DANGEROUS INTERNATIONAL FOREST TREE DISEASES 101 



than 4 years old may be killed in one season, whereas on young trees 

 beyond the seedling stage the effect is severely retarded growth. In- 

 fection of the leaves is by windborne spores. 



The fungus is present in Great Britain, Ireland, and western Eu- 

 rope. The manner of its introduction is unknown but it probably 

 came in on living plants, although it has been suggested that it came 

 on or with seed. Living material of western red and northern white 

 cedars should not be moved from region to region. 



Range: In North America it is prevalent from Alaska southward 

 along the coast ranges of British Columbia, western Washington 

 and Oregon, eastward to Idaho, Montana, and eastern British Co- 

 lumbia. In the eastern United States it is recorded in the Lake 

 States and Vermont. 

 Hosts: Cupressaceae — 



Thuja plicata D. Don (western red cedar) 

 T. occidentalis L. (northern white cedar) 

 Literature : 

 Boyce, J. S. Forest pathology. Ed. 2, 550 pp. New York: 



McGraw-Hill Book Co. 1948. (See pp. 148-151.) 

 Durand, E. H. The genus Keithia. Mycologia 5 : 6-11. 1913. 

 Weir, J. R. Keithia thujina, the cause of a serious leaf disease of 

 western red cedar. Phytopathology 6 : 360-363. 1916. 



Western Gall Rust (Woodgate Rust) 



W. G. ZlLLER 



Forest Entomology and Pathology Branch, Canada Department 

 of Forestry, Victoria, British Columbia 



Peridermium harknessii J. P. Moore, western gall rust, causes 

 branch galls and trunk cankers of hard (2- and 3-needle) pines in 

 North America. Trees of all ages are susceptible. 



In spring, conspicuous pale-yellow blisters (aecia), occasionally 

 preceded by droplets of clear, viscid liquid (pycnial exudate), emerge 

 from the living bark of the globose rust galls and the marginal swell- 

 ings of the trunk cankers. The aecia rupture a few days after their 

 appearance and begin to release clouds of orange-yellow aeciospores. 

 Carried by air currents, these spores may spread the disease for 

 hundreds of miles. Dissemination of aeciospores and infection of 

 young pine shoots continue for 1 to 3 months each year. 



Each new infection is followed by the formation of a well-delimited 

 gall sometimes accompanied by a small witches'-broom at the court 

 of infection. The time it takes from when a rust gall develops un- 

 til it produces its first crop of aeciospores varies between 2 and 4, 

 and rarely 6, years. Thereafter the gall continues to enlarge, produc- 

 ing a new crop of spores each spring, until it has girdled the branch 

 it grew on, resulting in the death of the branch, 1 the gall, and the 

 rust fungus that caused it. Rust galls cause growth losses and tree 

 mortality but no cull, while the so-called "hip-cankers" of trunks 

 cause severe cull but rarely mortality or even growth losses. The 

 fungus may survive in cankers for as long as a century or two, 

 causing conspicuous swelling and deformation of the bole but pro- 

 ducing few if any aecia. 



