42 



(ascus) develops on fruit that has wintered over on the ground. Perithecia de 

 velop in stromatic cushions. Asci are oblong-clavate, 55/70 x 9", and 8-spored 

 Ascospores are curved, 12/22 x 4". infection of the fruit in many cases at least 

 arises from the conidia on canker areas. > "■ l^^" ' 



Black-Rot Canker (Sphaeropsis malorum). The Black-rot of apples is 

 more or less common and well-known, but it is not so well-known that this same 

 fungus produces cankers on the limbs of apple trees. The bark of affected areas 

 first becomes discolored, then it becomes roughened and thickened. Sometimes 

 the dead bark comes off, but usually the Umb is girdled completely and killed. 

 Infection takes place in spring through wounds, and the affected areas extend 

 slowly as the fungus grows outward in all directions till midsummer, when a definite 

 boundary forms between the dead and hving bark. It is beUeved that sun-scald 

 renders some varieties very susceptible to this canker (Figs. 4 and 5). Pycnidia 

 appear through bark which produce oblong-eUiptical brouTi spores, 22/32 x 10 14". 



Micro3coDic structure of the Black Rot fungus a. Section from a wintered SDecimen of crab apple showing, o. the 

 dark-colored mycelial threads among the cells of the fruit, 6; d.a thick-walled spore case (pycnidium), which has 

 pushed through the epidermis, c. and is giving off dark-colored spores e, through the protruding mouth ; 6, mature spores 

 germinating in water and giving rise to germ tubes, /. All highly magnified, (.^ter Longyear.) 



Twig-Blight Canker (Bacillus amylovorus). — Infection occurs usually at 

 blossoming, and the disease extends down the twigs and branches, causing first 

 a wilting, afterwards a blackening. Cankers are often formed at wounds made 

 by pruning. 



Blister Canker (Nummularia discreta) occurs in the Mississippi Valley and 

 perhaps elsewhere. Its appearance on the large hmbs is quite characteristic. 

 The canker areas are mottled, sunken, cracked and charred. The blackened areas 

 are dotted over with circular stromata. The mycehum penetrates both the bark 

 and the wood. The stromata and fruit bodies begin to appear in late summer. 



