In August 2012, symptoms of stunted growth and leaf epinasty, crumpling, and yellowing, were observed in basil plants (Ocimum basilicum) grown in a shadehouse in Calipatria in the Imperial Valley of California. Populations of the beet leafhopper (Circulifer tenellus) carrying curtoviruses (genus Curtovirus, family Geminiviridae) were detected in the Imperial Valley in May 2012. Together, this suggested a curtovirus etiology for this virus-like disease of basil. Total DNA extracts were prepared from leaves of nine representative symptomatic plants (BA1 through 9) and used in the PCR with the general curtovirus primer pair, BGv377 and BGc1509 (1,2). This primer pair directed the amplification of the expected ~1.1 kb DNA fragments from extracts prepared from all nine plants, and not from equivalent extracts from symptomless plants. The sequences of 1.1 kb fragments amplified from four plants (BA1 through 4) were determined, and BLAST analyses revealed 99% nucleotide sequence identities among these sequences, and 98% identities with the homologous region (V2/CP) of Beet severe curly top virus-Cfh (BSCTV-Cfh; GenBank Accession No. U02311). A second primer pair (BGv981 5′-AACGGTCAGGCTATGCCGTCTAC-3′ and BGc479 5′-GAAAGACCTCGCCTTCTTCTAGGG-3′) was designed to amplify the remainder of the viral genome. The expected size ~2.4 kb fragments were amplified from the extracts of the BA1 through 9 plants, and the fragments from the BA1 and 2 plants were cloned into the pGEM-T Easy Vector (Promega, Madison, WI) and sequenced. Using the sequences of the overlapping PCR-amplified fragments, the complete viral genome sequences of the BA1 and BA2 isolates were determined. The BA1 and BA2 sequences were 2,934 bp and were 99% identical to each other and to the sequence of BSCTV-Cfh (3). To confirm the infectivity of BSCTV in basil, the BSCTV-Cfh infectious clone, which originated from California, was used for agroinoculation and leafhopper transmission experiments in basil plants (cvs. Sweet aroma and Genovese). Basil plants agroinoculated with the BSCTV-Cfh clone developed stunted growth and leaf crumpling and curling symptoms, similar to symptoms observed in the symptomatic plants from the Imperial Valley. The presence of viral DNA in symptomatic plants was confirmed by PCR with the BGv377/BGc1509 primer pair. Basil plants inoculated with an empty vector control did not develop symptoms, nor was curtovirus DNA amplified from these plants by PCR. Beet leafhoppers were given a 48-h acquisition access period on BSCTV-Cfh-infected sugarbeet plants, followed by a 48-h inoculation access period on healthy basil plants. These plants developed curly top symptoms approximately 21 days after inoculation, indicating that BSCTV was transmitted to basil by the beet leafhopper. Together, these results establish that the cause of the disease symptoms in basil in the Imperial Valley of California was BSCTV. This is the first report of curly top disease in basil, which is the second member of the mint family (Lamiaceae) known to be infected by a curtovirus. The stunted growth induced in basil by BSCTV has the potential to cause yield and economic loss, particularly in open field or screenhouse production when beet leafhopper populations are high.
References: (1) L-F. Chen et al. Plant Dis. 94:99, 2010. (2) S. L. Dellaporta et al. Plant Mol. Biol. Rep. 1:19, 1983. (3) D. C. Stenger. Mol. Plant-Micro. Interact. 7:154, 1994.