EXPERIMENT STATION BULLETINS. 



355 



coming much wrinkled and shrunlccn. Just under the skin of such apples small 

 black dots or pustules may be seen even with the unaided eye. They are the 

 spore producing bodies of the fungus. A section through one of these bodies 

 (Fig. 8) shows it to be hollow with a projecting mouth from which the oval or 

 egg-shaped spores escape. This fungus, however, is not confined to the fruit 

 alone. In 1899 Paddock at Geneva, N. Y., demonstrated that it is the cause of 

 a canker on the branches of the trees.' Previous to that time these injuries 

 were ascribed to sun scald but it is now definitely known that the black 

 rot fungus is the true cause. Its first effect on the branches, which are attacked 

 in the spring, is shown by a discolored area of the bark. These areas gradually 

 enlarge killing the inner bark and later showing a definite boundary at the 

 margin of the dead portion. Spore producing bodies form in these diseased areas. 

 In time the bark becomes loose and rough or sloughs off exposing the bare wood. 

 In severe cases limbs may be completely girdled when all that portion beyond 

 the canker dies. The fungus apparently gains an entrance through cracks or 

 wounds and seems to prefer large branches, although it also occurs on twigs. 

 For this reason and because of its obscurity this disease is capable of doing much 

 injury to the trees before its presence is detected. The leaves are also affected 

 by the black-rot fungus which causes the appearance of brownish spots and may 

 lead to the premature shedding of the foliage. 



Fig. 8. — Microscopic structure of the black rot fungtis. A, section from a wintered specimen of crab 

 apple, showing, a, the dark colored mycelial threads among the cells of the fruit, b; d, a thick 

 walled spore case (pycnidium), which lias pushed through the epidermis, c, and is giving off dark 

 colored spores, e, through the protruding mouth; B, mature spores germinating in water and giving 

 rise to germ tubes, f. All highly magnified. (Original.) 



TREATMENT. 



Collecting and burning or burying the diseased fruit late in autumn will 

 greatly diminish the number of spores of this fungus and consequently the 

 liability of infection as the spores are given off from these affected apples. The 

 spraying methods used for scab will also serve to check this disease. Cankers 

 on large limbs should be scraped, painted with the copper sulphate solution A 

 and coated with tar or paint. Smaller branches should be cut off below the 

 cankered area and burned. 



TWIG-BLICnT, FIRE-BLIGHT. 



(Bacillus amylovorus (Burr.) DeToni.) (See Pear.) 



CANKER. 



Canker is a general term applied to diseased areas in the bark and wood of the 

 stem and branches of trees and causing more or less deepseated killing of the 

 tissues. Such areas usually appear roughened by the scaling or cracking of 



