APPLE POWDERY MILDEW. 21 



reaction might be expected from the application of sulphur dusts in 

 the production of sulphur spotting. There was no opportunity for 

 investigating this point in the work at Wenatchee, but since it was 

 demonstrated that the sulphur spotting of the fruit is the effect of 

 hot sunshine on sulphur fer se and not the result of toxic action of 

 sulphids in solution, there appears to be no reason why the injury 

 should be mitigated by the use of sulphur dusts instead of liquid 

 sprays. Confirmatory evidence on this point is furnished by Mr. 

 F. E. De Sellem, formerly State horticultural inspector in the 

 Yakima Valley, Wash., who reported to the writer very severe fruit 

 spotting as following the application of sulphur dust in 1917. 

 (PL II, fig. 6.) The dust was a mixture of powdered lead arsenate 

 and superfine sulphur, such as ordinarily has been used in recent 

 dusting operations in the Eastern States. A power dusting machine 

 was used in making the applications, and the climatic conditions 

 under which the injury occurred were reported to be similar to those 

 which obtained in the Wenatchee Valley when the injury occurred 

 there. 



Eecent observations of Hundley (14) indicate that in the Yucaipa 

 Valley, Cal., slight sulphur spotting may occur when the hot sun- 

 shine is delayed for as long a time as 60 days after spraying. Hund- 

 ley found fruit injury resulting from sulphur applied in several of 

 the forms used in the experiments at Wenatchee, and he records a 

 marked difference in susceptibility to injury in orchards at different 

 altitudes. He also found those trees most susceptible to spray injury 

 which were low in vitality either because of drought or some other 

 condition. 



Apparently the application of sulphur in any form as a spray 

 material for apples is dangerous in a hot, dry climate during the 

 summer season, for in the production of fruit " spotting " or sulphur 

 sunburn high temperature and burning sunlight are the controlling 

 factors. 



SPRAY MATERIALS. 



In these experiments the lime-sulphur solution appeared to spread 

 better than any of the other materials tested except colloidal sulphur. 

 Lime-sulphur solution possesses great powers of penetration through 

 the felted mycelium and spore coating, and it appeared that much 

 of its beneficial action was due not only to its spreading qualities but 

 to its immediate caustic effect, which destroyed the mycelium and 

 conidia of the fungus. This conclusion is in agreement with the re- 

 sults noted by Eyre and Salmon (13) in their investigations of the 

 fungicidal properties of certain sprays when used against mildews. 

 They demonstrated that, against mildews, solutions of polysulphids 

 act fungicidally as such and not by virtue of the sulphur which 



