284 FuHCjlddal Propcrtioi of Certain Spray -fluids 



growth was so vigorous that the abundant conidiophores had produced 

 masses of ripe, free conidia. This stage is denoted by the term " powdery " 

 in the details of the experiments given below.' 



During the work in 1916 it became evident that the "powdery" 

 patches on the older leaves of the plant were more easily killed than 

 those on the younger leaves. In the experiments with the different 

 solutions which are described in detail below, several instances will 

 be found recorded where the powdery stage on a young leaf survived 

 the application of a solution which killed the powdery stage on an older 

 leaf, — sometimes even at the next node. In the later experiments in 1916, 

 and in all those in 1917 and 1918, information is given in the Tables 

 as to the relative age of the leaves on which the mildew occurred. In 

 the experiments recorded below, just over 900 leaves, each usually 

 bearing several "powdery" patches of mildew, have been sprayed and 

 kept under close observation in the greenhouse. 



Taking everything into consideration, the best standard to adopt 

 for comparing the relative fungicidal powers of different solutions 

 was found to be that based on the "death-point" of the young powdery 

 conidial stage of the mildew found on the young, vigorous leaves of 

 the hop-plants used in the experiments. It is by this standard that the 

 fungicidal values of the solutions used have been fixed. In the experi- 

 ments carried out in 1918 to compare the fungicidal action of two given 

 solutions as closely as possible, the method was adopted of selecting 

 mildew-patches at the same stage of development on opposite leaves 

 at the same node, — the mildew-patches on leaves at the nodes- above 

 and below serving as controls. 



Some Biological Factor.s determining the Fungicidal Action of 



THE Solution. 



In some of the experiments the behaviour of the solution towards 

 the earliest stages of development of the mildew, i.e., those immediately 

 following infection, has been noted. Very frequently, in the case of 

 young and rapidly growing hop-leaves, the first sign of infection is the 

 presence of a convex "blister" or "hump"; this is green at first, with 

 no apparent sign .of the mildew, then later (as observed under a pocket- 

 lens) fine, branched hyphae appear straggling over its surface, which 

 increase until the "hump" becomes covered over with the yoimg, 

 delicate vegetative mycehum. Conidiophores then quickly begin to 

 appear, and after a few days a small, "powdery" patch results. 



The early stages of development of the mildew antecedent to the 



