In the summer of 2011, severe root and stem rot of begonia (Begonia × semperflorens-cultorum cv. Vodka Dark Red) was observed during a field trial. Seventy-eight percent of the plants had symptoms included foliar blight, blackened and rotting roots, rotting stems, and collapsing crown, often leading to plant death. Isolation from the diseased plant roots consistently recovered a Pythium-like species and 41 isolates were subcultured for identification. These isolates produced very similar single-strand conformation polymorphism (SSCP) fingerprints (2), which were distinct from those of other oomycete pathogens known to attack begonia (1). These isolates produced proliferous, ovoid to globose, terminal, and papillate sporangia which were 30.6 to 45.4 µm (av. 38.7 µm) in length and 20.5 to 35.4 µm (av. 28.2 µm) in width. Oogonia were produced in single culture grown in clarified V8 juice agar. These smooth-walled oogonia were mostly aplerotic and 28.9 to 36.8 µm (av. 33.1 µm) in diameter. Each contained a single oospore with a diameter of 23.7 to 34.4 µm (av. 26.9 µm). Single to multiple antheridia were attached lengthwise to each oogonium. These morphological characteristics match the description of Phytopythium helicoides (= Pythium helicoides) (3). The identity of these isolates was confirmed by sequencing the rDNA internal transcribed spacer (ITS) 1 and 2 regions. ITS sequence of the representative isolate 55C7 (GenBank Accession No. KC907734) had 97 to 99% homology with P. helicoides sequences in GenBank. Two isolates, 55C7 and 56A7, were tested for pathogenicity to begonia in the summer of 2012. Twelve plants per isolate were inoculated by injecting ground P. helicoides-colonized rice grains into the root soil using a long-neck funnel. Sterile rice grains were used on control plants. Aboveground symptoms including foliar blight, stem rot, and collapsing crown were observed 7 days after inoculation and the disease progressed for additional 6 weeks. At 7 weeks, all inoculated plants showed different symptom levels. Four and 10 plants inoculated with 55C7 and 56A7, respectively, were already dead. Begonia roots showed severe symptoms including blackening, stunted growth, and rotting. Seven of 12 control plants also had notable symptoms due to cross contamination. Isolates recovered from all symptomatic plants had identical SSCP fingerprints to those of isolates 55C7 and 56A7. To our knowledge, this is the first report of P. helicoides attacking begonia plants. The avenue of this pathogen entering the 2011 field trial remains unknown. The field trial in 2011 and pathogenicity test in 2012 indicate that this pathogen is potentially destructive to begonia. Additional research is warranted to identify the origin and dissemination of this pathogen to mitigate the risk to begonia production. References: (1) C. X. Hong et al. Plant Dis. 92: 1201, 2008. (2) P. Kong et al. FEMS Microbiol. Lett. 240:229, 2003. (3) A. J. van der Plaats-Niterink. Monograph of the Genus Pythium. Centraalbureau voor Schimmelcultures, Baarn, the Netherlands, 1981.