THE FUNGICIDAL PROPERTIES OF CERTAIN 

 SPRAY-FLUIDS. III. 



By E. HORTON and E. S. SALMON, 

 Research DepartiTient, S.E. Agric. College, Wye, Kent. 



As an aid to the elucidation of the problem of the exact fungicidal value 

 of a mixture of lime-suljjhur and arsenate of lead — a matter of great 

 importance to the practical fruit-grower — spraying experiments were 

 carried out during 1921 with certain spray-fluids containing arsenic or 

 lime-sulphur and its constituents. 



Method. In order to ascertain within narrow limits the fimgicidal 

 value of any solution, it is obviously necessary to maintain as fixed a 

 biological standard as possible. To ensure this, the fungus used in com- 

 parative experiments should be in the same stage of development and, 

 if it is a parasite, the host-plant used should also be " standardised " as far 

 as possible — since it has been shown(2) that the same stage of a fungus 

 may be more easily killed when on the older leaves of a plant than on 

 the younger. 



In all the experiments described below, the fungus used was Sphaero- 

 theca Huniuli (DC.) Burr., and the stage selected for spraying was the 

 young, "powdery," conidial stage produced on young leaves, at the 

 3rd to 9th node, of rooted cuttings of hop-plants {Humulus Lupulus, 

 Linn.) grown in an unheated greenhouse. To escape as far as possible 

 variation on the side of the host-plant, with possible consequent effects 

 on the vigour of the parasitic fungus, all the plants used were clone- 

 plants, i.e., plants raised vegetatively by cuttings taken from one indi- 

 vidual hop-plant. The general methods of sprajdng and of the examina- 

 tion of the sprayed leaves, etc. have been described in previous articles 

 (1, 2). In order to secure complete wetting of the fungus, calcium casein- 

 ate (1 per cent.) was added to the solutions used. 



Where care is taken to obtain in this way strictly similar biological 

 conditions of parasitic fungus and host-plant, it becomes possible, as 

 shown below, to determine within narrow limits the fungicidal value of 

 a solution. In all the experiments described below "powdery" conidial 



