Golden flecking of red ash privet-leaf blotch disease

Urban pest & disease

Golden flecking of red ash (Fraxinus pennsylvanica var. aucubaefolia)

privet-leaf blotch disease

Pathogen

privet leaf blotch-associated virus (PrLBaV)/privet idaeovirus, (Idaeovirus ligustri)

Bright chlorotic ringspots, irregular spots and lines in a “golden flecking” variety of red ash (Fraxinus pennsylvanica var. aucubaefolia). Examples of symptoms in ash species are provided below.

Infected by PrLBaV exhibiting “golden flecking” of leaves 

Privet (Ligustrum japonicum) infected by PrLBaV shows strong leaf blotching and yellow-cream rings. In privet, the symptoms usually appear only on a few branches, while in Fraxinus, the virus symptoms can be seen in most parts of the canopy.

Scattered canopy of the “aucubaefolia” variety of red ash (Fraxinus pennsylvanica var. aucubaefoliae)

Privet leaf blotch-associated virus (PrLBaV) was first described in association with yellow blotching of privet (Ligustrum japonicum) in Italy, and it has also been mentioned to affect Fraxinus excelsior (Navarro et al. 2017). The golden flecking of F. pubescens “aucubifolia” (syn. F. pennsylvanica var. “aucubaefolia”) has been already reported by Baur (1907) as a graft-transmissible disease. The “aucubaefolia” varieties of red ash are vegetatively propagated as decorative and valuable ornamental trees. However, they are often less resilient than uninfected ashes exhibiting growth retardation under suboptimal environmental conditions and being less tolerant against heat and drought stresses. Modes of virus transmission apart from grafting to susceptible woody host species (Fraxinus spp. and Ligustrum sp.) are not known yet. Leaf symptoms of PrLBaV infected F. excelsior are usually less prominent than those in F. pennsylvanica var. aucubaefoliae.

White flecking of leaves of PrLBV-infected ornamental red ash variety (F. pennsylvanica var. aucubaefoliae).

Red ash (Fraxinus pennsylvanica var. “aucubaefolia”), European ash (Fraxinus excelsior), and privet (Ligustrum japonicum) are natural hosts. White ash is (F. americana) also susceptible, showing symptoms after graft transmission of PrLBaV, suggesting that other species within the Oleaceae family may also be putative host plants.

Chlorotic spots and leaf crinkling in PrLBaV-infected European ash (Fraxinus excelsior).

The use of tested virus-free plant material is strongly recommended, especially when the ash trees are planted as street trees in cities, where they may face adverse environmental conditions (Bandte et al., 2022). Since other viruses have been already reported to infect Fraxinus spp. and privet (Büttner et al. 2023), reliable diagnosis of PrLBaV should be done by molecular methods such as RT-PCR with specific primers from symptom-bearing leaf material according to Navarro et al. (2017).

Not regulated within the EU

In Fraxinus spp. PrLBaV was confirmed in Germany, Poland, Sweden, and Norway. In Italy diseased privet plants were confirmed to be infected by the virus.

PrLBaV is a member of the genus Idaeovirus (family Mayoviridae), a group of positive single-stranded (+ssRNA) viruses with a bipartite genome encapsidated in icosahedral virus particles (Navarro et al. 2017). RNA1 contains a single open reading frame encoding a polyprotein with characteristic motifs of viral RNA methyltransferase, helicase and polymerase. The RNA2 is bi-cistronic encoding a putative movement protein and the viral coat protein, which is likely expressed through a subgenomic RNA. Further investigation is needed into the distribution and host range of PrLBaV, as no systematic surveys have been carried out for instance in tree species of the family Oleaceae, which includes susceptible hosts as ash and privet. Additionally, the modes of virus transmission in both natural and managed habitats need to be investigated. Our studies confirmed that some PrLBaV-infected Fraxinus spp. including the “aucubaefoliae” varieties of red ash show prominent leaf symptoms. It is uncovered that these trees are also infected by a putative novel cytorhabdovirus. However, weather both viruses are necessary to cause the ornamental leaf patterns associated with “golden flecking” needs further investigations in the future.

1.    Bandte M, von Bargen S, Landgraf M, Rybak M, Büttner C., 2022. Neue Erkenntnisse zu Viruserkrankungen an Bäumen und Konsequenzen für die Baumpflege. In: Dujesiefken D. (Ed.), Jahrbuch der Baumpflege 2022, Haymarket Media, Braunschweig, 143-161.

2.    Büttner C, Landgraf M, Fernandez Colino HL, von Bargen S, Bandte M, 2023. Virus diseases of forest and urban trees. Chapter 3. In: Asiegbu F & Kovalchuk A (Eds.): Forest Microbiology. Tree diseases and pests, Volume 3. Elsevier 2023, London, United Kingdom, 61-97.

3.    Baur E, 1907. Über infektiöse Chlorosen bei Ligustrum, Laburnum, Fraxinus, Sorbus und Ptelea. Berichte der Deutschen Botanischen Gesellschaft 25, 410 - 413.

4.    Navarro B, Loconsole G, Giampetruzzi A, Aboughanem-Sabanadzovic N, Ragozzino A, Ragozzino E, Di Serio F, 2017. Identification and characterization of privet leaf blotch-associated virus, a novel idaeovirus. Molecular Plant Pathology 18, 925-936. https://doi.org/10.1111/mpp.12450

5.    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

6.    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