First Report of Root Rot and Stem Blight Caused by Phytophthora palmivora in Eustoma grandiflorum.

Eustoma grandiflorum (Raf.) Shinn., commonly referred to as Lisianthus or Texas bluebell, is grown on a limited but increasing scale in Norwegian greenhouses. In autumn 1999, E. grandiflorum plants with brown rotten roots and wilting foliage were collected from a cut-flower production facility in southeastern Norway. Symptoms were difficult to distinguish from those caused by Pythium spp. or early attack of Fusarium spp. For diagnosis, root and stem segments surface sterilized by dipping in alcohol and hypochlorite were plated on potato dextrose agar. Phytophthora sp. was isolated consistently from diseased roots and stems. Suspected Phytophthora sp. isolates were transferred to selective agar medium containing hymexazol and identified by morphological characteristics and DNA sequence homology (1). The fungus was characterized by stellate mycelium, lack of oogonia in single culture, sympodial sporangiophores, and production of numerous papillate, ellipsoid sporangia that were caduceus with small pedicels in water. Optimum temperature for mycelium growth was 25 to 30°C, with limited growth at 10 and 35°C. The DNA sequence of the polymerase chain reaction amplified internal transcribed spacer (ITS1 and ITS2) regions of rDNA, confirmed the identification as P. palmivora. Pathogenicity studies were conducted by inoculating nine, 12-week-old E. grandiflorum plants grown in 10-cm pots with peat substrate at 24°C with artificial light (120 W/m²) for 10 h per day, with a zoospore suspension or an agar piece toward the plant stem. The pathogenicity study was repeated once. In both experiments, all plants except uninoculated controls, died within 3 weeks after inoculation. P. palmivora was reisolated from diseased plants. In later greenhouse experiments, more than 50% of plants showed disease symptoms when adding 1,000 zoospores of P. palmivora per pot. 'Ecco Yellow' was more susceptible than 'Flamenco Purple'. The pathogen was as aggressive as Fusarium avenaceum in pathogenicity studies. Since E. grandiflorum is grown on a limited scale in Norway, we lack experience with the economic impact of this disease in the cut-flower industry. To our knowledge, this is the first report of P. palmivora in E. grandiflorum. Reference: (1) D. C. Erwin and O. K. Ribeiro. Phytophthora Diseases Worldwide. The American Phytopathological Society, St. Paul, MN, 1996.