. Diseases of economic plants . Plant diseases. 84 DISEASES OF ECONOMIC PLANTS Upon twigs, cankers in the early stage appear as round or oval, slightly sunken, 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
. Diseases of economic plants . Plant diseases. 84 DISEASES OF ECONOMIC PLANTS Upon twigs, cankers in the early stage appear as round or oval, slightly sunken, 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-. FlQ. 31. —Black spot canker. After Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Stevens, Frank Lincoln, 1871-1934; Hall, John Galentine, 1870-. New York : Macmillan
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