**1. Introduction**

Citrus is a genus of the Rutaceae family that is flowering, small to moderate size of trees. The genotype of citrus is available on the Ensemble website (http://www. ensembl.org) [1]. Citrus usually grows in the tropical and subtropical regions and they produce hesperidium fruit with a leathery rind. Important crops harvested from these plants are Oranges, Lemons, Grapefruits, Pomelo, and Limes. Because of their taste and benefits for humans, most countries cultivate them. The amount of citrus production around the world is 96,812,090 tons in 2020 and the amount of area harvested is 521,000 hectares in 2020 [2]. Like other biotic organisms, plants have various kinds of diseases and pests. So citrus as the same like others can be infected by viruses, nematodes, bacteria, and fungi that can impact the amount of production. One of the agents that induce loss of production is viruses. Viruses are the smallest organism in the world and they have 16–300 nm in diameter. It usually includes nucleic acids and proteins that make a particle of a virus called a virion. For a long time, they are adopted specific organisms and cells and almost all of them require alive cells or organisms for replication. Viruses are intracellular parasites because they used protein synthesis machinery (ribosomes) in their hosts. In the other words, infected cells produce viruses' proteins and other components that viruses need to infect and live. Furthermore, they achieve their energy from energy-generating metabolic pathways' host [3].

Plant viruses are the most indispensable agent that causes huge economic losses. They are widespread and almost all plant species can be infected by viruses. Characteristics of plant viruses are the same as animal viruses but their host is different. Plant viruses cannot infect animals and animal viruses cannot cause diseases in plants. Due to these features, recently, biotechnologists use plant viruses for curing some diseases like cancer in humans. Until now, plant pathologists can detect various kinds of plant viruses in plants. Because of the importance of citrus among other crops, various kinds of viruses are introduced that we mention in this chapter. In this chapter, you will read about the most important citrus viral disease (Tristeza) and after that, you know about other virus diseases that are reported. Viroids are another casual disease in plants [4]. Viroids are a subgroup of viruses. Some characteristics of viroids are similar to viruses although they have some differences like coat protein that viroids do not have and their genome is naked. Usually, their genome is circular in host cells [5–7]and the replication ways are different. They follow a rolling circle mechanism for propagation [8, 9]. The genome of viroids does not translate, so they cannot produce any proteins. The infection of viroids is occurred by interaction with the host factors in order to replicate and exert their pathogenic effects [9]. this casual disorder can cause diseases in citrus and we review it in this chapter. We will read about Exocortis and cachexia both being induced by viroids and they are a significant effect on the production amount. We try to discuss other viroids that recently they are reported by researchers. Briefly, in this chapter, you know about different viruses and viroids that are important in the citrus industry.
