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Yellow Leaf Disease of Cantaloupe and Wax Gourd from Thailand Caused by Tomato leaf curl virus

Cantaloupe (Cucumis melo var. reliculatus) and wax gourd (Benincasa hispida Cogn.) plants showing a yellow leaf disease have been observed in central Thailand since 1993. The disease has been transmitted to cantaloupe and wax gourd plants by grafting and whiteflies (Bemisia tabaci Genn.) (K. Kittipakorn, unpublished data). Because of the leaf yellowing symptoms and transmission by whiteflies, a geminivirus was suspected. Three naturally infected cantaloupe and four naturally infected wax gourd plants showing leaf yellowing were collected from fields at Kamphaengsaen, Nakorn Pathom, Thailand. Virus cultures were maintained in cantaloupe and wax gourd plants. Inoculations were done with a whitefly vector. Symptoms in cantaloupe and wax gourd plants were the same as those observed in the field. Virus DNA associated with yellow leaf disease of cantaloupe and wax gourd plants was amplified by polymerase chain reaction with geminivirus-specific degenerate primers, described by Rojas et al. (2), that anneal within the AC1 open reading frame (ORF; replication associated protein gene) and the AV1 ORF (coat protein gene) of bipartite geminiviruses and the C1 and V2 ORFs of monopartite geminiviruses (2). Fragments (1.2 kbp) were amplified and cloned from affected cantaloupe and wax gourd plants. Three independent clones were sequenced and analyzed, and identical 32-base stem loop regions were found in the amplified fragments from both isolates, including the conserved nonanucleotide sequence TAATATTAC, which is present in all geminiviruses. The iterative sequence GGCGTC also was found in the intergenic region (IR) (434 bp) of the amplified fragments from both isolates. The B component could not be detected. The nucleotide sequences of the IR and V1 were compared with 28 well-studied whitefly-transmitted geminiviruses and revealed ≈96% sequence identity with DNA A of Tomato leaf curl virus of the genus Geminivirus from India (ToLCV-In 2) (1) for both isolates. The iterative sequence GGCGTC in the IR also was identical to the sequence in ToLCV-In 2. These results establish the provisional identity of the pathogen causing yellow leaf disease on cantaloupe and wax gourds in Thailand as ToLCV or closely related strains.

References: (1) M. Padidam et al. J. Gen. Virol. 76:249, 1995. (2) M. R. Rojas et al. Plant Dis. 77:340, 1993.