178 



DISEASES OF TREES 



ledge of the biology of this parasite, has, however, proved that 

 the secidium form may be dispensed with. Where spruces are 

 absent the sporidia germinate directly on the leaves of the 

 rhododendron to produce the uredo layers, and these serve 

 to maintain and spread the fungus during summer, until, in 

 autumn, teleutospore-layers are again formed on the leaves of 

 the youngest rhododendron shoots. These hibernate, and in the 

 following spring the germination of the teleutospores results in 

 the rupturing of the leaf-epidermis (Fig. 107). 



At first the development of the parasite in the spruce-leaf 

 resembles that of Chrysomyxa Abietis, but even in July and 



FIG. 107. Teleutospore-layer of C. Rhodo- 

 dendri on R. hirsutum. The development 

 of the promycelia has caused the epidermis 

 of the leaf to rupture. 



FIG. 108. Spermo- 

 gonia and secidia 

 of C. Rhododendri 

 on a spruce-leaf. 



August numerous small spots, the spermogonia, are to be 

 observed on the yellow parts of the leaves. Shortly afterwards 

 one sees the yellow vesicles of the secidia breaking through the 

 epidermis, and these bear a close resemblance to those produced 

 by the pine-blister-rust on the leaves of pines (Fig. 108). In 

 August and September, when the peridia burst at the apex, the 

 secidiospores are liberated in such numbers that if a diseased 

 spruce is shaken the air is filled with a dense cloud of spores. 

 In the course of the same year the diseased leaves die and drop 

 off. This distinguishes the parasite at once from Chrysomyxa 

 Abietis, which, in an immature state, hibernates on the tree. On 

 lateral branches it is usually only the leaves that are situated on the 

 upper side that are diseased. The leaves on the under side, being 

 preserved against infection by those above, escape the disease. 



