Assessing the virulence of four bark beetle‐associated bluestain fungi using Norway spruce seedlings

Abstract

The virulence of two isolates of each of four different bark beetle‐associated bluestain fungi was evaluated by wound‐inoculating 2‐ and 4‐year‐old Norway spruce seedlings. One isolate of Ceratocystis polonica killed 40% of the 2‐year‐old plants and 20% of the 4‐year‐old plants, whereas the other fungi (Ophiostoma piceae, Ophiostoma sp., an unidentified fungus with dark sterile mycelium) and the control treatments (inoculation with sterile agar and unwounded plants) did not kill any plants during the 11‐week incubation period. Only C. polonica and the Ophiostoma sp. caused any bluestain of the sapwood. The two C. polonica isolates caused significantly deeper bluestain penetration into the sapwood and longer necrotic lesions on the sapwood surface than all other isolates. The symptoms caused by the other fungi were similar to those on the sterile inoculated control plants. The virulence of the fungal isolates tested in this study agrees largely with results from a previous mass inoculation study using the same isolates inoculated into 40‐year‐old Norway spruce trees. Thus, inoculation of seedlings seems to be a reliable, inexpensive and convenient bioassay for determining the virulence of bark beetle‐associated bluestain fungi.