Fusarium wilt of palms occurs worldwide, caused by different Fusarium oxysporum ff. spp. including F. oxysporum f. sp. elaeidis, F. oxysporum f. sp. canariensis, and F. oxysporum f. sp. albedinis (3). Prior to 2010, F. oxysporum f. sp. canariensis was the only palm infecting species known to occur in the United States. In 2010, isolates of F. oxysporum were reported from dying Syagrus romanzoffiana and Washingtonia robusta in Florida. Based on morphological and molecular data, as well as the unique host species affected by the pathogen, this fungus was determined to be a new forma specialis of F. oxysporum, designated f. sp. palmarum (1). The pathogen infects foliar tissue, causing complete necrosis of the crown and leading to tree death within 2 to 3 months. In June 2012, the Texas Plant Disease Diagnostic Laboratory (TPDDL) received a plant sample from a dying W. robusta palm, exhibiting reddish-brown stripes on the petiole with chlorotic and necrotic leaves, from an established palm in the landscape from Harris County, Texas. Fungal cultures were obtained from symptomatic foliar tissue and identified as F. oxysporum based on morphology. Microconidia were oval to reniform, 1- to 2-septate, measuring 5 to 18 × 2.5 to 5 μm. Phialides were short with microconidia produced in false heads. Macroconidia were curved and slender with a foot-shaped basal cell, usually 3-septate, and 22 to 37 × 2.5 to 5 μm. Chlamydospores were roundish and ranged from 7 to 13 μm in diameter. Fungal colonies had white to purple mycelia when grown on potato dextrose agar. DNA from a single spore culture was extracted, amplified by PCR using primers corresponding to a segment of the translation elongation factor 1α (EF-1α) gene, and the PCR product sequenced (2). Using the sequence alignment tool (BLASTn) in GenBank, the TPDDL's sequence (GenBank Accession No. KC897693) was aligned with EF-1α regions from F. oxysporum f. sp. palmarum isolates previously entered into the database ([1]; accessions GQ154455[=NRRL53544] and GQ154456[=NRRL46589]), revealing 100% homology between the isolates. Based on host source and sequence similarity, the fungus was tentatively identified as F. oxysporum f. sp. palmarum. Pathogenicity tests were performed on three leaf seedlings of W. robusta and W. filifera. Fifteen plants of each species were inoculated with the suspect isolate (designated KB2012) and 10 control plants were mock-inoculated as described by (1). Plants were grown in a greenhouse for 8 weeks post-inoculation. During this time, 83% of inoculated plants developed foliar lesions and died or severely declined, and all control plants remained healthy. F. oxysporum was recovered in culture from 100% of the symptomatic plants. DNA was extracted from fungal cultures, and EF-1α was amplified by PCR and sequenced, as described above. The amplicon was determined to share 100% homology with known F. oxysporum f. sp. palmarum isolates, confirming this fungus as the cause of disease in W. robusta. This is the first report of this pathogen in Texas, as well as the first report outside of Florida. This is also the first documentation of W. filifera as a host of this pathogen.
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) G. W. Simone. Pages 17-19 in: Compendium of Ornamental Palm Diseases and Disorders, M. L. Elliott et al., eds. The American Phytopathological Society, St. Paul, MN, 2004.
