First report of Sclerotinia subarctica nom. prov. (Sclerotinia species 1) in the UK on Ranunculus acris

During a study of Sclerotinia sclerotiorum populations on Ranunculus acris (meadow buttercup) in the UK, dead and dying flowers collected from plants in a Herefordshire meadow were incubated at 20° C in damp conditions. Fungal growth typical of S. sclerotiorum (white fluffy mycelium / sclerotia) emerging from individual flowers was transferred to potato dextrose agar (PDA). DNA was extracted from cultures and the ITS regions of the rDNA amplified and sequenced. BLAST analysis showed that of 32 isolates selected at random, only 17 were S. sclerotiorum while 15 were identified as S. subarctica nom. prov. (= Sclerotinia species 1; Holst-Jensen et al., 1998; Winton et al., 2006, 2007). All 15 S. subarctica ITS sequences were identical (GenBank Accession No. GU018183). The identity of the S. subarctica isolates was further confirmed by the lack of an intron in the LSU rDNA compared with S. sclerotiorum (Holst-Jensen et al., 1998).

A formal description and host range for S. subarctica has yet to be published but it is reported to be morphologically indistinguishable from S. sclerotiorum (Holst-Jensen et al., 1998). However, the cultures of S. subarctica in this study produced fewer but larger sclerotia in vitro (ca. 5.8 ± 1.0 mm diameter) than S. sclerotiorum (ca. 4.1 ± 0.9 mm (Fig. 1). S. subarctica has only been reported on wild hosts in Norway (Holst-Jensen et al., 1998) and causing white mold disease on vegetable crops in Alaska (as does S. sclerotiorum; Winton et al., 2006). As oilseed rape is the major crop host of S. sclerotiorum in the UK, its susceptibility to S. subarctica was tested by inoculation of plants and detached leaves. Stem and leaf lesions were identical to those caused by S. sclerotiorum although stem lesions of S. subarctica were slower to develop (Figs. 2, 3). This is the first time that infection of oilseed rape by S. subarctica has been demonstrated. The presence of S. subarctica in the UK indicates that this pathogen is not confined to the ‘High North’ as suggested previously (Winton et al., 2007). Significantly, as S. subarctica causes symptoms very similar to S. sclerotiorum, it may currently be undetected in Sclerotinia-diseased crops in the UK and hence further work to establish its distribution on crops and wild hosts is merited. The presence of S. subarctica (and S. sclerotiorum) on R. acris also suggests that this wild host is a reservoir of inoculum for crop plants.