In the fourth edition of our blog series Plant virus snapshots, the whitefly-transmitted ipomovirus Cucumber vein yellowing virus (CVYV) is in the spotlight.
What is the Cucumber vein yellowing virus?
CVYV is an ipomovirus found worldwide, belonging to the Potyviridae plant virus family. It is mainly present in countries of the Mediterranean basin, as well as in some parts of Africa and Asia. The ipomovirus CVYV was first detected in the late 1950s in cucurbits cultivated in the Israeli Jordan valley. This plant virus naturally infects a wide range of cultivated cucurbit crops, such as cucumbers, melon, watermelon or zucchini. In Spain e.g., scientists also recorded its presence in weed species belonging to families other than the cucurbitaceae. Moreover, in Jordan, researchers reported wild cucurbits as hosts for CVYV. As like many other whitefly-transmitting viruses like ToLCNDV or TYLCV, the whitefly vector (i.e. Bemisia tabaci) enables CVYV to rapidly spread in both open and greenhouse crops – even more so in the latter, as the vector is more difficult to control in this situation. The Cucumber vein yellowing virus further appears to be closely related to the sweet potato mild mottle virus (SPMMV), much more than to any other species in the Potyviridae plant virus family.
Virus structure
The ipomovirus Cucumber vein yellowing virus is a typical member of the Potyviridae plant virus family. It consists of a single molecule of positive-sense single-strand RNA. The absence of a coding region for the helper component-proteinase seems to be a distinctive trait of CVYV. Furthermore, studies of the virus have revealed rod-shaped particles 740–800 nm long and 15–18 nm wide. Scientists have also recognised two main CVYV strains from Israel and Jordan, i.e. CVYV-Is and CVYV-Jor. The two strains induce similar vein-clearing symptoms in cucurbits like cucumber and melon, but CVYV-Jor infections in cucumber tend to be more severe.
Transmission
CVYV is transmitted by the whitefly Bemisia tabaci in a semi-persistent manner. The adults of this whitefly can be observed on the backs of the leaves like small insects, that measure a little more than 1 millimetre. Their yellowish body and white wings are due to the secretion of waxes that covers them. Adult insects acquire the virus by sucking on phloem sap for at least 30 min, and can further transmit it to another plant when feeding. The insect retains the virus for 6 hours and has a latent period of 75 minutes. At least 15 to 20 whitefly insects are required per plant for its transmission. CVYV does not circulate or replicate in the insect, and transmission efficiency generally decreases after 4-12 hours. Researchers have found no evidence so far for seed transmission for CVYV, or any other ipomovirus species.
Symptoms
Depending on the cucurbit affected, CVYV symptoms vary in their severity.In cucumber, CVYV causes pronounced vein clearing, chlorosis and general necrosis of the affected plant. Growers can also observe light to dark green mosaic on the fruit (Anonymous, 2001). In both cucumber and melon, symptoms include vein yellowing, vein clearing and stunting with a corresponding yield reduction. Additionally, in Spain sudden death was observed in melon crops. In watermelon, symptoms are often not expressed, but scientists have observed occasional splitting of fruits. Affected zucchinis have a wide range of symptoms however, ranging from chlorotic mottling to vein yellowing, but can also be symptomless. In Spain e.g., it further seems that mixed infections of CVYV and Cucurbit yellow stunting disorder virus (CYSDV) occurs, which boosts the symptoms of both viruses. Overall, CVYV also leads to pinwheel-shaped cytoplasmic inclusions (i.e. typical of the Potyviridae virus family) in infected plant cells.
Virus management and control
Preventive and cultural practices are mainly used to control CVYV. Commercial resistant cucumber varieties are available. Moreover, researchers have identified potential resistance sources in melon, but these are not used commercially. There are no report of CVYV resistance in watermelon yet. It is important to protect cucumber or melon seedlings from infection before transplanting in the field or in greenhouses. Growers should grow the seedlings in a whitefly free environment. In Spanish greenhouse crops, CVYV control further relies on e.g. the use of adequate glasshouse window screens, double doors, treatment of infected vegetable residues, the introduction of a rest period of at least one month between two cucurbit crops, as well as monitoring of Bemisia tabaci whitefly populations. When preventive and cultural management and control methods are not sufficient, CVYV control relies on keeping its whitefly vector Bemisia tabaci in check, such as with the following means:
- Use yellow chromotropic whitefly traps;
- Surveil and control whiteflies in early cultivation stages and seed beds;
- Set up the measures relating to cleaning plant remains and weeds in the greenhouse and surrounding areas. Pull out and immediately eliminate plants affected by CVYV during cultivation;
- Not abandoning the crops and carrying out treatment against Bemisia Tabaci before pulling them out is compulsory as a management method, as well as correctly eliminating the plant remains;
- Regarding chemical control, the Bemisia tabaci whitefly appears to develop resistance to all groups of plant protection products that have been developed for its control. Rotation of insecticides that do not lead to cross resistance should therefore be used to control Bemisia tabaci whitefly infestations;
- The predatory mite Amblyseius swirskii can limit Bemisia tabaci whitefly populations in cucurbit crops by feeding on its eggs;
- Identification of CVYV is possible by RT-PCR testing as well as nucleic acid hybrization;
Current status of Cucumber vein yellowing virus
CVYV still remains a threat for cucurbit production in the affected regions across the globe. Especially in the (eastern) Mediterranean basin, CVYV is a widespread and severe disease of cucurbits. While there are no exact estimates of yield losses for CVYV, this plant virus usually progresses very rapidly in its infection of plants. The logistic regression model best describes CVYV disease progress curves. Whitefly numbers and indoor temperatures tend to have a significant impact on CVYV epidemics. In the oriental Mediterranean basin, CVYV epidemics also occur mostly during the hottest crop seasons, when whitefly populations are high. In international trade, CVYV is most likely to be carried by infected vegetative host material, such as seedlings. However, CVYV is not known to be seed-borne. As of 2023, the EPPO does not consider CVYV to be a major plant pest. The use of CVYV-resistant commercial cucumber cultivars, and the measures applied against the Bemisia tabaci whitefly vector, may have helped reduce its impact, according to the EPPO.
