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First Report of Wilt and Stem Canker of Poinsettia (Euphorbia pulcherrima) Caused by Phytophthora nicotianae in Puerto Rico

Poinsettias are grown extensively in greenhouses in Puerto Rico, a five million dollar industry during the winter season of 2005. More than 2,000 ‘Freedom’ plants from an ornamental nursery near Aibonito exhibited severe wilt and dieback symptoms. The disease was found in 5 of 12 surveyed greenhouses where severity ranged from 50 to 100% and occurred regardless of the use of metalaxyl and mefenoxam. Symptoms during the growth phase included stunted plants, thin stems, chlorotic leaves, and brown roots. During flower bract development, symptoms consisted of leaf wilting often in sectors, stem canker, and purple-to-black lesions that extended from the stem to the petioles followed by soft rot. Isolations from symptomatic stem and root tissue were made on corn meal agar (CMA) (17 g/liter) amended with pimaricin (10 mg/liter), ampicillin (250 mg/liter), rifampicin (10 mg/liter), pentachloronitrobenzene (100 mg/liter), and hymexazol (50 mg/liter) (PARPH) (1). Pure cultures were obtained by hyphal-tip transfers onto CMA-PARPH. Colonies grown on acidified potato dextrose agar (APDA) at 25°C were arachnoid with abundant aerial mycelium and sporangia. Sporangia formed singly or in a loose sympodium on long sporangiophores and were papillate, mostly persistent, or caducous with short pedicel. The shape of sporangia varied from ellipsoidal to elongated ovoid, with a length/breath ratio of 16.5 × 14.2 μm. Large, spherical, terminal, and intercalary chlamydospores readily formed in APDA and PARPH. The isolate was identified as Phytophthora nicotianae on the basis of morphological and molecular characteristics. Molecular identification by sequence analysis of the internal transcribed spacer rDNA region confirmed the identity of the isolate P210.1 as P. nicotianae (GenBank Accession No. DQ485412). Pathogenicity tests were conducted with isolates P210.1 and IG grown on APDA for 7 days at 24°C. Isolates were then covered with sterile water for 48 h to induce sporangia and zoospore production at 24°C and continuous light. A suspension of 10 ml of approximately 120 sporangia ml^-1 were added to pots containing 2.2 kg of a 1:1 mixture of field soil and soilless media (Promix) composed of peat moss vermiculite and perlite. One healthy poinsettia rooted stem and one 2-month-old plant were transplanted individually into the inoculated soil and pots were flooded with sterile water for 24 h. A noninoculated control was included, each isolate and control were replicated five times. First symptoms (7 to 10 days after inoculation) included brown, limited root growth and lower stem discoloration. Plants (30- to 45-day-old) show a decline in vigor and rapid death of branches that are similar symptoms to those observed on infected poinsettias in commercial greenhouses. P. nicotianae was reisolated from infected root, crown, and stem tissue on PARPH media. A strict sanitation program was recommended to prevent recontamination. To our knowledge, this is the first report of P. nicotianae on poinsettia in Puerto Rico.

Reference: D. J. Mitchell and M. E. Kannwischer-Mitchel, Phytophthora. Page:31 in: Methods for Research on Soilborne Phytopathogenic Fungi. L. L. Singleton et al., eds. The American Phytopathological Society, St. Paul, MN, 1993.