First Report of Phytophthora nicotianae Causing Foliar Blight of Codiaeum variegatum in Florida.

Codiaeum variegatum (L.) A. Juss. (family Euphorbiaceae) is indigenous to the western Pacific islands. Commonly known to the ornamental plant industry as croton, this tropical species is a popular houseplant and well adapted to landscapes in South Florida, Hawaii, and milder areas in Southern California. In May 2015, a plant sample of C. variegatum ‘Petra’ from a local nursery was submitted to the Florida Extension Plant Diagnostic Clinic in Homestead, FL. The foliage exhibited dark dry brown spots. A Phytophthora sp. was initially confirmed by serological testing with a commercially available ImmunoStrip test (Agdia, Elkhart, IN). Based on closer examination, the pathogen was identified as P. nicotianae Breda de Haan by the presence of spherical to obturbinate, noncaducous, papillate sporangia with short pedicels. The sporangia averaged 40 × 29 μm with ranges of 11 to 60 μm long and 20 to 45 μm wide (n = 20) (Erwin and Ribeiro 1996). Phytophthora nicotianae-specific primers targeting part of the 18S rRNA gene, the internal transcribed spacer (ITS) 1, the 5.8S rRNA gene, and the ITS 2 were used to amplify around ∼680 bp, testing positive for P. nicotianae. Forward and reverse primers used were 5′-CCAATAGTTGGGGGTCTTATT-3′ and 5′-AATTCAAAAGCCAAGCCACC-3′, respectively. The PCR product was cleaned (Qiagen Purification Kit) and sequenced (GenBank Accession No. KT455618). The sequence from our isolate was nearly identical (99% nucleotide identity) to an isolate of P. nicotianae (KJ549640) collected from a diseased Mediterranean fan palm in the United States. To further support identification, phylogenetic analysis by the maximum likelihood method (Tamura-Nei model) was performed (MEGA 6.06). Our isolate grouped with other P. nicotianae isolates with high support (98% bootstrap values, 1000 replicates). Pathogenicity of the sequenced isolate was evaluated using 4-month-old C. variegatum ‘Petra’ plants and 3 additional cultivars including ‘Mamey,’ ‘Gold Star,’ and ‘Red Ice Ton.’ Plants were inoculated with a zoospore suspension (1 × 106 spores/ml) from the P. nicotianae sequenced isolate. Inoculum or autoclaved water was sprayed over the foliage until runoff. Three plants of each variety were sprayed and the experiment was repeated. Inoculated plants were covered with plastic bags to maintain humidity and placed in a shade house at 25 to 32°C. Plants were covered for one week until brown water soaked spots appeared on the foliage. The spots coalesced resulting in severe leaf blight and subsequent defoliation within 14 days. Symptoms appeared only on Petra and no symptoms developed on control plants. Based on serological testing, molecular analysis, and distinguishing morphological characters, this is the first report of P. nicotianae causing foliar blight of C. variegatum ‘Petra’ in Florida. A local nursery reported that approximately 35% of 500,000 C. variegatum ‘Petra’ containerized plants growing in two South Florida locations were affected with blight. Consistent outbreaks of foliar Phytophthora occur frequently during periods of hot and humid conditions, so we anticipate this disease to have a substantial impact on the tropical foliage industry. © 2016, American Phytopathological Society. All rights reserved.