During surveys of tomato (Solanum lycopersicum) fields in Niono, Mali, conducted in March 2011, unusual disease symptoms, including stunted growth, epinasty, and chlorosis of leaves and necrosis of leaf veins and stems were observed in multiple fields. The incidence of these symptoms was low (~1 to 5%), but they were distinct from those associated with known diseases in the region. A representative leaf sample with these symptoms was applied to filter paper (FTA cards, Whatman), and DNA and RNA extracts were prepared according to manufacturer instructions. RT-PCR tests for Tomato spotted wilt virus, Tobacco streak virus, Tomato necrotic spot virus, Tobacco/tomato mosaic viruses, Cucumber mosaic virus, Alfalfa mosaic virus, torradoviruses, and potyviruses, and PCR tests for begomoviruses, phytoplasmas, and ‘Candidatus Liberibacter’ infection were also negative. However, virus-like symptoms developed in all 16 tomato seedlings (cv. Early Pak 7) 7 to 10 days after mechanical (sap) inoculation with inoculum prepared from the FTA sample. No symptoms developed in mock-inoculated control plants (n = 3). Symptoms induced included stunted growth and severe epinasty of leaves, followed by necrosis of leaf veins, petioles, and stems. These symptoms were similar to those observed in plants in Mali. When RNA extracts prepared from leaves of these symptomatic plants were mechanically inoculated onto 24 tomato seedlings, similar symptoms developed in all plants, suggesting the causal agent might be a viroid. RT-PCR tests with RNA from symptomatic tomato leaves and universal (3) and various specific Pospiviroid primer pairs were negative. However, equivalent RT-PCR tests conducted with the pCLV4/pCLVR4 primer pair specific for Columnea latent viroid (CLVd) (2) generated a DNA fragment of the expected size (~370 bp). The sequence of this DNA fragment (GenBank Accession No. JQ362419) was 99% identical with those of CLVd isolates from the Netherlands (AY373446 and AY372396). In host range studies, the CLVd isolate from Mali induced symptoms in all 48 mechanically-inoculated tomato plants, whereas no symptoms developed (up to 90 days after inoculation) in inoculated Chenopodium quinoa, C. amaranticolor, Nicotiana benthamiana, N. tabacum (cvs. Havana, Glurk and Turkish), N. glutinosa, Datura stramonium, common bean (cvs. Topcrop and Pinto bean), pumpkin (cv. Small Sugar), pepper (Capsicum annuum, cv. Yolo Wonder) and cucumber (cvs. Emparator and Poinsett 76) plants (results of three independent experiments with six plants per experiment). Symptomless infections were detected in pepper (24 of 30), N. benthamiana (25 of 25), and N. tabacum cv. Turkish (11 of 24) plants by RT-PCR with the pCLV4/pCLVR4 primer pair. To our knowledge, this is the first report of CLVd infecting tomato in Mali. RT-PCR tests of seeds collected from CLVd-infected tomato, pepper, and N. benthamiana plants also detected CLVd (1). Thus, it is possible that CLVd was introduced into Mali in association with seed.
References: (1) O. Batuman and R. L. Gilbertson. Phytopathology 102:S4.9, 2012. (2) R. L. Spieker. Arch. Virol. 141:1823, 1996. (3) J. T. J. Verhoeven et al. Eur. J. Plant Pathol. 110:823, 2004.