Canary Island date palm (Phoenix canariensis) is native to the Canary Islands and widely grown throughout the world as an ornamental. At a home site in Austin, TX in May 2008 and a commercial site near Charleston, SC in December 2009, declining Canary Island date palms were observed. Symptoms included individual leaves with chlorotic or necrotic leaflets on one side of the leaf blade (one-sided wilt or death) and a distinct reddish brown stripe along the petiole and rachis. Cross-sections through the petiole or rachis exhibited discoloration of internal tissue. Fusarium oxysporum was isolated from the internal petiole or rachis tissue of each palm sample onto one-quarter-strength potato dextrose agar (PDA). Typical macroconidia in pale orange sporodochia, microconidia in false heads on short monophialides, and chlamydospores were observed (2). Macroconidia were mostly 3-septate, slightly curved, and ranged from 3.8 to 4.2 × 42.9 to 46.5 μm. Microconidia were single cell, oval to reniform, and ranged from 2.5 to 2.9 × 7.2 to 7.8 μm. Single-spore isolates grown on full-strength PDA (12-h light and 26°C) produced abundant white-to-pale lavender mycelia with a purple pigment in the agar. One isolate from each location (PLM-385B from Texas and PLM-511A from South Carolina) was selected for pathogenicity tests and molecular characterization. The translation elongation factor 1-α gene (EF-1α) was amplified in each isolate by PCR using the ef1 and ef2 primers (1). Products were sequenced and queried for similarity against the NCBI database and the FUSARIUM-ID database (http://isolate.fusariumdb.org/index.php) (1) using the BLAST search tool. In both databases, both isolates matched F. oxysporum f. sp. canariensis strain NRRL 26035 (GenBank Accession No. AF008485; FD_01211) at 100% sequence similarity. Sequences for PLM-385B and PLM-511A have been deposited in the NCBI database (GenBank Accession Nos. HM591537 and HM591538, respectively). Pathogenicity of these two isolates was tested on three-leaf Canary Island date palm seedlings. There were five replicate palms per isolate and control treatment. All potting mix was shaken from the roots and three groups of five seedlings were placed in small buckets. Twenty-five milliliters of a 106 conidia ml–1 suspension was pipetted down among the leaf bases and the excess drained onto the roots. Control palms received sterile water. Seedlings were covered with plastic for 48 h and then transplanted into separate growing containers. Ten weeks after inoculation, initial symptoms of a leaf wilt (off-color and folded over) were observed on some of the inoculated palms. After 4 months, all palms inoculated with PLM-511A were dead and three of the five palms inoculated with PLM-385B were dead. The pathogen was reisolated from diseased palms. All five control palms remained healthy. While the symptomatic palm in Texas had been in the home site approximately 2 years, which implied the palm could have been already infected when transplanted, the palm in South Carolina had been planted in 1990. To our knowledge, this is the first report of Fusarium wilt of Canary Island date palm in Texas and South Carolina. Previously in the United States, the disease had only been noted in California, Florida, and Nevada.
References: (1) D. M. Geiser et al. Eur. J. Plant Pathol. 110:473, 2004. (2) J. F. Leslie and B. A. Summerell. The Fusarium Laboratory Manual. Blackwell Publishing, Ames, IA, 2006.
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