REMEDIAL MEASURES. i6i 



Obviously it is of no use to apply such topical 

 remedies to a parasite while it is spending the 

 greater part of its life inside the tissues of the 

 host. Further, questions of expense of the 

 materials employed and of the labour of apply- 

 ing them help to limit the adoption of such 

 measures. 



Among the various kinds of powders employed, 

 finely divided sulphur, or a mixture of sulphur and 

 lime, have been used with success in some cases 

 e.g. against Hop mildew and other epiphytic 

 Erysipheae, and against red spider, aphides, etc., 

 the gaseous sulphur dioxide evolved being the 

 efficacious agent. In other cases pyrethrum or 

 tobacco powder, wood ashes, etc., have been em- 

 ployed against insects. Such powders are applied 

 by hand or by means of bellows, and are very 

 easily manipulated in most cases, though, like all 

 such applications, the dangers of concentration at 

 particular spots owing to uneven distribution, or 

 of dilution and washing off by rain, have to be 

 incurred. 



Far more numerous are the various liquids 

 which have been employed for washing, spraying, 

 or steeping the affected parts of diseased plants. 

 Water alone, or aqueous decoctions or emulsions 

 of various kinds e.g.., quassia, tobacco, soap, or 

 aloes, have been widely employed against insects 

 such as green fly, red spider, etc. In greenhouses, 

 where the leaves can be washed by hand or 

 thoroughly syringed, and the concentration and 

 time of action thoroughly controlled, such liquids 



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