286 Fungicidal Properties of Certain Spray-fluids 



The explanation of the fact that the powdery patches on the older 

 leaves were easier to kill than those on the younger leaves may be that 

 the former were all older, and that older patches are more vulnerable, 

 or possibly that the vigour of the leaf directly affects the vigour and 

 power of resistance of the powdery mildew. 



It is clear from the cases mentioned above — and others will be found 

 described below — that the mildew shows in its different stages of 

 development very different powers of resistance to the same solution, 

 being very hard to kill in the stages just following infection, and com- 

 paratively easy to kill when in its powdery conidial stage on the "older' 

 leaves. 



By the selection of only those patches of mildew in the same stage 

 of development and on young, vigorously-growing leaves it is possible 

 to keep a sufficiently fixed standard by which to measure satisfactorily 

 the fungicidal value of different solutions. Where, however, two solu- 

 tions have to be compared under as strictly similar biological conditions 

 as possible, it is necessary to use mildew-patches in the same stage of 

 development on leaves at the same node. It is much to be hoped that 

 in future work dealing with the "powdery mildews" and their fungicides 

 exact indication will be given both of the stage of development of the 

 mildew-jmtches used and of the position of the leaves bearing the 

 mildew. 



Materials used. 



Distilled water has been used in the preparation of the various 

 solutions, and with the exception of a few instances which are referred 

 to, soft soap was added to ensure the proper wetting of the mildew. 



Soaf. In nearly all cases where soap has been used alone or in 

 conjunction with other substances, the kind known commercially as 

 " Chiswick Soft Soap " has been employed. The sample was a moderately 

 firm type of soft soap practically neutral in character. The total alkali 

 was found to be equal to 12-8 % KOH (or 9-1 % NaOH). 



In the other cases where soap was used it was that known commer- 

 cially as "Cook's Soap," a rather fluid type of soft soap which exhibited 

 a neutral or slightly acid character. The total alkali was found to be 

 equal to 11-6 % KOH (or 8-36 % NaOH). 



Saponin. The material used was the ordinary white powder sold 

 commercially. 



Ammonium hydrosulphide. This was prepared by saturating a 4 % 

 solution of ammonia in water with sulphuretted hydrogen gas. The 



