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Introduction of the New World Squash leaf curl virus to Squash (Cucurbita pepo) in Egypt: A Potential Threat to Important Food Crops

Squash plants showing leaf curling, yellow mottling, and reduced fruit set were observed in fields in Giza, Egypt in spring 2005. These particular symptoms had not been observed previously in zucchini squash plants in Egypt, but were reminiscent of those caused by begomoviruses (Geminiviridae) that are known to occur in the region, including Watermelon chlorotic stunt virus. Squash plants were heavily infested with the whitefly Bemisia tabaci (Genn.), the only known vector of begomoviruses. Total nucleic acids were isolated from symptomatic squash leaves using the cetyltrimethylammoniumbromide method, and extracts were subjected to polymerase chain reaction (PCR) analysis using two sets of PCR primers. One primer set (prAV2644 and prAC1154) was designed to amplify a fragment that contains the entire viral coat protein (Cp), while the second primer set (prBV1855 and prBC656) was designed to amplify the common region (CR) of DNA-B of begomoviruses (1). The expected size fragments were cloned and the sequence was determined for five clones each. Unexpectedly, the Cp and the CR-B fragments shared their highest nucleotide sequence (nt) identity among well-characterized begomoviruses to the bipartite Squash leaf curl virus (SLCV) native to the western United States. A third primer set (prAC344 and prAV1134) (1) was subsequently used to amplify the remainder of the putative SLCV DNA-A. The fragment was cloned and the DNA sequence was determined. Assembly of the overlapping DNA-A fragments resulted in a complete DNA-A component sequence of 2,636 nt, which is identical to the expected size of the SLCV DNA-A component (GenBank Accession No. DQ285019). Comparison with the latter sequence indicated that the Egyptian squash isolate shared 98% nt identity with SLCV. The sequence for the DNA-B fragment (1,162 nt) shared 94% nt identity with SLCV and was deposited in GenBank as Accession No. DQ285020. The high-shared nt identity with SLCV (2) from the United States suggests that this isolate, herein SLCV-EG, has been introduced into Egypt. The relatively low DNA-B nt sequence identity was a not a surprise since this component is normally less conserved even between strains of a single begomoviral species. Introduction of SLCV is not only potentially significant to the domestic production of crop species in the Cucurbitaceae but also for legume crops. SLCV has a broad host range that also includes members of the Fabaceae, which includes species that contribute significant sources of protein for much of Egypt's population. The virus thus far is thought to be present only in Lower Egypt, however, it could feasibly threaten legume and cucurbit crops if it spreads to Upper Egypt. To our knowledge, this is the first begomovirus of New World origin to become established in the Old World.

References: (1) A. M. Idris and J. K. Brown. Phytopathology 88:648, 1998. (2) S. G. Lazarowitz. Virology 180:70, 1991.