208 FUNGOUS DISEASES OF PLANTS 



the outset the apothecium is closed, but opens by a circular or 

 transverse split, and the edges are often torn or bent back as 

 distinct lips or lobes. The apothecia are usually tough and 

 leathery. The asci and paraphyses form a very closely adherent 

 layer, in which the paraphyses overlap above the summit of 

 the asci, forming a rather definite epithecium. Rhytisma is the 

 only genus which is here of importance. 



XVI. THE BLACK SPOT OF MAPLE 

 Rhytisma Acerinum (Pers.) Fr. 



Klebahn, H. Bemerkungen iiber Rhytisma acerinum und iiber die Arbeit 

 des Herrn. Dr. Julius Miiller iiber die Runzelschorfe. Bot. Centrbl. 58 : 

 321-323. 1894. 



Muller, J. Zur Kenntniss des Runzelschorfes und der ihm ahnlichen Pilze 

 Jahrb. f. wiss. Botanik 25: 607-627. pis. 27-29. 1893. 



The black spot of the maple (Acer spp.) is a fungus of very 

 wide distribution, but the amount of injury caused is so slight 

 that it cannot be considered of much economic importance. The 



affected areas of the leaf are so 

 conspicuous, however, as to attract 

 the attention of all interested in 

 parasitic fungi (Fig. 81). The 

 fungus occurs upon a number of 

 species of Acer, the first evidences 

 of the spot being manifest by yel- 

 low, thickened areas soon after the 

 leaves have attained full size. A 

 cross section shows that beneath 



the cuticle there are produced in 



Fig. Si. The Black Spot of Maple . ,-, -u„_t „ ;j;„ 



great quantity on short conidio- 



phores arising from a stromatic tissue unicellular, curved conidia, 

 and these conidia serve to spread the fungus, it is believed, during 

 the same season. This stage is referred to the form genus Melas- 

 mia. The' tough blackened structures, which appear in the affected 

 spots as the season advances, consist in reality of sclerotioidal 

 masses of fungous tissue, black without but white within, penetrat- 

 ing all medullary parts of the leaf. These areas are much thicker 

 than the normal leaf. After the fall of the leaf further growth or 



