142 PLANT DISEASES 



fungus in the tissues of the leaf. At a later stage the fruit 

 appears, mostly on the under side of the leaf, as long, 

 shining, black streaks. 



Hartig and Somerville, The Diseases of Trees (Engl, ed.), 

 p. io8, figs. 



DISCOMYCETACEAE 



Sycamore Leaf Blotch 

 {Rhytisma acerinum, Fries.) 



The large black patches so common on the upper surface 

 of living leaves of the sycamore {Acer pseiidoplatanus), 

 maple {Acer cavipestre)^ A.platanoides, and^i. rubrum^ are of 

 fungous origin, and when present in considerable numbers 

 on almost every leaf on the tree, do considerable injury 

 by preventing the leaves from performing their proper 

 functions. The patches are yellow when they first appear 

 in June or July, soon changing to pitch-black, having the 

 surface wrinkled or corrugated. The patches become 

 thickened and scab-like, due to the formation of a thin 

 stroma, on the surface of which a crop of minute bodies called 

 spermatia are produced in the autumn. These spermatia 

 resemble very minute conidia in appearance, but have not 

 been seen to germinate, and their function is unknown. In 

 damp weather they ooze out from below the surface of the 

 cuticle, and cover the surface of the black patches. During 

 the winter, after the leaves have fallen, an ascigerous form 

 of fruit is produced, the spores becoming mature in the 

 spring. 



Preventive Means. — It has been experimentally proved 

 that the spores produced in asci will give origin to the 

 black blotches, when placed on young, living leaves, and 



