1 7 6 



DISEASES OF TREES 



and is frequently so prevalent on the youngest shoots as to kill 

 a large proportion of the leaves, which subsequently drop off. 



The fungus is autcecious, the uredo and aecidium layers being 

 entirely absent. Its teleutospore-layers alone are developed on 

 the spruce-leaves. In the month of May the sporidia germinate 

 on the delicate leaves of the young shoots, inside which they 

 produce the mycelium, which contains numerous drops of yellow 

 oil. By the end of June the part of the leaf that is infested may 

 be recognized by its pale yellow colour. The diseased portion 

 may occupy the base, middle, or apex of the leaf. As autumn 

 approaches it constantly becomes deeper lemon yellow in 

 colour, while the rest of the leaf remains green. In autumn 

 formation of the teleutospore-layers begins on both the 

 under sides of the leaf. These take the form 

 of oblong, somewhat prominent cushions, 

 which are at once to be recognized by their 

 more golden yellow colour. In this condi- 

 tion the fungus hibernates on the tree, and 

 in the following spring the teleutospore-layer 

 still further develops (Fig. 106), until finally 

 the epidermis ruptures longitudinally and ex- 

 poses a golden yellow stroma. The promy- 

 celia with their sporidia next develop from 

 the cells of the teleutospores, as is shown 

 in the case of C. Rhododendri in Fig. 107, 

 and as this occurs in the month of May, at a 

 time when the young shoots of the spruce are 

 forming, the sporidia have the opportunity 

 of germinating directly on the young leaves. 

 It is probable that spruces which are very backward in growth 

 at the time when the spores ripen escape infection, and this 

 explains why many individuals of a wood remain entirely free 

 from the fungus while others are very badly attacked. Such 

 cases have frequently inspired non-professional minds with the 

 belief that the fungoid disease is dependent upon a pre- 

 disposition to disease on the part of particular spruces. After 

 the sporidia drop off the teleutospore-layers wither, while the 

 leaves themselves soon afterwards die and fall from [the tree. 

 As a rule the tree suffers but little from the loss of leaves, 



FIG. 106. A spruce- 

 leaf attacked by 

 C. Abietis, the 

 golden yellow 

 spore-layers of 

 which have not 

 yet ruptured the 

 epidermis. 



