1NISK, Hogskoleveien 12, N-1432 As, Norway; 2Washington State University, Pullman, WA 99164, USA

Background and objectives
Acquired resistance to pathogen attack has been observed in a number of angiosperms, including tobacco, cucumber and different monocots [1]. When plants are pre-treated with a pathogen, a nonpathogen or an avirulent form of a pathogen, they may become resistant to subsequent infection by a virulent pathogen. This acquired resistance can be expressed locally (at or very near the pre-treatment site) or systemically (e.g. in another leaf). In the experiment reported here we observed enhanced resistance to subsequent fungal infection in Norway spruce (Picea abies) trees that were pre-treated with a pathogenic blue-stain fungus associated with the aggressive bark beetle Ips typographus. This is the first demonstration of acquired resistance to pathogens in gymnosperms.

Materials and methods
42 trees of similar size (about 16 m height) were selected from a 31-year-old monoclonal Norway spruce plantation near Kongsvinger, Norway and used in a dose-response experiment involving three different dosages of fungal and sterile pre-treatment inoculations. On 17 June, 1997 six trees each were pre-treated with a high (100 inoc./m2), medium (50 inoc./m2) or low (10 inoc./m2) dosage of Ceratocystis polonica or sterile malt agar inoculations, and six trees were left untreated as controls. 3 weeks later, all 42 trees were mass-inoculated with C. polonica at a density of 400 inoc./m2 to determine their resistance. Both pre-treatment and mass inoculations were made with a 5-mm cork borer on a 1.2-m stem section on the lower bole. Trees were harvested 15 weeks after mass inoculation, and the percentage sapwood area that had been blue-stained by the fungus and the proportion of dead and live cambium circumference were measured on two thin stem disks cut from within the inoculated stem section.

Results and conclusions
Control trees that received no pre-treatment before mass inoculation were heavily colonized by C. polonica (91% blue-stained sapwood, 87% dead cambium). Pre-treatment with fungal and sterile inoculations both caused enhanced tree resistance to mass inoculation. Compared with the control trees, fungal colonization success was reduced by 97% (sapwood blue-stain) and 84% (dead cambium) in trees pre-treated with the medium dosage of fungal inoculations, which was the most effective pre-treatment. The high fungal pre-treatment dosage reduced tree symptoms by 87 and 76%, respectively, while the low dosage gave reductions of 17 and 30%. Sterile pre-treatment inoculations were less effective in protecting the trees; the high dosage reduced tree symptoms by about 50% relative to the control, the medium dosage gave reductions of 15-31%, while the low dosage gave no reductions at all.

Because both pre-treatment inoculations and mass inoculations were applied to the same section of the stem, it is difficult to determine whether the acquired resistance we observed was localized or systemic. It may have been systemic, however, as there was a general reduction in symptoms within the pre-treated stem section even though only a very small part of it was affected by the pre-treatment inoculations. Furthermore, we have observed an increased number of traumatic resin ducts in the current annual ring of pre-treated trees, and these ducts extend several metres above the pre-treatment sites 18 weeks after pre-treatment. Based on a preliminary study, 1 week between pre-treatment and mass inoculation seems to be insufficient to induce enhanced tree resistance. 6 weeks between pre-treatment and mass inoculation seems to give better protection than 3 and 9 weeks. Our next research priority is to elucidate, at the biochemical and histological level, the resistance mechanisms that lead to acquired resistance in Norway spruce.

1. Hammerschmidt R, Kuc J, eds, 1995. Induced Resistance to Disease in Plants. Dordrecht: Kluwer.