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 inaequalis) and pear 

 (Venturia pyrina), downy mildew (Plasmopara viticola) 

 and black-rot (Guignardia bidwellii) of grape, early blight 

 (Alternaria solani) and late blight (Phytophthora in- 

 festans) of potato, brown-rot (Sclerotinia fructigena) and 

 leaf-curl (Exoascus deformans) 

 of the peach and the shot- 

 hole disease (Cylindrosporinm 

 padi) of the cherry. The ap- 

 ple-scab as it appears on leaves 

 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. 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). 



