Cucurbit yellow stunting disorder

Cucurbit yellow stunting disorder is a virus in the Crinivirus genus, which includes lettuce infectious yellows virus and potato yellow vein virus. The natural hosts of CYSDV are restricted to Cucurbitaceae: watermelon, melon, cucumber, courgette. In addition lettuce has been found to be an experimental host The virus is spread by the sweet potato whitefly (Bemisia tabaci).

Symptoms
Cucumbers and melons affected by CYSDV show severe yellowing symptoms that start as an interveinal mottle on the older leaves and intensify as leaves age. Chlorotic mottling, yellowing and stunting occur on cucumber and yellowing and severe stunting on melon.

Symptoms on cucurbit crops are said to be indistinguishable from those caused by Beet pseudo-yellows virus In experimental transmission experiments, chlorotic spots along the leaf veins of the melon cv. ‘Piel de Sapo’ were noticed after 14–20 days. Sometimes, initial symptoms also consisted of prominent yellowing sectors of a leaf. Symptoms evolved later to complete yellowing of the leaf lamina, except the veins, and rolling and brittleness of the leaves.

Control
The control of CYSDV centres on the control of its vector, the sweet potato whitefly (Bemisia tabaci), and elimination of sources of infection. In particular, cucurbit seedlings for planting should come from disease-free stocks.

Chemical control
Chemical control of populations of B. tabaci to levels that result in a signiﬁcant drop in disease incidence has proved difﬁcult. In general, chemical control of the vectors of Closteroviridae has not been effective in preventing the spread of the diseases they cause. Some of the difficulties are the wide host range of the vector, the presence of the whiteﬂy on the undersides of leaves, the extreme motility of adults and the ability of B. tabaci to develop resistance to most classes of existing insecticides. Many conventional insecticides such as organophosphorus compounds, carbamates and pyrethroids have effectively reduced whiteﬂy populations, but provided only partial virus control even when sprayed as frequently as 2–3 times a week.

Imidacloprid, a systemic insecticide that can be applied to soil and foliage, is used to control whiteﬂies, but resistance is now reported. Insects resistant to aldicarb and buprofezin were also detected. The parasitic wasp Encarsia formosa and the fungus Verticillium lecanii can be used as biological agents against B. tabaci, but are unlikely to affect virus transmission.

Roguing
Roguing infected cucurbit plants and removing overwintering crops early in the spring prior to the emergence of adult whiteﬂies may prove useful. To be effective, this sort of control measure should be applied over a whole area and preferably where there is no continuous production in glasshouses, which are often the sites of whiteﬂy activity and active virus spread throughout the year. Weeds in and surrounding glasshouses should also be destroyed as they could act as hosts for B. tabaci.

Mulching
In Israel, covering the soil with a mulch of sawdust, fresh wheat straw or yellow polyethylene sheets has markedly reduced populations of B. tabaci. Whiteﬂies are attracted to the yellow colour and are killed by the heat. The fading of the mulch colour and changes in the ratio of canopy to mulch area is believed to cause a reduction in control.

Interplanting
Interplanting with a species that is a good host for the vector, but not the virus may reduce virus incidence.

Netting
In Lebanon, insect-proof nets and sticky yellow traps are used for control. Growing plants under physical barriers, such as low mesh tunnels and shade-cloth, may also have a positive effect.

Resistant cultivars
No resistant cultivars of susceptible hosts are currently available commercially.