Chlorosis and vein yellowing of ash trees

Urban pest & disease

Chlorosis and vein yellowing of ash trees (Fraxinus spp.)

Pathogens

Fraxinus gammacytorhabdovirus 1, Fraxinus gammacytorhabdovirus 2

Chlorosis, chlorotic spots and line patter, vein yellowing and vein thickening of ash leaves; Studies on the association of virus and symptoms need to be confirmed.

Leaflets of Fraxinus ornus trees infected with the novel gammacytorhabdoviruses (FraxGCRV1 and FraxGCRV2).    

a-b) chlorosis; c) vein yellowing and vein thickening; d) distinct chlorotic lesions

Two novel cytorhabdoviruses (Fraxinus gammacytorhabdovirus 1, FraxGCRV1; Fraxinus gammacytorhabdovirus 2, Frax GCRV2) have been confirmed in ash trees in urban street trees in Germany. Cytorhabdoviruses are known to be transmitted by aphids (Cytorhabdovirus lactucanecante, Cytorhabdovirus medicagonis), planthoppers (Cytorhabdovirus gramineae, Cytorhabdovirus hordei), leafhoppers (Cytorhabdovirus oryzae) or whiteflies (Cytorhabdovirus caricae). Some cytorabdoviruses are also transmitted by vegetative propagation and some can be transmitted mechanically from infected sap. Seed transmission has not been reported. Details on transmission mode of cytorhabdoviruses affecting ash trees have still to be studied.

Leaflets of Fraxinus excelsior trees infected with cytorhabdovirus fraxini; a) chlorotic spots; b) chlorosis

Common ash tree (Fraxinus excelsior) and flowering ash trees (Fraxinus ornus) are so far the only known natural host species of FraxGCV1 and FraxGCV2 (Rehanek et al. 2024, Bejerman et al. 2023). The non-native Fraxinus ornus is increasingly planted in German cities as a climate-tolerant tree. To our knowledge, no screening or bioassay has been performed to determine the host range of this virus.

The use of virus-tested plant material is highly recommended. Therefore, reliable diagnosis of the two novel cytorhabdoviruses should be developed for instance by molecular tools such as virus-specific RT-PCR. So far no commercial serological tests (DAS-ELISA) are available to confirm the infection of ash trees with FraxGCRV1 and FraxGCRV2, respectively.

Not regulated within the EU

Germany, Europe-wide distribution is very much possible but has not been investigated to date.

FraxGCRV1 and FraGCRV2 are single stranded, unsegmented RNA viruses with a negative orientation, and are recently identified as two closely related members of the proposed novel genus Gammacytorhabdovirus, family Rhabdoviridae (Bejerman et al. 2023). The phylogenetic analyses of open reading frames of viral RNA-polymerase, glycoprotein, and nucleoprotein confirm that the virus is a member of the genus Cytorhabdovirus. Studies on morphology of the virus need to be carried out, although it is likely that the novel gammacytorhaddoviruses affecting ash trees form bacilliform particles (60-75 nm) in the cytoplasm of infected mesophyll cells. The viruses have only recently been identified, therefore many studies are still lacking also on transmission and epidemiology.

1.    Bejerman, N.; Dietzgen, R.; Debat, H., 2023. Novel Tri-Segmented Rhabdoviruses: A Data Mining Expedition Unveils the Cryptic Diversity of Cytorhabdoviruses. Viruses, 15, 2402. https://doi.org/10.3390/v15122402

2.    Rehanek M, Al Kubrusli R, Köpke K, von Bargen S, Büttner C, 2024. Detection of viruses in special stands of common ash reveals insights into the virome of Fraxinus excelsior. Forests 15:1379. https://doi.org/10.3390/f15081379 

3.    Rumbou, A.; Vainio, E.J.; Büttner, C., 2021. Towards the Forest Virome: High-Throughput Sequencing Drastically Expands Our Understanding on Virosphere in Temperate Forest Ecosystems. Microorganisms, 9, 1730. https://doi.org/10.3390/microorganisms9081730

4.    Vainio, E.J., Rumbou, A., Diez, J.J., Büttner, C., 2024. Forest Tree Virome as a Source of Tree Diseases and Biological Control Agents. Curr. For. Rep. 10, 153–174. https://doi.org/10.1007/s40725-024-00214-8