Natural occurrence of Phytophthora infestans on woody nightshade (Solanum dulcamara) in Ireland

Woody nightshade (Solanum dulcamara) is a common hedgerow plant in the UK and Ireland, a herbaceous perennial and one of only two native Solanum spp. S. dulcamara is a known host of Phytophthora infestans (Erwin & Ribeiro, 1996), but infection is rarely reported and there is only one British record, from Harpenden, England (Hirst & Steadman, 1960). There have been no published reports from Ireland.

On 2 September 1998, leaf lesions similar to those of P. infestans on potato were found on a naturally occurring S. dulcamara at the Agriculture & Food Science Centre, Belfast. The plant was growing within 100 m of an experimental potato plot inoculated with P. infestans.

When two infected leaves of S. dulcamara were incubated for 24 h under high humidity, fungal growth developed around the lesion margins. This had a hyaline mycelium bearing lemon-shaped sporangia which released motile zoospores after chilling in water, consistent with P. infestans. Three isolates were obtained from this plant during September 1998. Growth on rye agar was indistinguishable from that of local potato isolates of P. infestans.

Detached leaflets of S. dulcamara and S. tuberosum were inoculated with the woody nightshade isolates and kept in a humid chamber. Both hosts became infected and profuse sporulation occurred within 7 days. The fungus isolated was confirmed as P. infestans by morphological, biochemical and molecular characteristics. Inoculations of attached leaves of potted S. dulcamara plants resulted in necrotic lesions with a few sporangia; sporulation was mostly on fallen leaves. P. infestans was reisolated and identity confirmed as before. In September 1999, numerous leaf lesions were again observed on the same plant. White sporulation occurred on the abaxial surface and comprised sporangia characteristic of P. infestans.

The three 1998 isolates were A1 mating type, sensitive to metalaxyl, mitochondrial haplotype IIa, characteristics typical of P. infestans from potato in Northern Ireland (Carlisle et al., 2001). A 1999 isolate was A1, metalaxyl-sensitive, mtDNA haplotype Ia. All were monomorphic and homozygous for glucose-6-phosphate isomerase and peptidase (Gpi 100/100, Pep 100/100), as confirmed with single-spore isolates. RG-57 fingerprint analysis confirmed the close similarity of the woody nightshade and potato isolates.

P. infestans in Ireland belongs to the new population sensu Spielman et al. (1991), which may infect a wider host-range than the old US-1 clonal lineage. However, S. dulcamara infections have only been found when blight is already widespread in potato fields and there is no evidence to suggest that woody nightshade acts as an overwintering host in Ireland.