Cucurbit yellow stunting disorder virus

Pictures

Picture	Title	Caption	Copyright
Title Symptoms Caption Cucurbit yellow stunting disorder virus; symptoms on infected honeydew melon plants. The disease progresses from initial mottling of leaves, to leaves that are completely yellow, with veins sometimes remaining green. The oldest leaves develop symptoms first, thus these show the pronounced yellow colouration. USA. Copyright ©Mike Matheron/University of Arizona/Bugwood.org - CC BY 3.0 US	Symptoms	Cucurbit yellow stunting disorder virus; symptoms on infected honeydew melon plants. The disease progresses from initial mottling of leaves, to leaves that are completely yellow, with veins sometimes remaining green. The oldest leaves develop symptoms first, thus these show the pronounced yellow colouration. USA.	©Mike Matheron/University of Arizona/Bugwood.org - CC BY 3.0 US

Identity

Preferred Scientific Name

• Cucurbit yellow stunting disorder virus

English acronym

• CYSDV

Taxonomic Tree

• Domain: Virus
•     Unknown: "Positive sense ssRNA viruses"
•         Unknown: "RNA viruses"
•             Family: Closteroviridae
•                 Genus: Crinivirus
•                     Species: Cucurbit yellow stunting disorder virus

Notes on Taxonomy and Nomenclature

Cucurbit yellow stunting disorder virus (CYSDV) is a species of the Crinivirus genus, one of three genera of the Closteroviridae family (Martelli et al., 2005). CYSDV isolates can be divided into two distinct groups. One group contains isolates from Spain, Lebanon, Jordan, Turkey and North America and the other of isolates from Saudi Arabia. Nucleotide identity between isolates of the same group is greater than 99%, whereas identity between groups is about 90% (Rubio et al., 1999).

Description

Flexuous, filamentous virus particles typical of the Closteroviridae have been found in infected plants. The length distribution of CYSDV particles has shown two peaks at 825-850 nm and 875-900 nm (Célix et al., 1996). Using an improved method for particle measurement, Liu et al. (2000) have recorded lengths of 800-850 nm for CYSDV. Its bipartite positive sense single-stranded RNA genome has been completely sequenced; RNA-1 contains 9126 nucleotides and RNA-2 7281 nucleotides (Livieratos and Coutts, 2002; Aguilar et al., 2003; Coutts and Livieratos, 2003). The coat protein gene contains 756 nucleotides and encodes the coat protein of 28.5 kDa (Livieratos et al., 1999). CYSDV was first classified with the Closterovirus species with bipartite genomes exemplified by Lettuce infectious yellows virus (Célix et al., 1996), as then named. These have now been transferred to a new genus Crinivirus.

Distribution Table

The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

Risk of Introduction

Cucurbits are important crops in the EPPO region, both in the field and under glass, and CYSDV causes a serious disease notably on cucumbers and melons in Spain, Portugal, Turkey and the Middle East. Within Europe, cucumber and gherkin production is significant. In 2000, 1.66 million t were harvested in the EU. The Netherlands and Spain were the biggest producers, harvesting 465,000 and 420,000 t, respectively, according to FAO. Economic losses from CYSDV that could be expected in glasshouse-grown cucurbits, especially cucumber, in northern Europe are difficult to predict, but are likely to be substantial. Spread of the pest is likely to be much facilitated by the presence of its vector Bemisia tabaci in glasshouses in many countries of the EPPO region. Control of CYSDV is difficult due to the ability of the vector B. tabaci rapidly to become resistant to insecticides. A breakdown of efficacy of insecticides could result in serious problems. There is a strong probability that CYSDV will become a serious problem in other Mediterranean countries and in northern Europe, if introduced.

Hosts/Species Affected

The natural hosts of CYSDV are restricted to the Cucurbitaceae: watermelon, melon, cucumber and courgette. In addition, the following experimental host plants have been identified: Cucurbita maxima and Lactuca sativa. For further details, see Célix et al. (1996), Wisler et al. (1998), Berdiales et al. (1999), Abou-Jawdah et al. (2000), Desbiez et al. (2000), Kao et al. (2000) and Louro et al. (2000).

Host Plants and Other Plants Affected

Symptoms

Cucumbers and melons infected by CYSDV show severe yellowing symptoms that start as an interveinal mottle on the older leaves and intensify as leaves age (Abou-Jawdah et al., 2000). Chlorotic mottling, yellowing and stunting occur on cucumber (Louro et al., 2000) and yellowing and severe stunting on melon (Kao et al., 2000). No description of symptoms on courgette has been provided by the authors reporting the natural infection (Berdiales et al., 1999). Symptoms on cucurbit crops are said to be indistinguishable from those caused by Beet pseudoyellows virus (BPYV; Wisler et al., 1998).

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 (Célix et al., 1996).

List of Symptoms/Signs

Biology and Ecology

The life cycle of CYSDV is strongly dependent on its vector, the whitefly Bemisia tabaci. In Portugal, the first symptoms of CYSDV in a field plot of cucumber were associated with heavy infestations of B. tabaci (Louro et al., 2000). High populations were also associated with symptoms on melon in the USA (Kao et al., 2000). The spread of the virus may be related to the increase in distribution of the polyphagous B. tabaci (B biotype) (Bellows et al., 1994). This moves readily from one host species to the next and is estimated to have a host range of around 600 species. Transmission of CYSDV by biotype B is greater than by biotype A (Wisler et al., 1998). However, biotype Q transmits as efficiently as biotype B (Berdiales et al., 1999).

Within the EPPO region, biotype B is present and widespread in the field in many countries bordering the Mediterranean basin as well as in Slovakia and Ukraine. In northern European countries, it is of limited distribution and confined almost totally to glasshouse crops. Biotype Q is specific to Portugal and Spain (EPPO/CABI, 1997). B. tabaci infests polyethylene covered glasshouses where melons and cucumbers are grown along the south-east coast of Spain. It is displacing Trialeurodes vaporariorum as the dominant whitefly in this area and is associated with the change in the agent causing yellowing diseases of cucurbits from Beet pseudoyellows virus (BPYV) to CYSDV (Célix et al., 1996).

Acquisition periods of 18 h or more and inoculation periods of 24 h or more are necessary for transmission rates of CYSDV of over 80% in tests using melon. However transmission was noted after acquisition and transmission periods of 2 h (Célix et al., 1996). CYSDV persists for at least 9 days in the vector with a 72.2-h half-life. This is the longest retention time of all whitefly-transmitted viruses of the Closteroviridae (Wisler et al., 1998).

Means of Movement and Dispersal

Within cucurbit crops, natural spread of CYSDV is by its vector, Bemisia tabaci. Adults of B. tabaci do not fly very efficiently but, once airborne, can be transported long distances in air currents. Internationally, infected young plants of cucurbits intended for planting are a likely pathway to introduce or spread the disease. Also, all stages of the whitefly vector can be carried on plants for planting. There is not, however, known to be a significant movement of cucurbit plants for planting from areas where the disease occurs.

CYSDV is not known to be seedborne.

Plant Trade

Vectors and Intermediate Hosts

Impact

Since the late 1970s, cucumbers and melons grown in 16,000 ha of polyethylene-covered glasshouses in south-east Spain have been seriously affected by yellowing diseases transmitted by whiteflies. The first epidemics were caused by Beet pseudoyellows virus (BPYV) transmitted by Trialeurodes vaporariorum. Since the early 1990s, in addition to BPYV, CYSDV has been associated with these diseases. Surveys undertaken in 1994/1997 have shown that CYSDV has displaced BPYV as the major virus pathogen. No figures are available on losses caused by CYSDV. In Lebanon, the incidence of CYSDV has been high in summer and early autumn cucurbit crops grown in polyethylene tunnels along the coast. Yield reductions of 40-60% have been reported by farmers. Incidence was much higher in unscreened tunnels than in screened tunnels (Abou-Jawdah et al., 2000).

Diagnosis

Similarities to Other Species/Conditions

Symptoms on cucurbit crops are very similar to those of Beet pseudoyellows virus (BPYV; Wisler et al., 1998).

Prevention and Control

Cultural Control and Sanitary Methods

The control of CYSDV is dependent on the control of its vector Bemisia tabaci, and elimination of sources of infection. In particular, cucurbit seedlings for planting should come from disease-free stocks. 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. 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 with a species that is a good host for the vector, but not the virus may reduce virus incidence. In Lebanon, insect-proof nets and sticky yellow traps are used for control (Abou-Jawdah et al., 2000). Growing plants under physical barriers, such as low mesh tunnels and shade-cloth, may also have a positive effect.

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 (Berdiales et al., 1999). 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 (Nakhla and Maxwell, 1998). Imidacloprid, a systemic insecticide that can be applied to soil and foliage, is used to control whiteflies, but resistance has been reported (Elbert and Nauen, 2000). Insects resistant to buprofezin were also detected (Anon., 1996).

Biological Control

The parasite Encarsia formosa and the fungus Verticillium lecanii can be used as biological agents against B. tabaci, but are unlikely to affect virus transmission.

Phytosanitary Measures

CYSDV was added to the EPPO A2 action list in 2004, and endangered EPPO member countries are thus recommended to regulate it as a quarantine pest. There are, as yet, no specific measures against CYSDV in Europe, and in particular there are no restrictions on the movement of cucurbit seedlings from areas where the disease occurs. There is a potential danger that infected seedlings could move from countries where CYSDV occurs to other parts of the region, thus spreading the virus. Possible measures would be the same as those proposed for Cucumber vein yellowing virus (CVYV; OEPP/EPPO, 2005a).

Host-Plant Resistance

No resistant cultivars of susceptible hosts are currently available commercially. Preliminary attempts have been made to obtain select resistant melon genotypes (Sese et al., 1999; Marco et al., 2003).

References

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Distribution Maps