7.02.04 - Phytoplasma and virome of forest and urban trees
Plant Viruses (a comment by Carmen Büttner and Risto Jalkanen)
A lack of knowledge of the presence and frequency leads to the impression that viral diseases in forest trees are rare and therefore not important. The opposite is true.
Continuing surveys of forested areas, public greens and of young seedlings in nurseries seem to confirm the dispersal of viruses in many plants with virus associated symptoms. Viruses induce alterations in a tree's metabolism and alter plants predisposition. Any infected plant has to be considered as an infection source. Young plants from nurseries might later be planted in public gardens and expand infections over a large area. From the economic point of view one should be aware that virus diseased plants may increase production costs because of the possibly decreased growth of infected stock plants and that may damage subsequent field performance. Viruses are responsible for far greater economic losses than generally recognized, because due to extremely high diversity in the symptom appearance they are very different from those attributed to bacteria and fungi. Losses are often more insidious and less conspicuous and therefore go unnoticed and untreated
Plant viruses play a central role in the plant health status of forest trees because of high degree of disease that leads to extensive tissue damage. Viral pathogens are present in plants in every ecosystem and induce substantial losses worldwide in agriculture, horticulture and forestry . There is an urgent need for reliable methods for virus detection and identification in forest trees as well as tools for disease management to result in higher yields and quality of wood products. Knowledge concerning virus characteristics (identification of viruses) followed by their epidemiology are the first steps to develop appropriate phytosanitary strategies to produce virus free plants and to keep tree seedlings free of plant viruses . The mode of transmission has to be considered as an important factor affecting the spread and impact of a virus infection within a forest.
Viruses have been recovered and identified from many deciduous fruit crops and a scattered number of forest trees.
To date, viruses from 17 different tree species have been identified. The latest EMARaV has recently been classified by the international Committee on Taxonomy of Viruses (ICTV)). Interestingly some genera, such as Carpinus and Sambucus, are susceptible to only one or two virus species while others such as Betula and Fraxinus are hosts for multiple virus species. Because of the economic and ecologic importance of forest trees for industry, fuel and in future in compensating impacts of climate change, there is a need to increase research on plant viruses. Obviously viruses have infected trees and caused disease for centuries before they were first detected and demonstrated to be the causal agents. As stated above, virus symptoms often go unrecognized, because they produce visible symptoms in some varieties, but remain latent in others. Conversely some viruses produce distinctive symptoms whereas others are more difficult to detect visually. As an example, studies confirmed a wide distribution of CLRV in Finnish birch species causing significant reductions in vitality and yield of trees whereas CLRV infection in olive trees is symptomless and does not cause significant damage. Furthermore, symptoms caused by mineral deficiency, ozone, salt stress or drought can be confused with chlorotic and necrotic discolorations induced by biotic agents such as fungal or viral pathogens and sucking insects. Grafting and budding failures, which still are frequent in fruit and ornamental trees, were often considered due to incompatibility problems associated with a particular rootstock–scion cultivar combination. In reality viruses are frequently responsible for these complete or partial incompatibilities and bud-take problems. This problem is often observed in fruit trees affected by viruses.
- Büttner C., von Bargen S., Bandte M., Mühlbach H.-P., 2013. Forest diseases caused by viruses. In: Infectious forest diseases. Gonthier P., Nicolotti G. (eds), CABI, p. 50-75.
Phytoplasmas (a comment by Michael Kube)
Phytoplasmas are characterised as obligate intracellular parasites of the plant phloem. They are assigned to the provisory taxon 'Candidatus Phytoplasma' in the class Mollicutes. These phytopathogenic bacteria are associated with diseases in more than a thousand plant species. Beside the strong body of research on phytoplasmosis in crop plants, phytoplasma infections have also been diagnosed in forest trees. With a few exceptions, such as 'Ca. P. pini', most phytoplasma strains have been grouped according to phylogenetic analyses and named with respect to symptoms or colonised trees. A few prominent examples comprise Paulownia witches' broom, Ash yellows, Black locust witches' broom, Chestnut witches' broom, Eucalyptus little leaf, Poplar witches' broom, Elm yellows and Alder yellows. Today, only limited knowledge is provided on strains and frequency of infection in forest. In particular, the percentage of mild or tolerant infections cannot be estimated at all. Hence, the general impact of phytoplasmoses in forest on wood productivity and quality as also its impact on the lifespan of trees remains unclear due to this this situation. This gap of information remains remarkable considering that infection rates up to 100% are observed for some common trees such as Alnus spp. indicating the need of phytoplasma research on forest trees today.