First Report of Fusarium Wilt Caused by Fusarium oxysporum f. sp. palmarum on Canary Island Date Palm in Florida.

Canary Island date palm (Phoenix canariensis Chabaud) is an ornamental grown throughout the world. In the fall of 2009, a single plant of this species was observed with Fusarium wilt symptoms at a commercial property in Orlando, FL. Individual leaves had chlorotic or necrotic leaflets on one side of the leaf blade and a reddish brown stripe along the petiole and rachis. Petiole cross-sections exhibited discolored vascular tissue. Fusarium oxysporum was isolated from this tissue, with typical characteristics of macroconidia in pale orange sporodochia, microconidia in false heads on short monophialides, and chlamydospores (3). All colonies on potato dextrose agar had pale pinkish-salmon-colored mycelia. Macroconidia were mostly 3-septate, slightly curved, and ranged from 4.1 to 4.6 × 39.5 to 43.7 μm. Microconidia were unicellular, oval to reniform, and ranged from 3.1 to 3.3 × 7.1 to 7.5 μm. Single-spore isolates (PLM-509 and PLM-510A) were selected for molecular characterization, with PCR conducted using ef1 and ef2 primers (2). Resulting products were sequenced and queried for similarity against the NCBI and the FUSARIUM-ID databases (2) using BLAST. In both databases, the isolates did not match F. oxysporum f. sp. canariensis. Rather, the isolates matched F. oxysporum f. sp. palmarum, and of particular interest, were NRRL 46589 (GenBank Accession No. GQ154456) and NRRL 46592 (GenBank Accession No. GQ154468), which the isolates matched with 100% similarity. These NRRL isolates were from the same commercial property but different locations on the property as this Canary Island date palm (1). Prior to this identification, F. oxysporum f. sp. palmarum had only been associated with Fusarium wilt of queen palm (Syagrus romanzoffiana) and Mexican fan palm (Washingtonia robusta) in Florida (1). Furthermore, in September 2008, two of four Mexican fan palms in a concrete planter (not in the ground) at the same location as the diseased Canary Island date palm (in the ground) were symptomatic for Fusarium wilt and the pathogen was confirmed as F. oxysporum f. sp. palmarum. Therefore, pathogenicity studies were conducted on three-leaf seedlings of P. canariensis and W. robusta using PLM-510A. There were five replicate palms per isolate and control treatment, and they were inoculated by the same methodology used previously for seedlings (1). After 3 months, all inoculated W. robusta were dead and one of five inoculated P. canariensis was dead. After 6 months, three more P. canariensis had died for a total of four of five inoculated palms. The pathogen was reisolated from diseased palms of both species. All control palms remained healthy. The sequence for PLM-510A has been deposited in the NCBI database (GenBank Accession No. HQ727681). To our knowledge, this is the first report of Fusarium wilt of Canary Island date palm caused by F. oxysporum f. sp. palmarum. References: (1) M. L. Elliott et al. Plant Dis. 94:31, 2010. (2) D. M. Geiser et al. Eur. J. Plant Pathol. 110:473, 2004. (3) J. F. Leslie and B. A. Summerell. The Fusarium Laboratory Manual. Blackwell Publishing, Ames, IA, 2006.