-ORCHARD SPRAYING EXPERIMENTS II. / 



the changes resulting from the chemical combination of the two 

 substances than to the simple addition of the arsenate. 



In connection wdth the above experiment \\"allace advances 

 a theory accounting for the fungicidal action of the arsenate. 

 He suggests that such action may be due largety to physical 

 rather than chemical causes. Assuming that the leaf has been 

 well sprayed and is thoroughly coated with a thin layer of in- 

 soluble arsenate, it would be possible for fungus spores to 

 germinate in water on the leaf above this layer and still pre- 

 vent the penetration of the germ tubes. This protective action 

 might be reduced with the growth of the leaf, when the newer 

 surfaces would be unprotected and the chances of infection 

 greater. 



On this hypothesis reliance must be placed on a spray mate- 

 rial actually preventing spore germination, rather than on one 

 merely presenting obstacles to germ tube penetration. 



Other facts ehcited by the work in cpiestion were the changes 

 wrought in lime-sulphur used alone or with lead arsenate, in 

 the presence of carbonic acid. This chemical compound is 

 injected into the problem when the carbonic acid gas sprayer is 

 used. It was found that under such conditions the soluble sul- 

 phur was precipitated before its application, but the resulting 

 products were no less effective than the solutions applied by 

 means, other than the gas sprayer, after changes produced in 

 them by exposure to the air had occurred. This appeared to. 

 be true irrespective of the use of the gas sprayer with lime- 

 sulphur alone, or combined with lead arsenate. It was ob- 

 served, however, that with the use of the gas sprayer in a]iply- 

 ing the two materials combined, arose the tendency to produce 

 foliage injury, especially to the susceptible leaves of the peach. 



For the past two years the work at Highmoor in spraying 

 experiments has of, necessity been confined to trees of fairly 

 uniform conchtion, bearing a reasonable cro]). Tlicse condi- 

 tions have limited the work to the Ben Davis variety. .Although 

 other standard kinds are growing in the Highmoor orchards 

 they were either scattered in a way poorly adapted to experi- 

 mental work, or their poor condition rendered them unsuitable 

 material. It was therefore considered very dcsiral)lc that trials 

 along lines similar to those being conducted in the Station 

 orchard should also be made on other varieties in other or- 



