DANGEROUS INTERNATIONAL FOREST TREE DISEASES 75 



Larch Canker and Dieback 



H. ROBAK 



Forest Research Institute of West-Norway. Stend, Norway 



Dasyscypha icillkommii (Hart.) Rehm. A perennial — and often 

 girdling — stem and branch canker of larch in Europe. 



First symptoms are regular, elliptical to nearly circular bark de- 

 pressions, in the majority of cases a dead dwarf shoot or, sometimes, 

 twig in the center. Resin flow is often pronounced. Older, crusty 

 bark is not attacked unless through a branch already cankered. In 

 summer a callus formation takes place, the necrotic bark becoming 

 closed off from healthy tissue by a cork layer. During the resting 

 season of the tree, the fungus passes this layer and a perennial canker 

 results. After several years, large almost amphitheatrical, wounds 

 develop on the stems and coarser branches ; while at the same time a 

 more or less conspicuous convexity of healthy wood is being formed on 

 the opposite side of the axis. A younger axis is easily girdled ; smaller 

 trees are thus killed in almost their whole length, somewhat larger 

 trees getting lots of died-back branches and twigs, and young- to 

 medium-aged stands often declining as a result of the attack. 



In tiny, yellow-white pustules on young cankers are produced mi- 

 nute, hyaline, unicellular conidia — their role obscure, however, because 

 germination is never observed. Later they develop 1 to 4 mm. broad, 

 cupshaped or saucershaped apothecia, which are whitish with orange- 

 red hymenium. Asci are cylindrical, octosporous, with spores hyaline, 

 oblong to elliptical, normally unicellular, their extremities often some- 

 what pointed, size ca. 14r-30 X 6-10/x. 



Most probably ascospores infect mainly through recently dead or 

 weakened dwarf -shoots or twigs. Other ways of entrance may exist, 

 among them, perhaps, suction canals of aphids. 



The part played by the fungus in the development of the disease 

 is not always evident ; however, the following facts deserve attention : 



1. Dieback can occur without any cankers being observed. 



2. There are indications that spontaneous cankers of the said 

 type can develop also in absence of the fungus. 



3. In spontaneously developed cankers the callus shows cell 

 structures commonly identified as frost rings, and numerous ob- 

 servations confirm the importance of frost to initiation and fur- 

 ther progress of the canker formation. 



4. Cankers have been produced by artificial freezing, no Dasys- 

 cypha being present, 



5. Cankers have been produced also by inoculation of Dasys- 

 cypha mycelium into artificial wounds under conditions free of 

 frost. 



6. In most countries canker and fungus are almost generally 

 associated. 



7. Several investigators, among them the subscriber, agree that 

 purely saprophytically growing Dasyscypha strains are distinct 

 from those growing in cankers. 



The main host of larch canker is Larix decidua Mill. All geograph- 

 ical varieties are included, spontaneous forests showing sporadical and 



687-137 0—63 6 



