o:j 



ON THE RESISTANCE TO FUNGICIDES SHOWN 

 BY THE HOP-MILDEW {SPHAEROTHECA HU- 

 MULI (D.C.) BURR.) IN DIFFERENT STAGES 

 OF DEVELOPMENT. 



By E. S. salmon, 

 Mycologist to Ike South- Eastern Agricultural College, Wye, Kent. 



(With Plate XV.) 



In the testing of the resistance of any fungus to a chemical substance 

 both the environmental conditions and the stage of growth of the 

 fungus are, obviously, important factors. 



We know, e.g. from Clark's ^ experiments, that the spores of a fungus 

 when supphed with nourishment in the form of a decoction of sugar- 

 beet show a greatly increased power of resistance to the toxic effects 

 of copper sulphate. With (Edocephalwn alhidum the lethal strength 

 for spores in water is -0013 %, whereas with spores supphed with 

 nourishment it is 0-1 %, — about 80 times as strong. 



In a recently pubhshed paper^ deahng wdth the fungicidal action 

 of certain chemicals, principally sulphides, attention was drawn to the 

 fact that data as to the behaviour of spores placed in a fungicide have 

 little practical value as indicating the strength at which the same 

 chemical will be fungicidal when used against the growing fungus on 

 the plant. It was pointed out that while Foreman^ found that a 

 0-16 % solution of caustic soda prevents the germination of spores of 

 Botrytis cinerea and Sphaerotheca mors-uvae, experiments have shown 

 that a 0-3 % solution does not kill well-developed patches of the conidial 

 stage of S. Humidi. 



In all our spraying experiments with '"powdery mildews"' {Ery- 

 siphaceae) carried out during 1914 and 1915 we were careful to select 

 for spraying well-developed patches of mildew showing dense clusters 

 of conidiophores bearing chains of conidia — a stage of development 



1 Bot. Gazette, Vol. xxxiii (1902). 



- J. Vargas Eyre and E. S. Salmon, "The FiniLncidal Properties of Certain Spray-fluids, 

 Journ. Agric. Science, Vol. vii, 473 (1916). 



* Foreman, F. \\., "The Fungicidal Propertif>s of Liver of Ku.phur.'" J num. Agric 

 Science, Vol. m, 401 (1910). 



