Histopathology of Peronospora tabacina in tobacco

Background/Objective. Peronospora tabacina is an oomycete that causes blue mold in tobacco. Little is currently known about the pathogenicity of this pathogen, as available studies have focused on general aspects of the infection, and detailed histological analyses are scarce. Therefore, the objective of this study was to describe the histological changes that occur during P. tabacina infection in artificially inoculated tobacco plants. Materials and Methods. The infection process was analyzed by differential tissue staining with safranin-fast green in inoculated plants under greenhouse conditions. Tissue sections were made at 2, 4, 6, 9, 12, 18, 20, 24, 30, 36, 48, 60, 72, 96, 120, 148, 172, 196, and 220 hours after inoculation (hai). Results. The presence of sporangiospores attached to the leaf cuticle was observed between 2 and 48 hai, which germinated from 96 hai. The pathogen penetrated the leaves directly through appressoria at 96 hai. The first infectious hyphae emerged from 96-120 hai, and between 168 and 192 hai, intracellular hyphae developed and colonized the interior of the tissues, causing cell collapse in the lacunar and chlorophyll parenchyma, as well as structural alterations in vascular bundles and epidermal cells. The pathogen completed its infection cycle by producing oospores and sporangia at 192 hai and is considered a source of inoculum for the infection of new tissues. Conclusion. Peronospora tabacina is capable of directly penetrating tobacco leaf tissue through appressoria and developing a progressive infection both intercellularly and intracellularly, with severe alterations in leaf anatomy and completion of the cycle with the production of reproductive structures in a short period, reinforcing its potential as an aggressive pathogen under favorable conditions.