Istituto per la Protezione delle Piante del CNR, UOS di Portici, Via Università 133, 80055 Portici (NA), Italy
Plesso Didattico G. Scarabelli (Imola), Alma Mater Studiorum, Università degli Studi di Bologna, Bologna, Italy
Dipartimento di Scienze Agrarie (DiPSA) - Patologia Vegetale, Alma Mater Studiorum, Bologna University, Viale G. Fanin 42, 40127 Bologna, Italy
In winter 2012, some potted plants of African daisy (Arctotis × hybrida L., family Asteraceae) cv. Hannah, propagated by rooted stem cuttings and cultivated for commercial purposes in a greenhouse located at Albenga (Liguria region, Italy), were noticed for a rapid dieback, generalized reddening, following by an irreversible wilting. Around 130 plants on a total of 3,000 cultivated plants showed symptoms (4 to 5%). One gram of fresh leaves, each collected from three different symptomatic plants, was ground in 4 ml of cold (∼5°C) sodium phosphate 0.03 M buffer, containing 0.2% sodium diethyldithiocarbamate, 75 mg/ml of active charcoal, and traces of carborundum (600 mesh). The inoculum was rubbed on healthy indicator herbaceous plants and inoculated plants were maintained in an insect-proof greenhouse with natural illumination and temperatures of 24/18°C day/night. Healthy and buffer inoculated plants were also included in the test and used as negative control in the subsequent serological and molecular analysis. Sap-inoculated plants showed the following symptoms after 1 to 3 weeks: necrotic local lesions in Chenopodium amaranticolor and C. quinoa, yellowing and stunting following by systemic necrosis and death of the plants in tomato (Solanum lycopersicum cv. San Marzano), necrotic local lesions following by systemic necrotic patterns and leaf deformation in tobacco (Nicotiana tabacum cv. Xanthi nc.) and N. glutinosa, necrotic local lesions in petunia (Petunia × hybrida cv. Pink Beauty). No symptoms were recorded on buffer inoculated plants. Leaf samples from both symptomatic hosts and the three original symptomatic African daisy plants were tested by double-antibody sandwich-ELISA with polyclonal antisera against Cucumber mosaic virus (CMV) and tospoviruses (Tospovirus broad-spectrum, Serogroups I, II, and III, Bioreba AG, Switzerland). Positive reaction was obtained with Tospo-groups antibodies, but not with the CMV ones. Total RNA was extracted from infected leaves of African daisy with the RNeasy Plant Mini Kit (Qiagen, Valencia, CA) and subjected to reverse transcription (RT)-PCR by using the tospovirus universal primers BR60/BR65 that amplify part of the nucleocapsid protein gene (1). Target amplicons of 454 bp were produced for all samples tested. The PCR products were cloned and sequenced on both strands (one clone per amplicon cloned). The resulting sequences were 100% identical, so a single sequence was deposited in GenBank (HF913777). The sequence showed highest homology (99%) with the Tomato spotted wilt virus (TSWV) tomato isolate NJ-JN from South Korea (HM581936). The identity of the virus infecting African daisy was further confirmed by sequencing amplicons obtained by RT-PCR using primers partially covering the movement protein gene of TSWV (2). The sequence obtained (HF913776) showed the highest homology (99%) with three TSWV isolates: a tomato isolate from Spain (AY744493), a pepper isolate from South Korea (AB663306), and again the tomato NJ-JN isolate from South Korea (HM581936). To our knowledge, this is the first natural report of TSWV infecting African daisy plants. Moreover, since this ornamental is often cultivated with other flowering plants, it can act as reservoir for the virus that can infect other ornamentals and crops, considering that TSWV has a very broad host range (3). This result also represents the first finding of TSWV in the genus Arctotis, family Asteraceae, the greater botanical family of TSWV hosts (3).
References: (1) M. Eiras et al. Fitopatol. Bras. 26:170, 2001. (2) M. M. Finetti et al. J. Plant Pathol. 84:145, 2002. (3) G. Parrella et al. J. Plant Pathol. 85:227. 2003.