Institute of Forest Entomology, Forest Pathology and Forest Protection, Universität für Bodenkultur Wien (Uni BOKU Vienna), Hasenauerstraße 38, A-1190 Vienna, Austria

Background and objectives
Since the early 1980s symptoms of crown thinning, dieback of twigs and branches as well as top dying of Norway spruce (Picea abies (L.) Karst.) have been observed in some regions of Upper Austria. Symptoms first appear on individual branches or groups of branches and may then spread over the whole crown. The major characteristic of the observed symptoms was the dieback of the current year's shoot, thus differing from needle losses and/or discoloration resulting from air-pollution and deposition, which also affected the forests in the investigation areas [1]. The aim of our studies was to investigate the cause for shoot, twig and branch dieback, examining the possible role of fungi.

Materials and methods
Investigations were carried out in mature spruce stands at three sites in Upper Austria with extensive dieback of the current year shoots. All stands were characterised by poor podzolic soils over tertiary gravels (site "Kobernaußerwald") or old silicate bedrock respectively (site "Sonnenwald" and "Sauwald"), additionally impoverished by litter raking, grazing and lack of deciduous trees. By establishing an inventory of symptoms of damage the occurrence, distribution (exposition, elevation) and intensity of twig and branch dieback were estimated. Isolations were carried out from various sections of dying main branches, side branches (branches of the 2nd order) as well as from current year shoots of affected mature trees. All samples were surface sterilised and then plated out on the surface of 2% malt extract agar supplemented with 100 mg/l streptomycin sulphate and incubated at room temperature and diffuse daylight.

Results and conclusions
Terrestrial inventories in the "Sonnenwald" and "Kobernaußerwald" regions indicated that intensity of twig and branch dieback of Norway spruce is highest on west exposed upper slopes as well as on hilltops at about 700-1200 m. This altitude is known for its high incidence of days with foggy weather which favours the infection by S. conigenus. Similar results were obtained in an aerial photograph inventory of the region "Sauwald". Plots with highest degree of defoliation corresponded well with areas with very intensive infestation of old spruces by S. conigenus.

Main branches of affected trees first showed symptoms of needle browning on current year shoots followed by defoliation and severe shoot dieback starting at the tip of the individual branch progressing basipetally. Isolations from the current year shoots of affected branches confirmed that S. conigenus is the major cause for shoot dieback. The fungus was isolated from 70-83% of the current year shoots, showing disease symptoms. Other species suspected to be strongly pathogenic to Norway spruce were not detected. Sirococcus was also derived at lower incidence from dying main branches and side branches. At the latter it was found to be the second most frequent species. It was isolated from the living bark, the boundary between living and dead bark as well as from dead side branches at 17, 27 and 9% incidence. Similar results were obtained from main branches. Although a complex of factors including nutritional imbalances is to be regarded as the reason for twig and branch dieback, extensive and repeated infestation of shoots by S. conigenus might at least contribute on the occurrence of symptoms found on main branches. The status of S. conigenus as primary pathogen causing dieback of the current year shoots on spruce species is generally known. The more recent literature describes this fungus as a seed-borne pathogen and causal agent of a seedling disease affecting various conifer species. However, the results of this study indicate that S. conigenus can cause severe damage to mature Norway spruce, a fact that had already been reported by Hartig [2] and Rudolph [3] at the beginning of this century.

1. Neumüller A, 1994. Forstliche Schriftenreihe, Universität für Bodenkultur Wien, Vol.7, pp.171-90.
2. Hartig R, 1893. Forstlich-naturwissenschaftliche Zeitschrift 2, 357-59.
3. Rudolph H, 1912. Naturwissenschaftliche Zeitschrift für Forst- und Jagdwesen 10, 411-15.