Although the spread of the disease at Chen in go approximated 

 the general pattern described above, a block of trimmed trees within 

 the plantation appeared to cause a slight deviation in direction of 

 spread. These trees were on the windward side and downslope of 

 the plantation. Except for a fringe of infected needles on trimmed 

 trees bordering the nnt rimmed heavily infected trees, no evidence 

 of R. pseudotsugae infections was found. It is not known whether 

 the prevailing crosswind prevented viable spores from reaching these 

 trees or whether control was accomplished by the increased aeration 

 with subsequent rapid drying of otherwise susceptible needles. The 

 primary infection on four trees on the extreme western side of the 

 stand resulted in nearly 100 per cent infection of trees up to 200 

 yards to the East in three years. 



The plantations studied varied in size from approximately four 

 to ten acres. In all cases the disease had spread over the plantations 

 within three years after initial infection. Many trees which initially 

 escaped infection later became diseased. These apparently were less 

 susceptible to infection by R. pseudotsugae. 



On the basis of the present study, the rate of spread of R. pseu- 

 dotsugae in central New York plantations might be expected to 

 vary from about 100 to 250 feet per year in the direction of the 

 prevailing wind. It will be somewhat less in other directions, depend- 

 ing upon local topography, wind exposure and moisture conditions. 

 While the figures given for the spread per year are approximate, they 

 were obtained during a period of extremes in climatic conditions 

 such as may be expected to occur in central New York. The ex- 

 tremely hot, dry summer of 1955 was followed by an excessively cool, 

 moist spring and summer in 1956. By far the greatest spread of the 

 disease occurred in the 1956 season. The increase in intensity of 

 needle cast infection during cool wet seasons has been reported by 

 Weir (65, 66) and Boyce (5). 



Plantations infected for four successive years had approximately 

 55 to 75 per cent of the trees infected. In the Camden plantation 

 nearly 50 per cent of the diseased trees were estimated to be more 

 than 40 per cent defoliated. 



Prolonged studies of the spread of Rhabdocline needle cast in 

 the Northeast are prohibited due to the limited si/e of Douglas-fir 

 plantings, and the rapidity with which the disease envelops such 

 small stands. 



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