78 DISEASES OF ECONOMIC PLANTS 



which, during the first year, consist of small spots 2-10 

 mm., tan colored, each bearing several pycnidia. The 

 margins of the spots are sharply limited, and are usually 

 bordered with a narrow red line. In older cankers the 

 diseased bark becomes cracked, furrowed, and much 

 roughened. 



On the leaf this fungus causes small, irregular, light 

 brown, yellowish, or whitish spots, usually 1-2 mm. in 

 diameter, each of which may bear one or several pycnidia. 



The Ben Davis, Missouri Pippin, Limbertwig, North- 

 western Greening, Smith Cider, and Maiden Blush are 

 highly susceptible, while Winesap, Jonathan, and York 

 Imperial are almost immune. 



Ordinarily four applications of Bordeaux mixture suffice 

 to control the blotch. The first 'spraying should be made 

 three or four weeks after the petals have fallen ; the sec- 

 ond about four weeks later ; the third and fourth at inter- 

 vals of three weeks thereafter. Careful pruning to re- 

 move affected twigs is beneficial. 



Scurf {Phyllostida prunicola (Opiz) Sacc). — ^^This af- 

 fection of the bark of twigs causes it to shrivel. The dis- 

 eased part is thickly studded with the very minute pycnidia. 



Affected twigs should be cut out and burned. 



Rust [Gymnosporangium macropus Link). — The rust 

 of the apple may usually be readily recognized by its 

 characteristic yellowish orange spot on the leaf, or more 

 rarely upon the fruit or twig. This spot, yellowish green 

 at first, gradually becomes darker, approaching orange. 

 Upon close examination, the upper surface of one of 

 these discolored spots is seen to contain very numerous 

 pustules, at first honey yellow, finally black, smaller 



