Natural Occurrence of Phytophthora infestans on Black Nightshade (Solanum nigrum) in Wales.

There is only one published record of natural infection of black nightshade (Solanum nigrum L.) by Phytophthora infestans (Mont.) de Bary in England (3) and none from Wales. In August 2001, brown, necrotic leaf lesions with pale green margins were found on black nightshade weeds in a potato trial naturally infected with P. infestans at Henfaes Research Centre, University of Wales, Bangor. Although the plants were low growing with large, succulent leaves 4 to 5 cm long instead of having a more erect habit and smaller leaves, their identity was confirmed as S. nigrum; their atypical appearance may relate to the known phenotypic plasticity of this species (4). Infected leaflets incubated in moist chambers produced sporangia typical of P. infestans, and zoospores were released after chilling in water. Five isolates obtained from leaf fragments had growth on rye agar that was indistinguishable from that of P. infestans from potato. Detached leaflets of S. nigrum and S. tuberosum cv. Green Mountain inoculated with the S. nigrum isolates developed sporulating lesions under high humidity in 7 to 10 days; uninoculated controls remained symptomless. Inoculation of attached leaves of 10 potted S. nigrum plants resulted in seven plants developing necrotic lesions with a few sporangia 10 to 14 days later; sporulation developed mainly on lower leaves of plants that were older or had senesced. The remaining plants developed necrotic lesions with no sporulation, and P. infestans was reisolated from sporulating and nonsporulating lesions. All isolates were A1 mating type, metalaxyl-sensitive, and mitochondrial haplotype IIa, which are characteristics found commonly in isolates of P. infestans from potato in Wales (1). Single-sporangial isolates from each isolate were homozygous for glucose-6-phosphate isomerase and peptidase (Gpi 100/100, Pep 100/100). RG57 fingerprint analysis further established that all five black nightshade isolates were identical to each other and to some local P. infestans isolates from potato. P. infestans in Wales belongs to the new population (1), which may infect a wider host-range than the old US-1 clonal lineage. However, infected black nightshade was only found after late blight was widespread in potato fields. In subsequent years at the same site, weeds of S. nigrum have remained noninfected despite high levels of late blight pressure on adjacent potato plots. There is no evidence to suggest that this species acts as an overwintering host in Wales since it is an annual and lacks frost resistance. Field infection of S. nigrum by P. infestans has recently been reported in the Netherlands (2). Our observations confirm the potential of P. infestans to infect another solanaceous plant species. Alternative hosts may interfere with current disease control strategies because infected weeds would escape fungicide application and could serve as reservoirs of inoculum throughout the growing season. References: (1) J. P. Day and R. C. Shattock. Eur. J. Plant Pathol. 103:379, 1997. (2) W. G. Flier et al. Plant Pathol. 52:595, 2003. (3) J. M. Hirst and O. J. Steadman. Ann. Appl. Biol. 48:489, 1960. (4) B. S. Rogers and A. G. Ogg Jr. Page 30 in: Biology of Weeds of the Solanum Nigrum Complex (Solanum Section Solanum) in North America. USDA Publication ARM-W-23, 1981.