Root infection of Nicotiana by the blue mold pathogen Peronospora tabacina Adam

Peronospora tabacina, a biotrophic oomycete, infects leaves of tobacco and can cause severe crop losses under favorable environmental conditions. Systemic infection is sometimes observed, resulting in stunting of the plant. While researching blue mold resistance in N. langsdorffii, we found that P. tabacina can infect roots of this species and N. tabacum. Roots of four-week old seedlings were rinsed in water to remove the potting mix, and dipped in a P. tabacina suspension before transplant. Infection was first seen on leaves at 7 days post inoculation (dpi) on N. langsdorffii accessions that express a hypersensitive response (HR) at maturity (S-4-2, S-4-4). The infected leaf tissue became necrotic and turned brown, and the plants were dead by 14 dpi. In accessions manifesting a compatible interaction with P. tabacina, no symptoms were apparent until 10 dpi, at which time the leaves turned yellow. In addition, root drench with P. tabacina sporangia resulted in systemic infection on compatible hosts such as N. langsdorffii S-4-3 & N. tabacum KY 14, and foliar necrosis and severe stunting of N. langsdorffii accessions that express HR-type resistance. This finding provides evidence of another possible infection route for P. tabacina, and underscores the importance of avoiding blue mold contamination in seedling float-tray systems.