272 PLANT DISEASES 



the leaves, causing the leaves to crimp. Occurs on all varieties, 

 especially Roger's hybrids. Not so abundant as formerly. 



Control. It is said that an early spraying before the buds 

 open with sulfate of iron, 100 pounds to 50 gallons of water, is 

 very important. Later sprayings for black rot will also be 

 effective in preventing spread. 



BLACK-ROT (Guignardia bidwellii). The most serious disease of 

 grapes east of the Rocky Mountains, especially southward. At- 

 tacks all green parts. Produces a brown circular spot on leaves, 

 a black, elongated, sunken pit on petioles, canes, etc., and on the 

 berry a brown rot with shriveling and wrinkling ; finally the 

 berry becomes black and hard. 



Control. This disease may be controlled by timely applica- 

 tions of bordeaux mixture, 4-4-50. It is of great importance that 

 spraying be done before rain storms, as the berry enlarges so 

 rapidly. Spray (a) when the third or fourth leaf has unfolded ; 

 (6) as soon as the blossoms have, fallen; (c) when the berries are 

 the size of a pea ; (d) in about two weeks. In a wet season make 

 two more applications. After July 20 make the bordeaux 4-2-50, 

 or use ammoniacal copper carbonate. In case of dense foliage 

 all applications except the first two should be made by hand. 

 Attach trailers to the sprayer, and have two men following to 

 apply the spray directly to the clusters. About ten acres can be 

 sprayed in a day, and the total cost of labor and material should 

 not exceed 75 cents per acre for each application. 



CALIFORNIA VINE-DISEASE. An obscure disease, which destroyed 

 thousands of acres of vines in California. Cause not known, 

 and at present practically unknown and of no importance eco- 

 nomically. 



CROWN-GALL or BLACK KNOT (Bacterium tumefaciens). A tu- 

 merous, gnarled outgrowth on roots and stems, especially on Euro- 

 pean varieties. Frost injury often forms an infection court for 

 the bacteria. See p. 276. 



Control. Grub out and burn infected vines. 



DOWNY MILDEW or LEAF-BLIGHT (Plasmopara viticola). Appears 

 in white frost like patches on under side of leaf, the upper side of the 

 leaf showing a yellowish discoloration ; gradually spreads to all 

 parts of the leaf causing it to dry up. Attacks the berry, which 



