First and third authors: Forest Research Station, Alice Holt Lodge, Farnham, Surrey GU10 4LH, United Kingdom; and second author: Department of Horticulture, School of Plant Sciences, University of Reading, Reading, Berkshire RG6 6AS, United Kingdom
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Accepted for publication 10 March 1997.
The amount of defoliation of elm (Ulmus procera) caused by three Ophiostoma novoulmi Eurasian race isolates over 14 seasons of field trials was found to be strongly correlated with mean air temperature and mean number of sunshine hours over the 12-week period from inoculation to assessment, and with tree age. The coefficient of determination for the regression of percent defoliation on the environmental and tree factors was 0.76, P < 0.001 (33 df). Levels of defoliation were greatest when mean air temperatures exceeded 17°C with moderate light (5 to 7 h of sunshine), and lowest under conditions of either high light (>7.5 h of sunshine) at all air temperatures or low light (<4.5 h of sunshine) and air temperatures of less than 15.5°C. The model varied in its intercept for the three isolates, reflecting their different levels of aggressiveness. The role of environmental factors in the development of Dutch elm disease symptoms and the implications for elm resistance breeding are discussed.
© 1997 United Kingdom Crown Copyright