282 FUNGOUS DISEASES OF PLANTS 



Infection does not seem to be possible through uninjured 

 outer bark, but when the latter is punctured or broken, the 

 fungus promptly penetrates the living tissues. It is inferred, 

 therefore, that infection occurs through wounds and possibly 

 through lenticels. 



During the summer there is developed through the lenticels 

 from a stromatic mycelium the imperfect stage of the fungus. 

 This latter produces small, rod-like but curved spores, character- 

 istic of the genus Cytospora of the imperfect fungi. These spores 

 are discharged in long, twisted, brownish, thread-like masses. This 

 stage serves for the very rapid propagation of the disease. 



During autumn there may be produced from the stroma in the 

 inner bark the perithecial stage. The latter appear in clusters of 

 from ten to twenty. They are flask-shaped with long, slender 

 necks protruding above the surface. The asci are, according to 

 Murrill, 45-50 x gt^. They are constantly eight-spored, hyaline, 

 oblong, two-celled, and measure 9-10 x 4-5 p. 



Control. No practical method of controlling this fungus has 

 been suggested. Severe pruning is certainly advisable if the dis- 

 ease is detected when twigs or branches alone are infested, and it 

 is possible that systematic effort may hold the disease in check, 

 even where the conditions are favorable for its spread. Spraying 

 is perhaps impracticable. 



XLIX. BLISTER CANKER OF APPLE 



Nummularia discreta Tul. 



Hasselbring, H. Canker of Apple Trees. 111. Agl. Exp. Sta. Built. 70 : 225- 

 239. pis. 1-4. 1902. 



The blister canker of the apple is well distributed throughout 

 the apple-growing region of the Mississippi Valley, and doubtless 

 in other sections of the United States. It also occurs in Europe, 

 but has not been reported as a disease worthy of special consider- 

 ation. This canker, like others already described, is, however, a 

 source of constant danger on account of the fact that it is per- 

 ennial in the host and in time is sure to cause the death of large 

 limbs or of the entire tree. The blister canker has been termed 

 the Illinois canker, since it was first observed as particularly 



