84 



DISEASES OF ECONOMIC PLANTS 



Upon twigs, cankers in the early stage appear as round or 

 oval, slightly sunlven, dark areas which increase but slowly 

 in diameter. As soon, however, as 

 the fungus penetrates into the cam- 

 bium and sapwood the canker en- 

 larges very rapidly, making its most 

 rapid development through the cam- 

 bium, and advancing more tardily 

 through the overlying bark. The 

 resulting spots are nearly circular, 

 quite black, and from 5 mm. to 12- 

 14 cm. in diameter. When old, the 

 bark becomes brittle and a definite 

 fissure separates the dead from the 

 healthy portions, indicating cessa- 

 tion of advance of the fungus. 

 Acervuli appear first in the oldest 

 parts of the canker, later toward 

 the periphery, rupturing the epider- 

 mis and exposing the spore masses 

 which are at first creamy white, but 

 later black. ' After the spores have 

 matured the diseased bark becomes 

 separated from the wood and even- 

 tually falls away, leaving a scar. 

 Branches, or even trees, may be 

 killed by girdling cankers. 



Upon stored fruit the disease 

 appears first as light brown, cir- 

 cular, rotten spots. These later turn very dark or black, 

 and become depressed, dry, and tough. Acervuli simi- 



FiG. 31. — Black spot canker, 

 After Lawrence. 



