Traumatic resin canals as markers of infection events in Douglas‐fir roots infected with Armillaria root disease

In response to an infection, traumatic resin canals (TRCs) are formed in the roots of many conifers, which may be used to determine the timing and sequence of infection events essential for epidemiological studies of root diseases. Juvenile Douglas‐fir (Pseudotsuga menziesii) tree roots at coastal and interior sites in British Columbia were wounded at various times of the year or were inoculated with an isolate of Armillaria ostoyae, and root sections were taken to determine the timing and extent of TRC formation. Naturally infected Douglas‐fir were also examined to determine the extent of the TRCs in infected and uninfected roots on infected trees and in the lower stem. Wounds made in March and October had poor or no TRC formation while the summer wounds responded strongly and were associated with resin soaking. Roots wounded in October did not respond until the following year in all trees except one. Trees produced TRCs and resin soaked tracheids at all times in response to the fungal inoculations. The most striking difference between wounding or fungal inoculation was the multiple bands of TRCs produced in response to the fungus. TRCs at natural A. ostoyae infections were found 92% of time in roots at the stem junction and 74% of the time in the stem at soil line. TRCs were produced in uninfected roots on infected trees but disappeared with increasing distance from the initiating lesion. TRCs can be used to time yearly and seasonal root infections when they can be traced from an identified lesion.