Squash leaf curl virus
(leaf curl of squash)

WCMoV (syn? SLCV-E) and SLCV-CA (syn? SLCV-R) occur in agroecosystems in the south-western USA and northern Mexico including Arizona, California and Texas (USA) and Sonora (Mexico).

In 1999-2000, a newly described begomovirus, provisionally called Cucurbit leaf curl virus (CLCuV), was discovered in several cucurbit species from Arizona, California and Texas (USA) and Coahuilla (Mexico). During these growing seasons in Arizona and Texas, CLCuV was the most common begomovirus identified in field-infected melon, pumpkin and squash fields (Brown et al., 2000). SLCV-E-like isolates were still predominant in Hermosillo and Mayo Valley in Sonora, Mexico (JK Brown et al., unpublished data). CLCuV was most recently identified in field-infected cucumber from Coahuilla, Mexico and in watermelon from Arizona, USA.

Begomovirus-like symptoms have been observed in cucurbits (squashes and melons) in Costa Rica, Dominican Republic, Guatemala, Honduras and Nicaragua (JK Brown, pers comm) but only recently has the identity of one of the isolates been determined. In spring 2000, a newly described begomovirus species was identified in melon from Guatemala. The virus which is under more detailed investigation, has been provisionally named Melon chlorotic leaf curl virus (MCLCV) (Brown et al., 2001).

In the Philippines, Benigno (1977) and Dolores et al. (1988) refer to a Bemisia-transmitted leaf curl of squash, said to be seedborne and associated with Bemisia tabaci infestations. This reference predates the description of SLCV in the USA and is unlikely to refer to the same virus. A begomovirus of squash was also reported from China, SLCV-China (SLCV-CH) (Genbank Accession # AB027465) (Qinyan et al., 1999), and appears to be of Old World origin based on DNA sequence analysis; consequently it is a begomovirus species distinct from SLCV-like isolates from the USA and Mexico.

The distantly related, Old World Watermelon chlorotic stunt virus (WmCSV) has been described in watermelon in Yemen (Bedford et al., 1994) and Iran (Kheyr-Pour et al., 2000; Genbank Accessions # AJ245652 and AJ245653) and Sudan (Kheyr-Pour et al., 2000; Genbank Accessions # AJ245650 and AJ245651).

SLCV is on the EPPO A2 list, but has not been classified as a quarantine pest by any other regional plant protection organisation. This is no doubt partly due to the fact that its appearance as a significant pest is very recent. If introduced outside of its native habitat, it presents a severe threat to the cultivation of cucurbits (especially courgettes, melons and watermelons) in the open in southern Europe or under glass in northern Europe, wherever B. tabaci and especially the B biotype, occurs.

Host plants of B. tabaci from areas where SLCV occurs should come from a production nursery free from SLCV and B. tabaci (or treated against B. tabaci) during the last growing season. This applies especially to the ornamental Euphorbia pulcherrima or other ornamental species that are well known to carry B. tabaci inconspicuously.

 

The SLCV complex is transmitted in a persistent manner by the whitefly, Bemisia tabaci. Both B. tabaci and B. tabaci (B biotype) are effective vectors in the USA and Mexico. Virus can be acquired in ca 30 minutes to several hours. A latent period of ca 6-10 hours is required before transmission can occur. Whiteflies can continue to transmit the virus for 7-10 days or perhaps for the life of the vector. There is no evidence for transovarial transmission of SLCV (Cohen et al., 1983; Brown et al., 1986; Rosell et al., 1999).

SLCV can be present in spring, summer, autumn and winter crops if B. tabaci population levels are sufficiently high for dispersal of the virus. B. tabaci is typically abundant in the late spring, summer and autumn in the southern river valleys of Arizona and California, and in summer and fall in the Rio Grande Valley, Texas, USA (watermelon, melons, squash) and northern Coahuilla, Mexico (watermelon, cucumber, squash, melons). The vector is most abundant in squash, melons and cucumbers during the autumn, winter and spring in Sonora, Mexico. Susceptible cultivars develop symptoms within 6-12 days post-infection and can serve as virus reservoirs from which secondary spread can readily occur. Spread between crops is the most important means by which virus causes epidemics or routine crop loss.

Several wild cucurbit species are known to be experimentally susceptible to SLCV infection but only one wild cucurbit species (in Texas) has been found naturally infected with the virus (T Isakeit and JK Brown, unpublished data). No natural reservoirs in which SLCV overseasons have been identified in the USA or Mexico.

SLCV is not readily mechanically transmissible but can be transmitted by sap inoculation with difficulty. Severe mottle/mosaic and leaf curl develop on leaves that develop shortly after infection, and fruit is usually without symptoms (Brown and Nelson, 1986, 1989). Recommended indicator plant species are Cucurbita maxima, C. moschata or C. pepo (symptoms as above), and Cucumis melo, the latter to separate SLCV-CA from WCMoV. SLCV can be detected by PCR using universal primers for the core region of the coat protein gene (core CP) (Wyatt and Brown, 1996) followed by nucleotide (nt) sequencing of the cloned amplicon, and comparison with sequences of well-studied begomoviruses of cucurbits. If necessary, virus-specific PCR primers can be designed by taking into account the nt sequence of different SLCV strains (or species) in relation to each other and other closely related cucurbit-infecting begomoviruses. SLCV can also be detected in B. tabaci by DNA spot hybridization assay (Polston et al., 1990) or by PCR (Rosell et al., 1999) but unless component specific probes are employed or the nt sequence of the PCR amplicon is determined, respectively, accurate identification is not possible because several other closely related viruses may also be detected using these approaches.

SLCV is not readily mechanically transmissible but can be so transmitted with difficulty. Symptoms are recognisable as severe mottle/mosaic and leaf curl on newer leaves and fruit is usually without symptoms. Often, adult or immature B. tabaci whiteflies are present on leaves of infected plants (Brown and Nelson, 1986, 1989). Whitefly-mediated transmission to recommended indicator plants (Cucurbita maxima or C. moschata or C. pepo) is needed to separate SLCV virus from non-whitefly transmitted viruses that may occur in mixed infection.

It may be stressed that the SLCV complex, like Lettuce infectious yellows virus (EPPO/CABI, 1996), has only been recorded in North America. Several other B. tabaci-transmitted viruses are now known to attack cucurbits in other parts of the world, including the little-known Cucumber vein yellowing virus in the Middle East (Yilmaz et al., 1989; Mansour and Al Musa, 1993), which are not begomoviruses and have entirely different characteristics. The newly described begomoviruses in the USA and Mexico, CLCuV and MCLCV, may be confused with SLCV in certain hosts but in general they cause distinctive symptoms in their hosts. CLCuV causes chlorotic sectoring and leaf curling on Cucurbita and Citrullus species and a chlorotic mosaic/mottle on Cucumis species in contrast to SLCV-infected Cucurbita species which exhibit severe leaf curling and a striking mottling and distortion as well as stunting. MCLCV causes interveinal chlorosis, leaf curling and stunting in melon, compared with SLCV which causes a mild curling on new growth which in most cultivars disappears as plants mature, even though the virus remains detectable in infected plants.

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

SLCV is spread naturally only by its vector Bemisia tabaci which spreads it between fields (and presumably glasshouses) in infested areas. In international trade, it is unlikely to be carried by plants of its main cultivated hosts, since these are short-lived vegetable crops not normally transported except as seedlings. Young seedlings for transplanting might constitute a pathway, but would still be unlikely to be moved in intercontinental trade. The vegetables as such would not be likely to carry B. tabaci. So the main risk of movement is in B. tabaci on other host plants (for example, ornamentals), given the fact that the vector moves readily from one host to another and that the virus can persist in the vector for several weeks (to life) after virus acquisition (Cohen et al., 1983; Rosell et al., 1999).

In the field, disease control is mainly aimed at eliminating or excluding the whitefly vector, B. tabaci. Insecticides have been used in an attempt to reduce whitefly density. Protecting rows of seedlings with spun-bonded polyester as a floating cover has also been effective (Natwick and Durazo, 1985). Some cultivars of Cucurbita pepo have shown resistance to SLCV (McCreight, 1984, 1991).