**1. Introduction**

The contribution of global seed trade to modern agriculture is clearly emphasized in breeder's broad range of crop species that contribute to increased yield, long shelf life, pathogen resistance, and tolerance to extreme circumstances (e.g., salinity, drought, and arid climate

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conditions) around the world. However, spread of seed-borne viruses is an unfortunate side effect of this industry. Seed transmission of plant viruses has a great epidemiological significance causing disease outbreaks worldwide. Seed trade globalization has opened new pathways for the mobilization of crop produce between continents and countries. In the modern agriculture, new crop varieties are introduced into new growing areas and are cultivated in proximity to indigenous crops, which are exposed to the imported remote endemic diseases. Seed-borne plant viruses are a threat to world agriculture. Among them, species that belong to the genus *Tobamovirus* are considered a major peril to a range of cultivars especially to those belonging to the *Solanaceae* and *Cucurbitaceae* (cucurbit) families. These viruses have long been a threat to agriculture. For more than a century, tobacco, tomato, and pepper plants that belong to the *Solanaceae* family are infected by the *Tobacco mosaic virus* (TMV) [1, 2], *Tobacco mild green mosaic virus* (TMGMV) [3], *Tomato mosaic virus* (ToMV), [4] and *Pepper mild mottle virus* (PMMoV) [5]. Similarly, cucurbits are infected by the *Cucumber green mottle mosaic virus* (CGMMV) that was discovered in England in 1935 [6] and spread into neighboring countries in Europe and to the Middle East, Asia, and the Far East. In the last decade, CGMMV was detected in new countries and continents—Canada [7] and the USA [8] in North America and Australia [9]—recently reviewed in [10], achieving a global distribution and becoming a major threat to cucurbit industries worldwide. Among the main cucurbit-infecting tobamoviruses, CGMMV is the most economically important and currently considered a significant threat for the production of cucumber, melon, watermelon, gherkin, and pumpkin, which has been endangered by the globalized spread of the *Tobamovirus* (**Figure 1**). Additional *Tobamovirus* species that infect cucurbits need to be considered in seed transmission tests in countries that commercially produce seeds: the *Kyuri green mottle mosaic virus* (KGMMV) [11, 12], the *Zucchini green mottle mosaic virus* (ZGMMV) [13], and the *Cucumber fruit mottle mosaic virus* (CFMMV) [14].

The tobamoviruses are seed-borne, mechanically transmitted stable viruses. Infectious particles are primarily attached to the seed coat [4]. Indeed, viral inoculum is efficiently transmitted when it enters the embryo, and viruses attached to the seed coat may not survive germination when seed coat separates from the seedlings [15]. However, in nursery seedlings, the *Tobamovirus*-contaminated seed coat may affect the wounded roots, which occur upon transplantation [4]. Furthermore, low transmission rate to seedlings does occur when tobamoviruses contaminate the seeds. Seeds or seedlings are used in large scale continuously in regular farming. Therefore, even a low percentage of contaminated seeds can cause a multitude of infection foci [16]. Consequently, the primary infectious source can be spread rapidly by mechanical contacts, workers' hands, tools, greenhouse structure, and trellising ropes [17, 18] and the tractor path in open fields [19]. The infectivity of tobamoviruses is preserved in plant debris and in the contaminated soil and clay for months up to years [20].

The entry and the establishment of new *Tobamovirus* diseases inflict a major concern for growers around the world. Seed nurseries, especially those that produce grafted seedlings (a procedure that requires cutting and hand handling), and protected structures (e.g., greenhouses, net houses, walk-in tunnels), which are commonly used worldwide, mostly with monoculture Seed Transmission of Tobamoviruses: Aspects of Global Disease Distribution http://dx.doi.org/10.5772/intechopen.70244 235

conditions) around the world. However, spread of seed-borne viruses is an unfortunate side effect of this industry. Seed transmission of plant viruses has a great epidemiological significance causing disease outbreaks worldwide. Seed trade globalization has opened new pathways for the mobilization of crop produce between continents and countries. In the modern agriculture, new crop varieties are introduced into new growing areas and are cultivated in proximity to indigenous crops, which are exposed to the imported remote endemic diseases. Seed-borne plant viruses are a threat to world agriculture. Among them, species that belong to the genus *Tobamovirus* are considered a major peril to a range of cultivars especially to those belonging to the *Solanaceae* and *Cucurbitaceae* (cucurbit) families. These viruses have long been a threat to agriculture. For more than a century, tobacco, tomato, and pepper plants that belong to the *Solanaceae* family are infected by the *Tobacco mosaic virus* (TMV) [1, 2], *Tobacco mild green mosaic virus* (TMGMV) [3], *Tomato mosaic virus* (ToMV), [4] and *Pepper mild mottle virus* (PMMoV) [5]. Similarly, cucurbits are infected by the *Cucumber green mottle mosaic virus* (CGMMV) that was discovered in England in 1935 [6] and spread into neighboring countries in Europe and to the Middle East, Asia, and the Far East. In the last decade, CGMMV was detected in new countries and continents—Canada [7] and the USA [8] in North America and Australia [9]—recently reviewed in [10], achieving a global distribution and becoming a major threat to cucurbit industries worldwide. Among the main cucurbit-infecting tobamoviruses, CGMMV is the most economically important and currently considered a significant threat for the production of cucumber, melon, watermelon, gherkin, and pumpkin, which has been endangered by the globalized spread of the *Tobamovirus* (**Figure 1**). Additional *Tobamovirus* species that infect cucurbits need to be considered in seed transmission tests in countries that commercially produce seeds: the *Kyuri green mottle mosaic virus* (KGMMV) [11, 12], the *Zucchini green mottle mosaic virus* (ZGMMV) [13], and the *Cucumber fruit mottle mosaic virus*

The tobamoviruses are seed-borne, mechanically transmitted stable viruses. Infectious particles are primarily attached to the seed coat [4]. Indeed, viral inoculum is efficiently transmitted when it enters the embryo, and viruses attached to the seed coat may not survive germination when seed coat separates from the seedlings [15]. However, in nursery seedlings, the *Tobamovirus*-contaminated seed coat may affect the wounded roots, which occur upon transplantation [4]. Furthermore, low transmission rate to seedlings does occur when tobamoviruses contaminate the seeds. Seeds or seedlings are used in large scale continuously in regular farming. Therefore, even a low percentage of contaminated seeds can cause a multitude of infection foci [16]. Consequently, the primary infectious source can be spread rapidly by mechanical contacts, workers' hands, tools, greenhouse structure, and trellising ropes [17, 18] and the tractor path in open fields [19]. The infectivity of tobamoviruses is preserved

in plant debris and in the contaminated soil and clay for months up to years [20].

The entry and the establishment of new *Tobamovirus* diseases inflict a major concern for growers around the world. Seed nurseries, especially those that produce grafted seedlings (a procedure that requires cutting and hand handling), and protected structures (e.g., greenhouses, net houses, walk-in tunnels), which are commonly used worldwide, mostly with monoculture

(CFMMV) [14].

234 Advances in Seed Biology

**Figure 1.** *Cucumber green mottle mosaic virus* (CGMMV)-infected cucurbit species. (**a, b, e, i**) Mild mottling and mosaic on infected leaves of cucurbits: (**a–b**) cucumber (*Cucumis sativus*), (**e**) melon (*Cucumis melo*), and (**i**) watermelon (*Citrullus lanatus*) plants. (**c–d**) Mosaic pattern developed on cucumber fruits. (**f–h**) Mosaic pattern and mottling developed on melon fruits. (**j–k**) Yellowing fruits and spongy flesh (double-headed arrows) accompanied by necrotic lesions on stems and peduncles (single-headed arrows) of watermelon fruits.

crops (without crop rotations between growing cycles), are highly susceptible to *Tobamovirus* infection. When a new *Tobamovirus* disease emerges in a new country, the first response is to try to eradicate the disease, a strategy that is unfortunately prone to failure in most of the scenarios. The major reasons for this failure may be attributed to multiple contaminated seed entry events that occur in parallel, in multiple growing areas/farms, and late detection and response. The second strategy upon the emergence of a new *Tobamovirus* disease is based on management of the disease, which combines several approaches to contain the disease and reduce the disease damage below the economic threshold. A proper management needs to cover all aspects of plant growth that are involved in disease spread: disinfection of contaminated soil from the previous growing cycle, trellising ropes and the greenhouse structure, worker hands and clothes, etc. This approach however, cannot ensure low disease damage and further emphasizes the importance of proper management of commercial seed production, accompanied by the appropriate diagnostic approaches in order to eliminate *Tobamovirus*-contaminated seed lots.
