VEGETABLE PARASITES AND WEEDS 179 



the disease and be so thorough as to cover all susceptible 

 parts, else it cannot be fully effective. Such thorough 

 spraying requires a spray pump of high pressure provided 

 with a fine nozzle. 



Some of the common diseases preventable by proper 

 spraying are : scab of apple {Venturia maequalis) and pear 

 (Venturia pyrina), downy mildew (Plasmopara mticola) 

 and black-rot {Guignardia hidwellii) of grape, early blight 

 (Alternaria solani) and late blight (PhytopMhora in- 

 festans) of potato, brown-rot {Sclerotinia frudlgena) and 

 leaf-curl (Exoascus deformans) 

 of the peach and the shot- 

 hole disease (Cylindrospormm 



padi) of the cherry. The ap- ^^^ ^ ff% g^ ^' '^-'i 

 pie-scab as it appears on leaves M^.;,;::i:*'^»' ^mil 

 is shown in Fig. 86; as it ap- 

 pears on fruit in Fig. 81. A 

 section through a scabby spot, 

 magnified, is represented in 

 Fig. 82. 



„-- _ ... _,, Fig. 81. — Scab on the apple. 



329. Fungicides. —The most 

 important fungicides in common use contain some form of 

 copper or sulfur, although there are numerous other com- 

 pounds having fungicidal properties. The practical value 

 of a fungicide depends upon its ability to destroy the 

 fungus without serious harm to the host, coupled with 

 convenience of preparation or use, adhesiveness and cheap- 

 ness. The copper compound best meeting these standards 

 is bordeaux mixture; and lime-sulfur is the best sulfur 

 compound. In general, the bordeaux mixture has greater 

 fungicidal value but the lime-sulfur has replaced it for 

 certain uses, partly because it has also value for control- 

 ling certain insects (295). 



