Resistance to tobacco black shank in Nicotiana species

Black shank [caused by Phytophthora parasitica (Dast.) var. nicotianae (B. de Haan) Tucker] is one of the most devastating diseases in tobacco (Nicotiana tabacum L.). Phytophthora parasitica var. nicotianae race 1 is the most damaging race of this fungus. The objective of this study was to identify the Nicotiana species that are resistant to P parasitica var. nicotianae race 1 as part of the effort to develop Nicotiana-based plant varieties for applications in the production of plant-made pharmaceuticals (PMPs). The roots of approximately 26-d-old growth room-grown plants were inoculated with zoospores of an isolate of P parasitica var. nicotianae race I by a dipping procedure. The percentage of plants that were free of symptoms 10 to 14 d after inoculation was taken as a measure of resistance to the fungus. We evaluated a total of 97 Nicotiana accessions from 37 species from the USDA collection and other sources. Among the 97 accessions, three from N. debneyi Domin, two from N. repanda Willd. ex Lehm., and one each from N. megalosiphon Van Heurck & Mull. Arg., N. plumbaginifolia Viv., N. suaveolens Lehm., and N. sylvestris Speg. & Comes were found to be resistant. Their resistances were further confirmed after they were evaluated for an additional three times. These nine accessions were then challenged by another isolate of race 1. All accessions except for the N. sylvestris accession were found to be resistant. Nicotiana debneyi, N. megalosiphon, and N. suaveolens were not previously reported to be resistant to P parasitica var. nicotianae race 1. Our study indicated that the undomesticated Nicotiana species constitute a rich genetic source for resistance to the tobacco disease black shank.