STUDIES ON VIRUS INFECTION OF DISEASED MOUNTAIN ASH (SORBUS AUCUPARIA L.) IN NORTHERN GERMANY M FOHRLING, C BOTTNER and H RENNENBERG Institut of Forest Botany and Tree Physiology, Dept. of Tree Physiology, University of Freiburg, Am Flughafen 17, D-79085 Freiburg i. Br., Germany Background and objectives Our investigations focuse on diseased mountain ash (Sorbus aucuparia L.) trees and seedlings with virus-like leaf symptoms such as chlorotic ringspots, mottling and deformations at several forest stands and nurseries in north Germany. Mountain ash trees gained major importance within the last years since they are used for wood production on recultivation sites and for thr extraction of pharmaceutical compounds. It is known that virus infected trees suffer from a distinct loss of viguor and that these infections are not curable by chemical plant protection. Moreover, some plant viruses are not only mechanically transmissible but also by vectors, seeds and pollen which causes rapid spread of the disease especially if the affected species has a high natural reproduction rate such as mountain ash. The present investigations were carried out on selected north- and west-German stands to describe the distribution of the diseased trees and to identify the causal agent. Material and Methods Samples (bark and leaves) were taken on 30 sites at nurseries and altogether 118 stands of 16 major German forest areas located in four states. The transmissibility of the pathogen was tested by mechanical inoculation of plant sap to herbaceous indicator plants and grafting of scions, chips or buds of diseased trees to healthy mountain ash seedlings [1]. The enzym-linked immunosorbent assay (ELISA) and the immunological detection of double stranded RNA (dsRNA) were used for serological diagnosis. The ELISA [2] was carried out to check infection by apple mosaic virus (ApMV), apple chlorotic leafspot virus (ACIV), cherry leaf roll virus (CLRV), tobacco mosiac virus (TMV) or tomato spotted wilt virus (TSWV). DSRNA was isolated and detetected by immunoblotting [3]. Partly purified plant sap was adsorped on grids, negativiy stained and visually evaluated by electronmicroscopy [4]. Results and Conciusions Diseased mountain ash trees showing the described leaf symotoms are wide-spread in all visited forest districts and nurseries. Furthermore these trees degenerate and about 10% of them die within 5 years. Mechanical transmission of pathogens in plant sap by inoculation to herbaceous indicator plants failed. Phenolic compounds, polysaccharides and a low virus titer may have hampered the transmission. It is also possible that the infecting virus is not mechanical transmissible or the chosen indicator plants are no host plant even they are hosts for a wide scale of plant viruses. Six hundred graftings (budding, chip- and whip grafting) were carried out to confirm the transmissibility of the symptom inducing agent to woody seedlings. About 380 seedlings developed the described leaf symptoms one up to three years following the grafting. The transmission rate depended on used techniques, the cultivation conditions after the treatment and the quality of the used scions and buds. Applying the ELISA we found that the causal agent in diseased mountain ash resembles the TSWV, a tospovirus, as measurred by visualized antigen-antibody reaction. No serological reaction could be observed using the antisera specific to ACIV, APMV, CLRV and TMV. High molecular weight dsRNA (5-9 kbp) was isolated from diseased mountain ash trees; in healthy trees these structures were not detected. The dsRNA-structures are thought to be of diagnostic value, particullary for diseases of suspected virus ethiology, as the detected fragments are intermediate structures which originate from virus replication. Until now no virus particles have been visualized in electronmicroscopy. The investigations showed that the diseased montain ash trees are wide spread in north- and west-Germany. The causal agent is graft transmissible and resembles a tospovirus . Isolated dsRNA confirms the assumption of a virus ethiology. References 1. Bdttner C, Fuhrling M, 1996. Ann. Sci. For., 53, 383-88 2. Clark NF, Adams AN, 1977. J. Gen. Virol. 34, 475-83 3. Bdttner C, F0hrling M, Werner R, Mdhibach HP, Lukacs N, 1996. Gesunde Pfianzen 48, 95-103 4. Lesemann DE, Bozarth RF, Koenig R, 1980. J. Gen. Virol. 48, 257-64