First Report of Phytophthora Stem Rot on Gloxinia in Bulgaria

Severe stem base necrosis was observed on potted gloxinia (Sinningia speciosa) plants in a greenhouse in the Plovdiv Region of Bulgaria in the spring of 2006 and sporadically in 2007. Initial symptoms were water-soaked lesions at the stem base; eventually the lesions spread upward and downward until the entire stem was affected, resulting in withered leaves and plant collapse. Disease foci were sometimes apparent in rows of plants because of water splash. White, fungal-like colonies with arachnoid, aerial mycelia were obtained from affected plant tissue placed on potato dextrose agar (PDA) at 24 to 25°C in the dark. Pyriform to ellipsoid sporangia, 30 to 55 × 23 to 40 μm (average 38 × 27 μm), with a prominent papilla (sometimes two) and a short pedicel were observed. Chlamydospores were intercalary or terminal, spherical, and 15 to 55 μm in diameter. Oogonia and antheridia were not observed. On the basis of morphological features, the pathogen was tentatively identified as Phytophthora nicotianae (2). Pathogenicity was tested by placing 3-mm-diameter discs from 7-day-old PDA cultures onto wounded petioles of visibly healthy 3-month-old gloxinia potted plants (three replicates). Sterile PDA plugs were placed onto similar wounds of three control plants. The inoculated wounds were covered with Parafilm. Three days after inoculation, water-soaked lesions began to spread longitudinally in both directions on inoculated petioles. The pathogen was recovered from the inoculated tissue but not from the control plants. Molecular identification of the pathogen was achieved with PCR-restriction fragment length polymorphism of the internal transcribed spacer regions ITS1 and ITS2 of the ribosomal DNA (1). The restriction patterns obtained with the enzymes AluI, MspI, and TaqIα were identical to those of P. nicotianae, confirming the morphological identification. To our knowledge, this is the first report of P. nicotianae on gloxinia in Bulgaria. References: (1) D. E. L. Cooke and J. M. Duncan. Mycol. Res. 101:667, 1997. (2) D. C. Erwin and O. K. Ribeiro. Phytophthora Diseases Worldwide. The American Phytopathological Society, St. Paul, MN, 1996.