Squash Vein Yellowing Virus (SqVYV)

California Pest Rating for

Squash Vein Yellowing Virus (SqVYV)

Pest Rating: B

PEST RATING PROFILE

Initiating Event:

None. The risk of introduction of Squash vein yellowing virus to California is assessed and a permanent rating for SqVYV is herein proposed.

History & Status:

Background: In 2003 in Hillsborough County, Florida, an unknown virus was detected in squash plants (Cucurbita pepo) exhibiting vein yellowing symptoms and soon after in 2005, this virus was found to cause watermelon vine decline in watermelon plants in Florida (Webb et al., 2003; Adkins et al., 2007). In 2006, the virus was identified and characterized as a new species, Squash vein yellowing virus. SqVYV is a whitefly-transmitted member of the genus Ipomovirus in the family Potyviridae which induces necrosis of watermelon stems and petioles resulting in rapid wilt and death of plants at or near harvest. In the field, SqVYV is often detected in watermelon in mixed infections with other viruses (Adkins et al., 2013).

Squash vein yellowing virus was reported from California in 2015 following the fall of 2014 detection of diseased pumpkin plants grown from seed at the University of California Desert Research Extension Center in Holtville, California. Molecular analysis of pathogens associated with the diseased plants revealed mixed infections with the crinivirus Cucurbit yellow stunting disorder virus, the begomovirus Squash leaf curl virus and SqVYV. Symptoms of infection and association of a divergent strain of SqVYV were confirmed through pathogenicity trials and molecular diagnostic tests of infected pumpkin and squash plants. SqVYV-infected melon plants were also detected in commercial fields in the Imperial Valley (Batuman et al., 2015). Subsequently, during December 2014, official melon and pumpkin samples from the infected sites were collected by Imperial County Agricultural Commissioner’s staff and sent to CDFA Plant Pathology Laboratory for diagnosis. Tongyan Tian, CDFA plant pathologist, detected SqVYV from the samples using a RT-PCR protocol and sequence analysis. The detection of SqVYV in Imperial County marked the first report of an Ipomovirus in California (Batuman et al., 2015).

Hosts: The host range is limited to species in Cucurbitaceae with more dramatic symptoms produced in squash (inc. pumpkin) and watermelon. Plant hosts include two varieties of cucurbit weeds, namely, Momordica charantia (Balsam-apple) and Melothria pendula (creeping cucumber) (Adkins et al., 2008). The weeds may serve as reservoir hosts for SqVYV.

Symptoms: Initial symptoms consist of a slight yellowing of leaves. This is followed by browning and collapse of entire vines within weeks of the first symptoms. These symptoms appear as the fruit develops to a harvestable size. Infected fruit internally often exhibit discolored and necrotic blotches in the rind, discolored flesh (too red) and an off-taste (Baker et al., 2008). SqVYV-infected cucurbit weed hosts are asymptomatic (Adkins et al., 2008). In Puerto Rico, symptoms of watermelon vine decline on field-grown watermelon included leaf curling, mosaic, and internode necrosis. During the early stage of plant growth reduced vigor and general stunting occurred, and at the flowering stage, symptoms progressed to necrosis and wilting of vines (Acevedo, et al., 2013). Adkins et al. (2013) reported that symptoms of vine decline in watermelon appeared 12-16 days after inoculation regardless of plant age at time of inoculation and greenhouse or field location. However, younger watermelon plants exhibited more severe symptoms than older ones.

Damage Potential: In Florida, watermelon plants suffering from vine decline and fruit rot disease caused by SqVYV has resulted in severe losses in spring and fall plantings. During this period the disease may rapidly increase in incidence from 10 to >80% within a week (Adkins et al., 2007). The disease can result in total crop loss with collapsed vines and unmarketable fruit with discolored and necrotic rinds.

Transmission: SqVYV is transmitted by the whitefly Bemisia tabaci. The pathogen is not transmitted by aphids unlike other common cucurbit-infecting species of the family Potyviridae (Adkins et al., 2003). Experimentally, Adkins and others determined that whiteflies required 1-2 days to feed and acquire the virus from infected plants followed by 2 hours or 2 days to inoculate or transmit the virus to non-infected squash and watermelon plants. Transmission occurs in a semi-persistent mode by the whitefly which remains infective for 4-6 hours after acquiring the virus. Adkins et al. (2008) experimentally demonstrated that the whitefly vector was able to acquire SqVYV from inoculated cucurbit weed host Momordica charantia and subsequently transmit it to squash and watermelon to produce typical symptoms. While the virus has been artificially inoculated to plants under greenhouse conditions, the main mode of natural field transmission is through its whitefly vector.

Worldwide Distribution: North America: USA (California, Florida, Georgia, Indiana, South Carolina, Puerto Rico (Acevedo et al., 2013; Egel & Adkins, 2007; Adkins et al., 2013).

Official Control: Squash vein yellowing virus currently holds a temporary Q rating by the CDFA. No other official control for SqVYV has been reported.

California Distribution: Currently, Squash vein yellowing virus has only been detected in Imperial County.

California Interceptions: There are no official records of interceptions of Squash vein yellowing virus in California.

The risk Squash vein yellowing virus would pose to California is evaluated below.

Consequences of Introduction:

1) Climate/Host Interaction: Evaluate if the pest would have suitable hosts and climate to establish in California. Score:

– Low (1) Not likely to establish in California; or likely to establish in very limited areas.

– Medium (2) may be able to establish in a larger but limited part of California.

– High (3) likely to establish a widespread distribution in California.

Risk is Medium (2) – SqVYV has already been able to establish in Imperial County, southern California Its further spread to non-infected sites cultivated to cucurbits is limited by the distribution of its vector, Bemisia tabaci, which to date, has not been found in natural cooler climates of northern California counties.

2) Known Pest Host Range: Evaluate the host range of the pest. Score:

– Low (1) has a very limited host range.

– Medium (2) has a moderate host range.

– High (3) has a wide host range.

Risk is low (1) – The natural host range is limited to plant species in the family Cucurbitaceae (which are grown extensively in the lower Sacramento Valley and in limited production in San Joaquin and Imperial Valleys).

3) Pest Dispersal Potential: Evaluate the natural and artificial dispersal potential of the pest. Score:

– Low (1) does not have high reproductive or dispersal potential.

– Medium (2) has either high reproductive or dispersal potential.

– High (3) has both high reproduction and dispersal potential.

Risk is High (3) – The virus is able to thrive in climates that are favorable for its vector. Its potential for spread is always artificial being completely dependent on the distribution of its vector and infected plant materials. Therefore, factors that increase movement and activity of the vector and infected plants will also influence that of the virus.

4) Economic Impact: Evaluate the economic impact of the pest to California using the criteria below. Score:

A. The pest could lower crop yield.

B. The pest could lower crop value (includes increasing crop production costs).

C. The pest could trigger the loss of markets (includes quarantines).

D. The pest could negatively change normal cultural practices.

E. The pest can vector, or is vectored, by another pestiferous organism.

F. The organism is injurious or poisonous to agriculturally important animals.

G. The organism can interfere with the delivery or supply of water for agricultural uses.

– Low (1) causes 0 or 1 of these impacts.

– Medium (2) causes 2 of these impacts.

– High (3) causes 3 or more of these impacts.

Risk is High (3) – SqVYV infections could lower crop yield and value, increase production costs, trigger loss of market, and the virus is vectored by the whitefly, Bemisia tabaci which would require implementation of management strategies to minimize the risk of the introduction and establishment of the virus in non-infected regions within California.

5) Environmental Impact: Evaluate the environmental impact of the pest on California using the criteria below.

A. The pest could have a significant environmental impact such as lowering biodiversity, disrupting natural communities, or changing ecosystem processes.

B. The pest could directly affect threatened or endangered species.

C. The pest could impact threatened or endangered species by disrupting critical habitats.

D. The pest could trigger additional official or private treatment programs.

E. The pest significantly impacts cultural practices, home/urban gardening or ornamental plantings.

Score the pest for Environmental Impact. Score:

– Low (1) causes none of the above to occur.

– Medium (2) causes one of the above to occur.

– High (3) causes two or more of the above to occur.

Risk is Medium (2) – Infestations of SqVYV could significantly impact home/urban gardening of cucurbit host plants resulting in the imposition of additional official or private treatment programs in order to prevent spread of the virus and virus-carrying whitefly vector.

Consequences of Introduction to California for Squash vein yellowing virus

Add up the total score and include it here. (Score)

-Low = 5-8 points

–Medium = 9-12 points

-High = 13-15 points

Total points obtained on evaluation of consequences of introduction of SqVYV to California = (11).

6) Post Entry Distribution and Survey Information: Evaluate the known distribution in California. Only official records identified by a taxonomic expert and supported by voucher specimens deposited in natural history collections should be considered. Pest incursions that have been eradicated, are under eradication, or have been delimited with no further detections should not be included. (Score)

–Not established (0) Pest never detected in California, or known only from incursions.

-Low (-1) Pest has a localized distribution in California, or is established in one suitable climate/host area (region).

-Medium (-2) Pest is widespread in California but not fully established in the endangered area, or pest established in two contiguous suitable climate/host areas.

-High (-3) Pest has fully established in the endangered area, or pest is reported in more than two contiguous or non-contiguous suitable climate/host areas.

Evaluation is Low (-1).

Final Score:

7) The final score is the consequences of introduction score minus the post entry distribution and survey information score: (Score)

Final Score: Score of Consequences of Introduction – Score of Post Entry Distribution and Survey Information = 10

Uncertainty:

While SqVYV is established in the Imperial Valley and there have been no further reports of its spread to other intrastate regions, targeted surveys for the pathogen have not been conducted in other cucurbit production sites. The distribution and establishment of the virus is largely dependent on the distribution and established infestations of virus-carrying Bemisia tabaci. Subsequently, detections outside the Imperial Valley may alter the proposed rating for this virus pathogen.

Conclusion and Rating Justification:

Based on the evidence provided above the proposed rating for Squash vein yellowing virus is B.

References:

Acevedo, V., J. C. V. Rodrigues, C. E. de Jensen, C. G. Webster, S. Adkins and L. Wessel-Beaver. 2013. First report of Squash vein yellowing virus affecting watermelon and bitter gourd in Puerto Rico. Plant Disease 97:1516.

Adkins S., T. G. McCollum, J. P. Albano, C. S. Kousik, C. A. Baker, C. G. Webster, P. D. Roberts, S. E. Webb and W. W. Turechek. 2013. Physiological effects of Squash vein yellowing virus infection on watermelon. Plant Disease 97:1137-1148.

Adkins, S., S.E. Webb, D. Achor, P. Roberts, and C.A. Baker. 2007. Identification and characterization of a novel whitefly-transmitted member of the family Potyviridae isolated from cucurbits in Florida. Phytopathology 97: 145-154.

Adkins, S.T., S. Webb, C. Baker, and C.S. Kousik. 2008. Squash vein yellowing virus detection using nested polymerase reaction demonstrates the cucurbit weed Momordica charantia is a reservoir host. Plant Disease 92: 1119-1123.

Baker, C., S. Webb and S. Adkins. 2008. Squash vein yellowing virus, causal agent of watermelon vine decline in Florida. Plant Pathology Circular No. 407, Florida Department of Agriculture and Consumer Services, Division of Plant Industry.

Egel, D. S. and S. Adkins. 2007. Squash vein yellowing virus identified in watermelon (Citrullus lanatus) in Indiana. Plant Disease, 91:1056.2.

Batuman, O., E. T. Natwick, W. M. Wintermantel, T. Tian, J. D. McCreight, L. L. Hladky, and R. L. Gilbertson. 2015. First report of an Ipomovirus infecting cucurbits in the Imperial Valley of California. Plant Disease 99:1042. http://dx.doi.org/10.1094/PDIS-12-14-1248-PDN.

Webb, S. E., E. Hiebert and T. A. Kucharek. 2003. Identity and distribution of viruses infecting cucurbits in Florida. Phytopathology 93:S89.

Responsible Party:

John J. Chitambar, Primary Plant Pathologist/Nematologist, California Department of Food and Agriculture, 3294 Meadowview Road, Sacramento, CA 95832. Phone: 916-262-1110, plant.health[@]cdfa.ca.gov.

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