Response of planted and natural Douglas fir tree roots infected with Armillaria root disease: lesion type, time-to-callus, infection timing, and influence of site and environmental factors on lesion formation

Abstract

Armillaria root disease causes growth reduction and mortality in forest trees and agricultural plants. Root colonization is poorly documented but important for predicting interactions with environmental factors and disease spread. Nine planted and one older naturally regenerated Douglas fir (Pseudotsuga menziesii) sites were identified for sampling. Tree roots were pulled out of the soil, cleaned, and inspected for lesions arising from Armillaria root disease. A total of 1097 root systems were measured for host reaction, lesion type, age of infection, and size of the roots and stems. Callus tissue indicated successful lesion containment, and most lesions callused within five years. Patch lesions, girdled root lesions, or collar lesions on the lower stem differed in callus rates. In planted sites, girdled roots had the lowest callus rate coinciding with lack of induced bark hypertrophy ahead of lesions. Bark hypertrophy was more prevalent on smaller diameter roots and infection probably induced the phellogen to create more inner and outer bark, while older roots or stems had thicker bark initially. In planted trees, lower callus rate was also associated with longer growing season. In an older natural stand, induced bark hypertrophy was rare and without effect on callus formation. Callus rate was lowered with increasing infection intensity of the root system and with collar lesions. In both stand types, the main effect of tree age was increased callus rate, and for larger tree size it was lower callus rate. The study represents a macro examination of root lesions that affect the ability of trees to survive and grow or die.