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Susanne von Bargen, Kira Köpke, Artemis Rumbou, Carmen Büttner
Humboldt-Universität zu Berlin, Albrecht Daniel Thaer-Institute, Division Phytomedicine, Berlin, Germany
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AsMaV is a typical member of the emaravirus genus affecting Eurasian aspen and widespread in Fennoscandia. The virus has been confirmed as the causal agent of the mosaic disease of aspen (Rehanek et al. 2022) which is characterized by distinct leaf symptoms such as mosaic, mottle, yellow blotching, variegation, and chloroses along veins. The mosaic disease of aspen has been shown to be graft-transmissible in Populus tremula (von Bargen et al. 2020). AsMaV was confirmed as causal agent of the disease by detection of AsMaV using virus-specific RT-PCR in grafted scions and symptomatic rootstocks, in combination with HTS analyses, showing that AsMaV was the only virus detectable in the infected scions and rootstocks. Mechanical transmission to a set of herbaceous indicator plants failed to establish a stable AsMaV infection in Nicotiana benthamiana, N. tabacum cv. samsun, N. clevelandii, Cucumis sativus, Chenopodium quinoa, C. amaranticolor or Phaseolus vulgaris. However, the virus was detectable by virus-specific RT-PCR in P. vulgaris and N. benthamiana, that were directly rub inoculated with plant sap from P. tremula. Despite this, no symptoms were induced, and propagation to new indicator plants of the same species was unsuccessful. As other emaraviruses, AsMaV is considered to be naturally dispersed by eriophyoid mites. Although several mite species were found to infest mosaic-diseased aspen (Phyllocoptes populi, Eriophyes diversipunctatus), the vector has not been identified until now (von Bargen et al. 2020).
AsMaV-infected leaves of Eurasian aspen (P. tremula) showing vein yellowing (left) and yellow blotching (right)
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Eurasian aspen (Populus tremula) is the only known natural host species, which has been confirmed to be infected by AsMaV.
Mosaic symptoms in leaves of AsMaV-infected aspen (P. tremula)
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The use of tested virus-free plant material are highly recommended. Although AsMAV-induced leaf symptoms are very distinct in Eurasian aspen they may vary and can therefore be confused with leaf chloroses. Chloroses are known to be induced by other plant viruses known to affect Populus spp. (Büttner et al. 2023). However, reliable diagnosis of AsMaV is possible by virus-specific RT-PCR or RT-PCR using genus-specific primers in combination with Sanger-sequencing (Rehanek et al. 2022; von Bargen et al. 2020). In case, mosaic-diseased aspen trees show extensive signs of degeneration due to further stress factors in combination with the virus infection, it has to be considered - after evaluating ecological and economic aspects of the particular stand - to remove the trees, as they cannot be cured from the disease.
Leaves of AsMaV-infected Eurasian aspen (Populus tremula) showing chlorotic spots, variegation, and mottle.
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AsMaV (genus Emaravirus, family Fimoviridae) is a single-stranded RNA virus with a negative orientation. Its genome consists of five RNAs. The RNA1 (7.1 kb) encodes for the viral RNA dependent RNA polymerase (RdRP, 268.2 kDa) and it is involved in virus replication. The RNA2 (2.3 kb), RNA3 (1.6 kb), RNA4 (1.6 kb), and RNA5 (1.3 kb) encode for the glycoprotein precursor (GPP, 73.5 kDa), viral nucleocapsid protein (NP, 35.6 kDa), a putative movement protein (MP, 41.0 kDa), and a protein of unknown function (P28, 28.1 kDa), respectively (von Bargen et al. 2020). Sequence comparisons of RNA1-RNA4 confirm that the virus is a member of the genus Emaravirus (von Bargen et al. 2020; Rehanek et al. 2022). Studies on the morphology of the virus are missing. However, it is likely that AsMaV forms spherical particles of varying size (80-100 nm) in the cytoplasm of infected mesophyll cells as described for other emaraviruses (Rehanek et al. 2022). The main routes of virus transmission have not been investigated in detail so far. As mechanical transmission of this virus appears to be difficult, vector transmission by different gall mite species found to be associated with the mosaic disease of aspen has to be considered responsible for the wide natural dissemination of AsMaV in Fennoscandia. Furthermore, vegetative transmission may play a role, as Eurasian aspen propagates effectively by root suckers (von Bargen et al. 2020). The natural host range of the virus remains to be determined as AsMaV was only confirmed in Eurasian aspen displaying characteristic symptoms. Other poplar species showing similar virus-like symptoms were found to be infected by other viruses such as poplar mosaic carlavirus (PopMV) (von Bargen et al. 2020; Büttner et al. 2023). Since interspecies hybrids of aspen and poplar occur quite frequently in nature and are also used in forestry, it can be assumed that the natural host range of the virus extends to species related to Eurasian aspen.
AsMaV-infected Eurasian aspen (P. tremula) showing extensive dieback of twigs and branches in the lower parts of the canopy
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