THE INFLUENCE OF TEMPERATURE AND LIGHT ON DEFOLIATION LEVELS OF ELM BY DUTCH ELM DISEASE

The amount of defoliation of elm (Ulmus procera) caused by three Ophio stoma novo-ulmi Eurasian race isolates over 14 seasons of field trials was found to be strongly correlated with mean air temperature and mea n number of sunshine hours over the 12-week period from inoculation to assessment, and with tree age. The coefficient of determination for t he regression of percent defoliation on the environmental and tree fac tors was 0.76, P < 0.001 (33 df). Levels of defoliation were greatest when mean air temperatures exceeded 17 degrees C with moderate light ( 5 to 7 h of sunshine), and lowest under conditions of either high ligh t (>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 degrees C. The model varied in its intercept for the three isolates, reflecting their diff erent levels of aggressiveness. The role of environmental factors in t he development of Dutch elm disease symptoms and the implications for elm resistance breeding are discussed.