PATHOGENICITY OF 4 BLUE-STAIN FUNGI ASSOCIATED WITH AGGRESSIVE AND NONAGGRESSIVE BARK BEETLES

The pathogenicity of two isolates of each of four bark beetle-associat ed blue-stain fungi was evaluated after mass inoculation of about 40-y ear-old Norway spruce trees (Picea abies). Trees were inoculated with a different isolate of each fungus in 1995 and 1996 at a density of 40 0 inoculations per m(2) in a 1.2-m-wide band on the lower bole (about 270 inoculations per tree). Trees were felled 15 weeks after inoculati on. In 1995, Ceratocystis polonica was the only fungus that had staine d the sapwood (56.3% of cross-sectional sapwood area). It induced five times longer phloem necroses, 21 times more dead cambium, and 11 time s more dead phloem than any other fungus. In 1996, C. polonica induced less extensive host symptoms and an unidentified Ambrosiella sp. indu ced comparable symptoms to C. polonica in the phloem and cambium. No t rees showed any foliar symptoms 15 weeks after inoculation, but six ou t of eight trees inoculated with C. polonica in 1995 had only 0 to 25% functional sapwood and probably would have died if felling had been d elayed. This study confirms that C. polonica, an associate of the aggr essive bark beetle Ips typographus, is pathogenic to Norway spruce. Th e pathogenicity of the Ambrosiella sp., which is associated with a non aggressive bark beetle, seems moderate and varies between isolates. Th e two remaining fungi included in this study (Ophiostoma piceae and a dark fungus with sterile mycelium), which are associated with nonaggre ssive bark beetles, were nonpathogenic in both experiments. These resu lts are consistent with the hypothesis that aggressive bark beetle spe cies vector virulent fungi that may help them kill trees, but the resu lts also show that some nonaggressive bark beetles may vector phytopat hogenic fungi.