170 LEAF AND SEEDLING DISEASES 



favourable to the disease, and long summers assist in its 

 wide distribution. 



The mycelium lives . through the winter in the infected 

 needles lying on the ground. By the spring it is found to 

 be composed of brown thick-walled hyphae, which in April 

 and May give rise to perithecia spherical, ascus-bearing 

 fructifications, with a small aperture at the apex which 

 break through the epidermis like the conidial pustules. By 

 the beginning of June these perithecia are the same colour 

 as the conidial pustules, but are somewhat smaller (0-1 to 

 0-15 mm. in diameter). Some are half submerged in the 

 leaf tissue, some are nearly superficial. The pore at the 

 apex is obscure and difficult to find until ascospores, or 

 entire asci, are seen emerging from it. The asci are club- 

 shaped, 50 to 60 /x long, and each contains eight ascospores, 

 15-17/ui long, which are at first one-celled, but later two- 

 celled and colourless. They are forced out of the asci in 

 a single mass. 



The ascospores germinate in water in twenty-four hours 

 from both ends, and sometimes also from the middle. Pure 

 cultures have been grown from the spores, and such cultures 

 bear four-celled conidia, exactly like those in the conidial 

 pustules, after twenty days. Hartig observed a red colora- 

 tion at the border of the mycelium in gelatine cultures, and 

 presumed that this was due to the same cause as the red- 

 brown discoloration of the needles. In the forest the 

 ascospores are lifted by the wind, and are capable of infect- 

 ing the young leaves of the larch. Thus the life-cycle of 

 the fungus is completed. 



It has been noticed that the disease is particularly severe 

 in larch woods mixed with spruce. The explanation of this 

 is that the dead needles fall on to the spruce shoots and 

 remain there all the winter, and next year, as they are near 

 the level of the fresh larch needles, reinfection is facilitated. 

 Comparative immunity of larch woods at high altitudes is 

 attributed to the shortness of the season, which restricts 

 the number of conidial generations which can be produced 

 during the summer. 



When larch woods are heavily attacked by the leaf-cast 



