VIEW ARTICLE

Physiology and Biochemistry

Restricted Colonization by Peronospora tabacina and Phytoalexin Accumulation in Immunized Tobacco Leaves. K. Stolle, Department of Plant Pathology, University of Kentucky, Lexington 40546-0091; M. Zook, L. Shain, F. Hebard, and J. Kuc. Department of Plant Pathology, University of Kentucky, Lexington 40546-0091. Phytopathology 78:1193-1197. Accepted for publication 12 April 1988. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/Phyto-78-1193.

Nicotiana tabacum ‘Kentucky 14’ plants were stem injected with either sporangiospores of Peronospora tabacina (immunized) or with water (control). For challenge inoculations, filter-paper disks soaked with a sporangiospore suspension of this fungus were placed on the upper surfaces of leaves detached 28 days after stem injection. Nearly all (>99%) germinated spores penetrated both the immunized and the control tissue, as shown after staining with trypan blue. Light microscopy of cleared leaf tissue revealed that colonization of immunized tissue ceased within 5 days postinoculation (dpi) and that mycelium was confined within the chlorotic lesion area (4.2 ± 0.2 mm² per lesion). Colonization continued in control tissue for about 9 days, and the area per lesion was 314 ± 3 mm² at 7 dpi with hyphae extending about 1 mm beyond the lesion margins. The number of sporangiophores was reduced about 70% in immunized versus control tissue, but the number of sporangiospores per sporangiophore was unchanged. Immunized and control leaves accumulated the sesquiterpenoid stress metabolites phytuberol, phytuberin, rishitin, and capsidiol, but not solavetivone or lubimin, after challenge inoculation. Because the accumulation of these phytoalexins was low compared with reports of accumulations for other solanaceous plants and because they did not accumulate earlier in the immunized leaves than in the controls, it is unlikely that they are the primary cause of restricted colonization by P. tabacina in immunized tobacco.