Effect of defoliation on resistance response of Abies sibirica Ledeb. to inoculation with blue-stain fungi.
The effect of defoliation on the resistance response of Siberian fir (Abies sibirica) to fungal inoculation was investigated in 3 even-aged stands damaged by the Siberian moth (Dendrolimus superans sibiricus) in Krasnoyarsk Territory (Central Siberia, Russia). Histological changes in tissues accompanying the resistance response were examined in 30 fir trees (90-130 yr old) with different degrees of defoliation (50, 75, 100%) 6 wk after a single inoculation with Leptographium sp. (using the cork borer technique) in July (the swarming time of the insect pest Monochamus urussovii, a vector of blue-stain fungi). The rate and intensity of defence response of phloem and xylem tissues to inoculation and the percentage of trees infested by the insect-vector differed significantly with defoliation degree. Trees with 50% defoliation differed significantly from trees with more severe defoliation in all defence response parameters and in percentage colonization by the beetle. Trees 75 and 100% defoliated did not differ significantly in defence response activity and in the number of the trees attacked by M. urussovii. Strong differences in resistance to inoculation and colonization by the beetle was recorded between trees defoliated in the current year and in the previous year. All trees infested by M. urussovii were characterized by the epidemic level of frequency of blue-stain fungi associated with this insect pest (Leptographium sp., Ophiostoma sp. and O. curvicollis). There was a tendency for a decreased frequency of the pathogenic Ophiostoma sp. and a more frequent occurrence of the saprophyte O. curvicollis in trees defoliated in the previous year which had lost the defence ability against wounding and fungal infection.