Carrot virus Y

The symptoms of CarVY in carrot foliage may include chlorotic mottle (irregular small yellow patches), marginal leaflet necrosis or reddening, generalised chlorosis of leaves, and plant stunting. Necrosis occurs when a living organism's cells or tissues die or degenerate. Plant necrosis causes leaves, stems and other parts of the plant to darken and wilt. This is a result of the virus using the plant as a host, as the plant's energy is diverted to the virus rather than to the plant's growth. Over time, this energy depletion leads to necrosis in the infected area where the virus is replicating itself.[3] In plant chlorosis, tissues that are normally green become pale, yellow, or bleached. This results from chlorophyll failing to develop due to the infection of the virus.[4] Additionally, the infected plant can portray a ‘feathery’ appearance due to an increase in the subdivisions of the carrot leaflets.[1][5]

Leaf symptoms and plant stunting symptoms differ greatly between different carrot varieties. Some plants will develop obvious symptoms that can be easily seen from a distance. However, others can express symptoms that are so mild that they are difficult to see even upon close inspection of affected plants. In actively growing carrot crops, foliage symptoms of CarVY are easily confused with the symptoms of nutritional deficiencies. Additionally, growers routinely stop adding fertilizers at the end of each growing season to reduce post-harvest carrot disorders. This results in leaf yellowing symptoms, which can be confused with the symptoms that resemble those caused by CarVY.[5]

The roots of carrot plants also become severely deformed as a result of CarVY infection. When carrot plants become infected with CarVY at an early stage in the growth cycle, they portray stubby roots with severe distortion and knobbliness (the ‘Michelin carrot’ syndrome). Carrots that are infected later in the life cycle are thinner and longer than carrots infected earlier. Furthermore, in plants infected early, the tops of the roots tend to emerge from the soil and become exposed to the sun. This results in the ‘shoulders’ of the carrots turning a green color. Early infected plants also have severe distortion of the internal vascular cambium. Instead of its normal circular shape, carrots exhibit a star-like vascular cambium consisting of multiple contortions.[5]

Entire fields have been abandoned due to crop infection. The severe root defomalities associated with early virus infection make the carrots unmarketable. Carrots infected later in the growth cycle do not display such dramatic deformations, but are still considered less-desirable and are sold and a significantly lower price.[1]

The non-segmented, linear genome of CarVY is 1,754 kb.[7] CarVY is typically 11 nm wide and 770 nm long.[6]

Myzus persicae, the green peach aphid, is an effective transmitter of Carrot virus Y

CarVY is transmitted between plants by aphids, including those that do not normally colonize carrots. Aphids are soft-bodied insects that feed on plant sap. Their sucking mouthparts allow for viruses to be transmitted between plants. With their ability to reproduce asexually, they are able to multiply and colonize very quickly. This allows them to spread a virus very quickly. Myzus persicae (M. personae), a small green aphid most commonly known as the green peach aphid, is the most efficient transmitter of the virus.[8][2] By feeding on the sap of the carrot plant, aphids ingest the virus of the host, which they can later transmit to other uninfected plants that they will feed on. Other colonizing species, such as Aphis spiraecola (green citrus aphid) and D. apiifolia (hawthorn parsley aphid), can also be successful virus vectors. Non-colonizing species such as Lipahis erysimi (turnip aphid) and Acyrthosiphon kondoi (bluegreen aphid), may also serve as efficient transmitters.[8]

It is unknown if infected seeds can transmit the virus to other seeds. If CarVY is confirmed to be transmissible in future research, it is expected that it will only be at low levels, as Lettuce mosaic virus (LMV), a potyvirus, is only transmissible at very low levels. LMV is a potyvirus that affects vegetable crops. If seed transmission is confirmed, seed stock will need to be routinely tested before sowing.[5]

The first step in viral transmission is acquisition, which consists of the uptake of the virus from an infected source. Aphids are well designed for their roles as vectors. Their mouthparts consist of a needle-like stylet that is capable of piercing plant cells walls in order to feed on the plant’s sugary sap. Aphids can facilitate the uptake and delivery of virions into plant cells without causing too much irreversible damage to the host.[9]

The purpose of the second step of transmission is the stable retention of acquired virions in the vector at specific sites. A virus destined for inoculation is retained at sites within the stylet and food canal, or in the foregut. Potyvirus virions cannot be transmitted alone. Rather, they associate with a second viral-encoded accessory protein, termed “helper component” (HC). The virion and the HC associate with the cuticular lining of the aphid food canal. The “bridge hypothesis” helps explain how the HC functions as a bridge that attaches virions to the food canal of the aphid vector. The HC is a multifunctional protein, and mutations in the domains that make up the HC protein can affect transmission.[9]

The final step in transmission is inoculation. Inoculation consists of the release of bound or retained virions and their delivery to a site of infection. The needle-like stylet delivers virions to the new host plant. Salivation may be important in enhancing the release of bound virions and their delivery into plant cells.[9]

When it comes to the control of CarVY, the key measures include avoiding spread from nearby infected carrot crops, minimizing spread from volunteer carrots, and introducing non-host rotational crops.[5] One of the easiest control methods is done by planting crops that are not in close proximity to each other. This avoids spread from potentially infected carrot crops. Additionally, the spread from volunteer carrot crops can be eliminated by spraying volunteer crops with herbicide, or deep ploughing before re-sowing the land with carrots. This removes potential virus infection sources and prevents them from spreading to new plantings. Another control method consists of manipulating the planting date of carrot crops in order to avoid exposure to peak aphid populations. When plants are at their most vulnerable growth stage, the symptoms of the virus become more severe as the crops develop. Avoiding peak aphid populations reduces the chance of infection at early growth stages, as plants infected later produce less damaged carrots.[12][5]