W. MCRAE 



233 



Zoospores discharged in such conditions at the beginning of the new season 

 would form the starting point for infection of other parts of the tree. The 

 fungus also develops from within branches that have partially died back. 

 On splitting a branch in the vicinity of the place to which the die -back has 

 extended, there appears a clear line of demarkation between dead and hving 

 tissues ; it is 0-5 to 1 millimetre broad and is dark-brown. The dead wood 

 is dry and brittle, brownish in colour, and the vasa are easily visible to the 

 naked eye, while the living wood is moist and tough, yellowish in colour, and 

 the vasa are not so clearly seen. Sections show thick non-septate or sparingly 

 septate hyphse of a Phytophthora in the dead tissues as well as in the hving 

 tissues adjacent for a distance of a centimetre. Hyphse are found in the 

 cortex, phloem, xylem, medullary rays and pith. Some are inter-cellular, 

 but others pass into and through the cells of these tissues. From the distal 

 portions of the living part of a branch that has partially died back, fresh shoots 

 are produced in the early part of the growing season, and a few weeks later 

 they wilt. During the investigation of this point in 1916, shoots that were 

 produced about the middle of February showed signs of wilting about a 

 month later. The lowest 2 or 3 centimetres of the shoots became discoloured 



LIVING TISSUE. 

 DEAD TISSUE. 



BRO^M L'nS or oCMARnATION 

 BETwtCN DEAOA LIVING TI6SVJC§ 



NC-StPTATC MYCELIUM FOUND MtRC, 



Pig. 1. 



