First report of a resistance-breaking strain of Tomato spotted wilt virus from Gerbera jamesonii in Apulia, Southern Italy

In 2010, samples of greenhouse-grown Gerbera jamesonii showing severe malformations of the flowers and necrotic spots on the leaves were received from a farm near Bari (Apulia, southern Italy). As the leaves expanded, the initially distinct necrotic pinpoints coalesced into large necrotic spots, covering the entire leaf lamina. Disease symptoms appeared after a heavy and persistent infestation of Frankliniella occidentalis, probably facilitated by the compressed coconut debris used as substrate, which can offer a suitable site for the pupal stage of this insect (Moritz et al., 2004). Estimated disease incidence in the gerbera plants grown in the greenhouse was 50% in cvs Sporza and Dune, 20% in cv. Lancaster and 10% in cv. Poseidon. Tomato spotted wilt virus (TSWV) but not Impatiens necrotic spot virus was detected in all samples tested by dot blot hybridization with digoxigenin-labelled riboprobes (Finetti-Sialer and Gallitelli, 2000). Two viral isolates obtained from cvs Sporza and Dune and a local isolate of TSWV were mechanically inoculated onto three plants each (at the second true leaf stage) of tomato cvs UC82, Faino, Diaz and Messapico, the latter two carrying the Sw5 resistance gene to TSWV. Isolates Sporza and Dune but not the local TSWV strain overcame the resistance and induced systemic necrosis. Tomato cvs UC82 and Faino were systemically infected by the two virus isolates. These results show that the TSWV isolates Sporza and Dune are of the resistance-breaking (RB) type (Ciuffo et al., 2005). This is the first report of a RB strain of TSWV in gerbera in Italy.