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First Report of Crown Rot Caused by Phytophthora tropicalis on Gloxinia in North Carolina
Heather A. Olson and D. Michael Benson, Department of Plant Pathology, North Carolina State University, Campus Box 7616, Raleigh, NC 27695
Corresponding author: Heather A. Olson. firstname.lastname@example.org
Olson, H. A., and Benson, D. M. 2010. First report of crown rot caused by Phytophthora tropicalis on gloxinia in North Carolina. Online. Plant Health Progress doi:10.1094/PHP-2010-0708-03-BR.
Gloxinia [Sinningia speciosa (Lodd.) Hiern] is a popular houseplant that frequently is given as a gift because of its large brightly colored blooms and velvety foliage. In June 2007, during a survey of species of Phytophthora on greenhouse ornamental plants in North Carolina, wilting and crown rot (Figs. 1 and 2) were observed on recently transplanted gloxinias in a commercial greenhouse in Johnston Co. Severely affected plants were completely necrotic with collapse of the entire plant. Roots were necrotic with sloughing of the root cortex when gently pulled. Isolations were made directly from symptomatic roots and crown tissues onto corn meal agar (CMA) amended with pimaricin, ampicillin, rifamycin, PCNB, and hymexazol (PARPH) (5). A species of Phytophthora was isolated consistently, and axenic cultures were obtained by transferring single hyphae tips to fresh CMA.
Initially, isolates were characterized morphologically. When incubated at 22 to 25°C on CMA, sparse, appressed, radiating colonies grew rapidly. Sporangia formed abundantly on umbellate sporangiophores in 5% clarified V8 broth after incubation under continuous light for 24 to 48 h (Fig. 3). Sporangia were papillate and caducous with pedicels ranging from 71 to 109 µm in length. Sporangia were obovoid, limoniform, or ellipsoid and had tapered bases, and they measured 38 to 60 µm long by 22 to 33 µm wide with a mean L:B ratio of 1.9. Most sporangia were radially symmetrical, but approximately 25% were bilaterally symmetrical. Sparse chlamydospores were formed in liquid culture and averaged 32 µm in diameter. Isolates from gloxinia were paired with A1 and A2 isolates of P. tropicalis on rapeseed-extract malt agar (6) and were grown at ambient temperature (18 to 24°C). All isolates from gloxinia were the A2 mating type because oospores were formed when paired with an A1 isolate of P. tropicalis from verbena in North Carolina. Oogonia were abundant, tan to reddish-brown in color, spherical, smooth-walled, and averaged 32 µm in diameter (Fig. 4). Antheridia were amphigynous and round to cylindrical in shape. Oospores were spherical and nearly plerotic with a mean diameter of 27 µm. Isolates had poor to no growth at 35°C, a physiological characteristic distinguishing several species of Phytophthora. In addition, isolates were sensitive to mefenoxam at 1 µg ai/ml in vitro. Based on these morphological characters, isolates tentatively were identified as P. tropicalis (1,2,3).
Morphological identification was confirmed by amplifying and sequencing the internal transcribed spacer (ITS) region of the rDNA from two isolates using the ITS4/ITS6 primers. The ITS sequences of both isolates were identical and showed 99.7% similarity over 752 base pairs to the type isolate of P. tropicalis Aragaki & Uchida (GenBank accession no. FJ801385). Closer analysis revealed that the differences between the gloxinia isolates and the type isolate were the result of two unresolved heterozygous sites in the sequences from the gloxinia isolates.
Pathogenicity of the two sequenced isolates to gloxinia was confirmed. Commercially produced gloxinia plugs, cv. Symphony Trumpet Indigo, were transplanted and grown for 3 weeks under drip irrigation. Nine replicate plants were inoculated with each isolate by placing one rice grain colonized by mycelium in each of three holes 1 cm deep in the potting mix of each container. Nine non-inoculated plants served as controls. In addition, to evaluate a management option for Phytophthora crown rot of gloxinia, nine replicate plants were drenched with dimethomorph (48 ml/100 liters Stature SC; BASF, Research Triangle Park, NC) 3 days prior to inoculation and then every 14 days thereafter. Plants were maintained under standard greenhouse conditions for gloxinia and were fertilized weekly with 20-10-20 (N-P-K). Within 14 days, initial foliar symptoms were observed on several plants. At 45 days, all inoculated plants had collapsed completely from crown rot . Plants treated with dimethomorph did not develop symptoms. The pathogen was re-isolated from symptomatic crown and root tissues of all inoculated plants and was determined to be P. tropicalis based on morphology. To our knowledge, this is the first report of P. tropicalis on gloxinia although it has been reported to be associated with other ornamental plants (4).
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