Seasonal development of Dutch elm disease on white elms in central Ontario, Canada. I. Following wound inoculation

Abstract

Field inoculation experiments were carried out to determine the pattern of Ceratocystis ulmi infection of white elm (Ulmus americana). From April 7 to October 27, 1971, four methods of artificial inoculation were used to infect healthy, naturally occurring elm saplings approximately 56 km north of Sault Ste. Marie, Ontario. External and internal symptoms of Dutch elm disease, internal spread of C. ulmi, and external damage by the disease were correlated with inoculation methods and seasonal morphological development of elm.The results suggested that the most susceptible period for successful artificial infection of white elm in 1971 was early June to late July. The beginning of the most susceptible period corresponded with the initiation of leaf expansion and large size of springwood vessels in the outer rings. It coincided with a change in accumulated degree days above 5.6 °C from minus to plus. A decline in susceptibility was found after August 12 when leaves were fully matured and 1971 buds had developed. Inoculations before June 3 resulted in the expression of wilting symptoms, always about June 24. This indicates that the disease development is highly dependent on host morphological and physiological conditions.The slit method of inoculation most closely simulated natural beetle infection. The modified Hagborg method of inoculation gave results similar to those of the slit method but required more effort and time in practice. The drill and chisel methods appeared too severe and resulted in the death of all trees inoculated from April 7 to August 12.