THE TWO RUST DISEASES OF THE SPRUCE. I9I 



infect the young leaves of the spruce. An abundant intercellular 

 mycelium is developed in the tissue of the leaf which sends 

 haustoria into the cell cavities. The hyphae contain numerous 

 yellow oil-drops, and in consequence yellow bands appear on 

 the leaf. Soon afterwards teleuto-sori are produced which 

 take the form of elongated yellow cushions on both the under 

 surfaces of the leaf (Fig. 6). During the winter the sori are 

 covered, but in the following spring the epidermis is ruptured 

 and the teleutospores project as an orange-yellow mass (Fig. 7). 

 Each teleutospore is cylindrical and consists of 8-12 superimposed 

 cells of which only the terminal one produces a promycelium 

 (Fig. 8). At about the middle of May the sorus becomes brighter 

 yellow and the promycelia grow out. Each produces four small 

 spherical sporidia, which become easily detached and are 

 distributed by the wind. When the sporidia have been shed, 

 the sorus loses its bright colour and shortly afterwards the 

 diseased leaf falls. On coming into contact with the young 

 needles of the spruce the sporidium produces a germ tube which 

 bores through the epidermis and so brings about infection. 



It frequently happens that certain spruces in a wood remain 

 free from the disease, while others are badly attacked. This 

 may be explained by the fact that infection of the young leaves 

 only takes place at a certain stage in their development ; trees 

 which are in a backward condition when the sporidia are ripe 

 are not infected, while at the same time others may have passed 

 the susceptible stage and thus escape the disease. 



As a result of the attack the infected leaves die and fall 

 from the tree, and thus a considerable defoliation may take place. 

 The disease, however, usually fails to maintain itself through a 

 long series of years on any one tree, and in consequence felling 

 is not desirable. 



In the last issue of the Quarterly Journal of Forestry 

 (ix., 1915, p. 253), which we have just received on going to 

 press, Dr Somerville states that on April 21st specimens of 

 spruce branches badly attacked by Chrysomyxa abietis were 

 sent to him by Mr Neil MacGregor, Bridge of Dye, Banchory, 

 Kincardineshire. The specimens were from trees about 

 20 years old which were planted under old larch and Scots 

 pine, and were situated about 7 miles from the nearest part 

 of the Durris woods, in which Dr Somerville discovered the 

 disease in 191 1. 



