In western North America, where R. pseudotSUgae is endemic, 

 sporadic epidemics develop which last lor about two years and then 

 decline. The needle cast disease in the northeastern United States 

 and in Europe, where the host and pathogen are out oi their natural 

 range, develops into severe epidemics. The disease is now in its 

 eighth year in some of the New York plantations and its damaging 

 effects are rapidly increasing. Long periods of severe Rhabdocline 

 epidemics are known for many European plantations of Douglas-fir. 

 Infected trees in New York plantations become about 50 per cent 

 defoliated after three years (Fig. 1) and may be nearly completely 

 defoliated after five years. Weir (66), in Montana, reported that 

 trees that had been attacked for several consecutive seasons were 

 almost completely defoliated and died or "simply existed" without 

 growing. Heavily-infected trees in Scotland were defoliated after 

 five years, and death often occurred after three years of defolia- 

 tion (70). 



R. pseudotsugae, both in this country and in Europe, has been 

 observed on 2-year-old nursery stock and on trees 30 years old or 

 more (5, 66). Most Douglas-fir plantations in New York were estab- 

 lished in the late 1930's, hence the effects of the fungus could be 

 determined on only a relatively few age classes. 



Both height and diameter growth of trees afflicted with needle 

 cast were reduced in the study plantations. Estimations of growth 

 reductions were complicated by the extreme susceptibility of the 

 trees to late spring frosts. Certain trees in the study group were 

 frost-hardy but susceptible to R. fiseiidotsugae so that estimates of 

 effect of disease on diameter growth were obtained by sectioning 

 (Fig. 2). After the second or third year of heavy infection by R. 

 pseudotsugae, the reduced number of needles and living portions 

 of infected needles resulted in a marked slowdown in growth. 



Since the initial report of Rhabdocline needle cast disease was 

 made (66) in Montana and Idaho, the known distribution of the 

 disease has increased materially. Martin (33) reported its occurrence 

 in Oregon and British Columbia in 1923. It was first discovered in 

 Scotland in 1922, but was not reported until 1926 by Wilson and 

 Wilson (70). It was believed to have been introduced into Scotland 

 on transplants from the west coast of North America as early as 1914. 

 Disease quarantine laws were advocated by von Tubeuf (60) in 

 Germany and by Van Vloten (62) in Holland but the disease had 

 already become established in these countries. 1 'he natural spread 



17 



