LEAF AND SEEDLING DISEASES 177 



by the uredo -stage, which can spread from leaf to leaf 

 throughout the summer, but by the aecidial stage, which 

 cannot spread by itself. Some sporidia will blow up from 

 the ground and infect a few larch needles, and these needles, 

 which bear spores as early as three weeks after expanding, 

 soon wither and die. But that is the end of it as far as the 

 larch is concerned. Once the aecidiospores have attacked 

 the birch, the fungus can spread from birch to birch as long 

 as the summer lasts, and no limit is set to its distribution. 

 Probably this is the reason why the fungus is found so much 

 more commonly on the birch than on the larch. The disease 

 on the larch is only serious when seedlings are attacked. 

 Massee (1903) reports a case where rows of larch seedlings 

 were severely damaged by this rust. The owner, believing 

 in mixed planting as a panacea for plant diseases, put in 

 rows of birch seedlings between the larch, with the result 

 that any one familiar with the life-history of the fungus 

 would have anticipated. Of course birch is the worst con- 

 ceivable tree to bring into the neighbourhood of this rust, 

 and where the disease is troublesome in a nursery it is 

 advisable to destroy the birch as far as possible in the 

 immediate vicinity. As the mycelium spreads entirely 

 inside the leaf and not on the surface, spraying can only 

 be effective if the mixture is on the leaf at the time of 

 infection, so as to prevent the germination of the sporidia. 

 This requires such thorough and constant application to be 

 successful that it is outside the scope of practical sylvi- 

 culture. 



M elampsora tremulae, Tul, &c.(=Coeomalaricis). Coeoma 

 is another aecidial form which closely resembles Peri- 

 dermium, except that it possesses no peridium, and is 

 consequently far less conspicuous. The small red spots or 

 cracks which it produces in the epidermis of larch needles 

 and cotyledons are no doubt often overlooked. The uredo- 

 stage is borne on species of willow and poplar, and four 

 distinct species, as well as many sub-species, are recognized 

 in the uredo- and teleuto -forms, though there are no features 

 by which these species can be distinguished in the coeema 



1888 



