NEW DISEASE OF THE SILVER FIRS IN SCOTLAND. I15 



A. nobilis), but soon darken, becoming finally dark-brown or 

 grey-black. They remain hanging on the branches usually 

 for about a year, and towards the end of this time take on a 

 somewhat bleached appearance. In cases of slight infection 

 less than half the leaves may be attacked, but in severe attacks 

 only a few green leaves remain on each infected shoot, dis- 

 tributed amongst the shrivelled ones, and sometimes forming 

 an irregular ring just below the terminal bud (Fig. i). In cases 

 in which the majority of the leaves are attacked, the shoots 

 remain thin and almost always die during the course of the 

 summer. In Abies nobilis the diseased shoots are generally 

 twisted and curved (Fig. 3), but this character is not so marked 

 m A. pecti7iata. 



The disease has only been seen once on A. Finsapo, 

 A. cephalonica, and A. Pindrow ; in these cases the infection 

 is slight and the branches do not show any marked curvature. 



In some cases a few diseased leaves are still found hanging 

 on the portion of the shoot developed in the previous year. 

 These have probably hung on for more than one year after 

 infection. They may, however, be leaves which did not become 

 infected in the spring, but were only attacked some months after 

 their development. 



When leaves are examined soon after infection, they are 

 found to contain large, dark-coloured, septate, branched hyphae. 

 These develop very abundantly and soon bring about the 

 destruction of the mesophyll; the xylem of the vascular bundle 

 becomes brown, but retains its outline after the mesophyll is 

 destroyed. The epidermis and hypodermis retain their form 

 somewhat longer, but at length become brown. The cuticle 

 still persists after the whole of the leaf tissue is disorganised 

 (Fig. 8). 



Two types of fructification have been found on the diseased 

 leaves, but both appear identical to the naked eye. They 

 usually occur on the upper surface of the leaf, but have 

 occasionally been found on the under side. They appear as 

 dull-black, rounded structures, projecting slightly above the 

 surface, and are first developed near the margin, later spreading 

 over the whole of the leaf. The pycnidia are usually produced 

 first and, when ripe, contain small, oval or oblong, hyaline 

 spores. This stage was described by Bubak and Kabat as 

 Phoma bohemica. 



