io6 



DISEASES OF TREES 



marks from one to two centimetres in diameter. In August 

 these begin to turn black (Fig. 50), and the leaves usually fall 

 somewhat prematurely, so that, by the end of September, the 

 trees are, for the most part, leafless. 



Not till some time during winter or the following spring do 

 the numerous, somewhat prominent, vermiform apothecia appear 

 on the black blotches of the rotting leaves. During damp warm 

 weather these open by a longitudinal fissure. The disease is very 

 easily produced artificially by laying such black portions of the 

 leaves of the previous year on young maple-leaves during wet 



weather, or in a moist chamber, in 

 the month of May. The filamentous 

 spores which escape germinate, 

 and produce fresh blotches. As 

 this fungus agrees very closely with 

 the next genus, Hysterium> both as 

 regards origin of the perithecia 

 and the development of the black 

 stroma, I will not pursue this part 

 of the subject further here. 



The injury, which consists in the 

 reduced power of assimilation of the 

 leaves, is not so great as to warrant 

 the expense of instituting preven- 

 tive measures. These would con- 

 sist in raking together and removing 

 the leaves in autumn. In gardens 

 and parks, where this is done from 

 other motives for instance, in the 



English Garden in Munich one never meets with an example of 

 Rhytisma, whereas along the country roads and forest paths in 

 the immediate neighbourhood of the city, where the leaves are 

 left to lie in ditches and hollows, the disease occurs with great 

 intensity. 



RHYTISMA PUNCTATUM 



This species of Rkytisma^ which closely resembles the pre- 

 ceding one, is also to be met with on the leaves of Maples. 

 They are to be distinguished, however, by the appearance of the 



FlG. 50. Part of a leaf of the 

 Norway maple, showing R. 

 acerimim. The black blotches 

 are surrounded by a pale brown 

 zone of dead tissue. 



