APPLE DISEASES 107 



troduced parasites, especially those of such alarming habits as 

 the gooseberry-mildew fungus and others. 



The radiating threads which compose the blotches are hyphse 

 of the fungus Leptothyrium pomi. Likewise the black specks 

 belong to this fungus ; they are sclerotial bodies. It is unknown 

 just how the causal fungus passes the winter. It has been 

 suggested that it hibernates on the apple-twigs, presumably as 

 specks or sclerotial bodies. In the late spring each sclerotial 

 body undergoes certain developmental changes which result in 

 a pycnidium. Conidia, developed within the pycnidia, serve 

 to bring about the first infections in the summer. On the sur- 

 face of the apple a conidium germinates in the presence of mois- 

 ture with the result that a radiating growth of mycelium is 

 initiated. Apparently inoculations do not occur prior to the 

 month of July. The threads extend themselves superficially, 

 or at most penetrating the cuticle but to a slight degree. As 

 the hyphse grow, they branch uninterruptedly until there is a 

 prostrate, soot-colored mat of fungus threads; these compose 

 the blotches, whence the name sooty-blotch. The cells of the 

 radiating hyphse become enlarged and function in propagation 

 by breaking away and causing new infections. This probably 

 composes the inoculum during midsummer ; for at this season 

 no fruiting structures and no spores are to be found. When 

 the fungus develops the fly-speck stage, the sclerotial bodies may 

 possibly break away from the skin of the apple and may sub- 

 sequently be washed to new quarters, where secondary infections 

 may occur. This point, however, needs confirmation. In all 

 cases the fungus is highly favored by damp situations and it is 

 most abundant in seasons of considerable late summer rainfall. 

 Little difficulty is experienced in dry seasons ; in some years 

 sooty-blotch and fly-speck are practically unknown. 



Control. 



Fortunately the disease may be kept under control by the 

 methods of treatment employed for other more serious diseases 



