CORESTA Meeting, Agronomy/Phytopathology, Louisville, 1991

Morphology of Peronospora tabacina reproductive structures

USDA-ARS-SAA, Crops Research Lab, Oxford, NC, USA
Since the economically destructive blue mold epidemics in Europe in 1960 and North America in 1979, attention has focused on the pathogen Peronospora tabacina (P. hyoscyami f. sp. tabacina) and its control. Asexually produced sporangiospores are abundant and easily spread by wind. Oospores, produced by sexual reproduction, are of unquantified importance to pathogen survival and development. A new, atypical oospore was first observed in tobacco leaves in 1979 in North America. The new oospore (called verrucosae) is spherical, hyaline (blue-staining), with medium thickness walls, echinulate, and 17-36 microm in diameter. The typical oospore (called effusae) is spherical, red-brown (non-staining), thick-walled with irregular blunt elevations and ridges, and 22-25 microm in diameter. Authors' objective was to characterize Peronospora species infecting tobacco. They collected 25 tobacco leaf samples infected with Peronospora from 14 countries from 1986-89. Isolates of Peronospora were reproduced and preserved for inoculation studies. Leaf tissues were examined microscopically. All samples had sporangiophores and sporangiospores typical of P. tabacina. Effusae oospores were observed in seven samples, verrucosae in 13 and both morphological types in six. In a greenhouse inoculation experiment only effusae oospores were formed and by 13 of 25 isolates. To date it appears two species of Peronospora infect tobacco. More evidence is required to confirm this conclusion.