Intensification and lateral spread of Arceuthobiumlaricis in a young stand of western larch with stocking control

Abstract

Initial intensification and lateral spread of Arceuthobiumlaricis (Piper) St. John were measured for three levels of stocking, 1680, 549, and 272 stems/ha (2.4 × 2.4, 4.3 × 4.3, and 6.0 × 6.0 m spacing) of young, single-layer, western larch (Larixoccidentalis Nutt.) monoculture, following artificial inoculation. Intensification was slow for the first 6 years following inoculation, but showed 5- to 10-fold increases in treatment means for the next 3-year period. Lateral spread from the inoculated trees to neighboring suscepts was not observed during the first 6 years, but was observed in all three treatments between years 7 and 9 and averaged 3.5–4.9 m for the three spacing treatments. Maximum lateral spread during the period was 6.4 m. We conclude that A. laricis can intensify rapidly on young western larch once an inoculum source is established. The lowest stocking level (highest spacing level, 6 × 6 m) used in this study did not prevent lateral spread of the pathogen. These data attest to the value of prevention or early detection and removal of infections for the protection of western larch stands from subsequent losses.