**3.1 Transmission of viruses, phytoplasmas and proteobacteria in host plants by dodder**

Majority of agriculturally important plant viruses and phytoplasmas are dodder transmissible and among which *Cuscuta* species, *C. campestris* and *C. subinclusa,* are the most common. This is because the *Cuscuta* absorption system for host fluids is directional and has proven to be very effective and has shown rigorous sinking of resources during the host fruit development [46]. Some researchers in their studies with beet curly top virus and cucumber mosaic virus have shown that dodder assimilates virus particles along with nutrients from the host phloem and accumulate in the haustorium of the parasite [56]. While some phytoplasma like Aster yellows move from the phloem of the dodder towards phloem of the healthy host by a 'temporaryreversal' of phloem nutrient flow; others like in mosaic-type viruses like cucumber mosaic virus and beet curly top viruses, that move from the parenchyma of the haustorium to the host occurs through plasmodesmatal connections or from the bare


#### **Table 1.**

*Examples of plant viruses, phytoplasmas and proteobacteria transmitted by different dodder species.*

#### *Parasitic Plants as Vectors for Pathogens DOI: http://dx.doi.org/10.5772/intechopen.100187*

protoplasmic connections of dodder [57]. Regardless of the association between host and the parasite along with the directional movement of nutrients in the phloem, several other factors might play their part. For instance, an inhibitor in the sap of dodder have been proposed as contributing factor for poor transmission of *Tobacco mosaic virus* in some hosts [57, 58]. It is demonstrated that tobamoviruses (type spcies: *Tobacco mosaic virus*) are neither persistent nor multiply in dodder, whereas, *Cucumoviruses* (type spcies: *Cucumber mosaic virus*) persists and multiplies, causing disruption of growth in dodder. Hence, there are around 200 species of dodder, out of which some like *C. campestris* parasitises more than 100 diverse plant species and are capable of transmitting viruses between host species [57]. Moreover, several other parasitic angiosperms such as broomrape (*Phelipanche aegyptica*) can transmit viruses between taxonomically different plant families. However, whether the virus was persistent or developing inside the parasitic plant has not been thoroughly investigated [8].

Although the transmission of phytoplasma is quite similar to plant viruses, they are quite understudied. Most interactions of parasitic plants with phytoplasma necessarily are experimental in laboratory or greenhouse with special reference to dodder mediated transmission. Dodder acquires the phytoplasma cells from the infected plant via haustoria in the direction of the source of inoculum to the healthy host and progresses in the direction of the growing points [9]. However, the efficiency of transmission depends on different combinations of phytoplasma and dodder species. In an experimental trial, it was seen that rubus stunt and cotton phyllody were transmitted in higher frequencies by *C. europea* and *C. campestris*, whereas, other several phytoplasmas causing pear decline, stone fruit yellows and *Picris echioides* yellows by *C. odorata, C. reflexa* and *C. campestris*, respectively were transmitted less effectively [59]. Transmission of plant viruses and phytoplasma to healthy plants via parasitic plants as vectors seems unlikely to cause novel primary virus infection chain, as evidence of parasitic seed-virus/ phytoplasma transmission is missing, but can have impact on existing primary or secondary infection [16]. In addition, it should be taken into consideration that in general, parasitic plants are known to have a diverse natural host range, which can provide exceptionally high risk of novel virus or phytoplasma transmission between donors and recipients in natural as well as managed vegetation. During the years 1940 to 1960, many dodder-transmissible viruses or phytoplasmas were found and vividly studied [57]. However, these studies now have rapidly decreased and has just limited to experimental hosts (**Table 1**) to offer possibility of studying the nature of different virus transmission to taxonomically same or varied crop species [64].
