Tussilago farfara: a new natural host of stolbur phytoplasma

Coltsfoot (Tussilago farfara) is a perennial herb with a creeping rhizome and long runners. It is an invasive weed that prefers heavy clay soils throughout Europe, North Africa, Asia and the USA. Bois noir (BN), is caused by a phytoplasma belonging to the stolbur group (16SrXII-A). It is spread in the main Italian viticultural areas and it is common in vineyards of the Abruzzi region (Romanazzi et al., 2007). BN is transmitted to grapevines by its vector, Hyalesthes obsoletus, which can feed on BN-infected wild plants.

In 2007, during a survey carried out in a cv Chardonnay vineyard at Atri (TE) in the Abruzzi region in which 30% of plants were infected with BN (Romanazzi et al., 2007), naturally ocurring T. farfara plants were collected. Most of these showed typical symptoms ascribed to phytoplasma infection, such as foliar reddening and rolling (Fig. 1). The DNA was extracted from leaf samples of five symptomatic and three symptomless T. farfara plants using the DNeasy Plant Mini Kit (Qiagen, Hilden, Germany). The samples were amplified with the P1/P7 universal primers and nested-PCR was carried out with group-specific primers: R16(I)F1/R1, R16(III)F1/R2 and R16(V)F1/R1. Five-µl aliquots of the R16(I)F1/R1 amplicon were digested with 3 U restriction endonuclease MseI (New England BioLabs), overnight at 37°C. Further molecular characterization was carried out by amplifying the tuf gene in nested-PCR and digesting the amplicon with 3 U HpaII (New England BioLabs), overnight at 37 °C (Langer and Maixner, 2004).

All five symptomatic samples of T. farfara were infected by stolbur, showing an identical restriction pattern to that in the reference strain (STOL, kindly provided by C. Marzachì, IVV-CNR, Turin, Italy). No other phytoplasmas were detected. The pathogen was not found in samples from the three symptomless plants. However these developed disease symptoms when inoculated by grafting with samples from infected symptomatic ones. Leaf samples from these artificially inoculated plants tested positive for the presence of the phytoplasma. The molecular characterization of the stolbur isolates revealed tuf type II infections (Fig. 2) (Langer and Maixner, 2004).

Several infections with phytoplasma have been detected on different yellows diseased weeds in Italy (Credi et al., 2006; Borgo et al., 2008), although stolbur infection of T. farfara has not previously been reported. Information on weeds that are potential sources of the BN phytoplasmas in vineyards is important for disease management and this is the first report of stolbur infection in symptomatic T. farfara in Italy, and worldwide.