ASSESSING THE VIRULENCE OF 4 BARK BEETLE-ASSOCIATED BLUESTAIN FUNGI USING NORWAY SPRUCE SEEDLINGS

The virulence of two isolates of each of four different bark beetle-as sociated bluestain fungi was evaluated by wound-inoculating 2- and 4-y ear-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 unident ified fungus with dark sterile mycelium) and the control treatments (i noculation with sterile agar and unwounded plants) did not kill any pl ants during the 11-week incubation period. Only C. polonica and the Op hiostoma sp. caused any bluestain of the sapwood. The two C. polonica isolates caused significantly deeper bluestain penetration into the sa pwood and longer necrotic lesions on the sapwood surface than all othe r isolates. The symptoms caused by the other fungi were similar to tho se on the sterile inoculated control plants. The virulence of the fung al isolates tested in this study agrees largely with results from a pr evious 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.