Viral, Mycoplasma and Viroid Plant Disease

Primary reference(s)

FAO and IPPC, 2016. ISPM 27 Diagnostic protocols for regulated pests. DP 12: Phytoplasmas. Accessed 4 November 2020.

Hammond, R.W. and R.A. Owens, 2006. Viroids: new and continuing risks for horticultural and agricultural crops. Online. APSnet Features. doi: 10.1094/APSnetFeature-2006-1106. Accessed 4 November 2020.

Nakashima, K. and N. Murata, 1993. Destructive plant diseases caused by mycoplasma-like organisms in Asia. Outlook on Agriculture, 22:53-58.

Rubio, L., L. Galipienso and I. Ferriol, 2020. Detection of plant viruses and disease management: relevance of genetic diversity and evolution. Frontiers in Plant Science, 11:1092.

Additional scientific description

The terminology ‘outbreak’ is used if the disease occurs in a limited location or area, and ‘pandemic’ if it occurs in multiple regions and at world scale.

Plant diseases are a major concern globally, given their potential economic impact on populations that rely on income from selling crops. The impact in places where there is increased food scarcity is of particular concern, as it exacerbates the insecurity and poverty felt by populations affected by these diseases. Viruses, viroids and mycoplasmas (MLOs) are among the main disease agents causing significant crop losses, depending on crops and local conditions.

Mycoplasma lack cell wall(s) bounded by unit membrane(s) and have cytoplasm ribosomes and strands of nuclear material (Team Agri, 2017). Mycoplasmas are transmitted through insects that feed on plants, including plant hoppers and psyllids (Team Agri, 2017). Plant viruses are small microorganisms without cell walls, containing nucleic acids surrounded by protein coats and replicating only inside living cells (Gergerich and Dolja, 2006). Mycoplasmas are bacteria lacking cell walls and being categorised under the genus Mycoplasma. They are highly resistant to antibiotics and can be saprophytic or parasitic to plants (Britannica, 2019). Viroids are microorganisms that consist of a small circular RNA molecule, and are smaller than viruses and capable of causing certain plant diseases (Britannica, 2020).

The occurrence of mycoplasma in plants is a particularly difficult hazard to address, since they are ‘the perfect parasite’ and adapt to their host in a way that serves all their nutritional needs (Cleanroom Technology, 2008). While mycoplasma generally do little harm, in the right circumstances they can provoke serious responses from their host causing illness and death in plants (Cleanroom Technology, 2008). Mycoplasmas – and associated wall-less prokaryotes – first came into the global scientific consciousness in the 1960s when Japanese workers noted an increasing prevalence of these organisms in the plants they were working with (Arora and Sinha, 1988). Since then, the association of mycoplasma and other plant virals/viroids with negative agricultural outcomes has shot up, with 100 diseases now being associated with these vectors (Arora and Sinha, 1988).

Metrics and numeric limits

Not identified.

Examples of drivers, outcomes and risk management

Viruses are among the major causes of plant diseases in many plants, some being highly significant. Spread of viruses take place through the movement of infected plant materials or through insect vectors such as aphids, white flies and mealybugs. Some nematodes and fungi are also reported to transmit viruses. Thus, integrated management approaches should consider the control of insect vectors as well as use of virus-free planting materials. Diseases of economic importance caused by viruses include tobacco mosaic virus, cassava viruses, banana bunchy top virus, tomato yellow leaf curl virus, cucumber mosaic virus, potato virus Y, plum pox virus and potato virus X, citrus tristeza virus and barley yellow dwarf virus (Gergerich and Dolja, 2006).

Mycoplasmas and viroids are not considered among the major causes of plant diseases but where circumstances are conducive, they can cause outbreaks or in some cases epidemics causing economic losses under local conditions. Spread of mycoplasmas and viroids occur mostly through either movement of infected plant materials or vectors such as leafhoppers in the case of mycoplasmas, and aphids in the case of viroids. For effective control and integrated management of these pathogens, use of pathogen-free clean planting materials, crop hygiene and control of vectors are important measures (Team Agri Info, 2017).

Major mycoplasma diseases include witches’ broom of woody plants, X disease of peaches, coconut lethal decline, grapevine yellows, apple proliferation, while viroids causing significant diseases include potato spindle tuber viroid, citrus infected with citrus exocortis viroid and avocado sunblotch viroid.