difficult to kill. Our suspicions of this 

 fact were first aroused by the occurrence of minute tufts of conidiophores 

 at fresh places on the leaf two or three days after the spraying had 

 taken place. These fresh little patches of mildew could not have been 

 due to re-infections subsequent to the spraying, since a conidium takes 

 about five days from infection to develop mycelium and conidiophores. 

 The point was then studied more closely by means of hop-leaves 

 inoculated with conidia of the hop-mildew. It was found that very 

 soon after being inoculated the young hop-leaf, when rapidly growing, 

 shows small raised "blisters" or "humps" at the places where it has 

 been infected. At first the raised blister is green all over, and shows 

 no trace to the naked eye of a white mildew, although usually, under 

 a lens, a few delicate hyphae may be seen radiating from the centre of 

 the "hump." After a day or two, each "hump" shows a whitish 

 growth beginning to spread over its surface, due to the formation of 

 branched mycelial hyphae, and soon afterwards the first conidiophores 

 appear. An exactly similar appearance may be found on a young 

 Rose-leaf when attacked by the Rose-mildew {S. pannosa). 



Spraying experiments on hop-leaves bearing numerous "humps" 

 showed that the mildew in this very early stage of development has 

 a greater power of resistance to sulphide solutions. Thus in experiments 

 it was found that a certain stock solution of ammonium sulphide when 

 diluted 1 to 100 with water was completely fungicidal to the hop-mildew 

 when in its "powdery" conidial stage; this solution did not, however, 

 prevent the youngest stage of the mildew, described above, from 

 developing and forming conidiophores. At double the concentration 

 however, the sulphide solution in question became fungicidal for this 

 earliest stage of development. 



The details of two experiments may be given here. In Exper. 1 the fungicide 

 used was a certain concentrated solution of ammonium sulphide diluted 1 to 100 with 

 water (and containing I per cent, soft soap). Three seedling hop-plants were used, 

 and the leaves on each which wpr(> sprayed bore the hop-mildew in the stages of 

 development noted below: 



