Fusarium oxysporum f. sp. palmarum

Taxonomy

Domain	Eukarya
Kingdom	Fungi
Phylum	Ascomycota
Subphylum	Pezizomycotina
Class	Sordariomycetes
Subclass	Hypocreomycetidae
Order	Hypocreales
Family	Nectriaceae
Genus	Fusarium
Species	Fusarium oxysporum

Scientific Name

Fusarium oxysporum f.sp. palmarum

Common Name

Fusarium wilt of queen and Mexican fan palms

Author: Jiaming Yu, University of Florida

Reviewed by: Jeffrey Rollins, University of Florida

Pathogen

Fusarium wilt of queen and Mexican fan palms is caused by the fungal plant pathogen Fusarium oxysporum f. sp. palmarum. It is known to attack Syagrus romanzoffiana (Queen palm) and Washingtonia robusta (Mexican fan palm) (1, 2). In 2011, this fungus also was reported to attack Pheonix canariensis Chabaud (Canary Island date palm) in Florida (3), but infection of this host is rare (1).

Symptoms and Signs

Initial symtoms appear on the leaflets, occurring on at least one or on all leaflets located on one side of the rachis. Affected leaflets turn chlorotic or necrotic (i.e., yellow or brown in color), while the leaflets on the opposite side of the rachis appear healthy and green. On the petiole and rachis, a reddish brown or dark brown stripe appears on the same side as the affected leaflets. The stripe could be along the full length or only a portion of the petiole and rachis. Cross sections of the petiole or rachis reveal discoloration of the interior vascular bundles. Once one side of a frond dies, symptoms will progress from the frond tip to base on the other side, until the entire frond is dead. Eventually the whole palm will die, but the dried fronds do not usually fall off, instead remaining attached to the palm. Once the palm exhibits symptoms of F.o. palmarum infection, it can die within several weeks to months (1).

Ecology and Spread

Fusarium oxysporum is a soilborne fungus. Chlamydospores produced by this fungus can survive in plant debris and soil for years. Transmission of disease depends on the movement of infected trees or infested soil and is also possible through the use of contaminated pruning tools (1). Initial infection is believed to occur from chlamydospores in soil or plant debris located in the upper 5-30 cm layer of soil. The fungus invades through root pneumathodes. It then grows inside the aerenchyma and proceeds into the cortex, before finally recaching and penetrating the xylem vascular tissue. At the same time, adjacent parenchyma and schlerenchyma cells are colonized and abundant chlamydospres are produced (1, 4). Generally in the dry season, the disease spreads slowly with proper pruning practices and minimal irrigation. Conversely, disease spread increases in the presence of excessive irrigation and /or failure to adequately sterilize pruning tools (1).

Geographic Distribution

USA: Florida, Texas (1, 2, 3)

Management

There are no known methods to save symptomatic trees or prevent infection. Chemical controls have not been shown to be successful or cost effective. Currently, the control of this disease in palms is focused on methods to reduce disease spread. An important aspect of this control involves proper sterilizaiton of tools between pruining trees. Before planting, consider choosing non-host palm species, especially if there is a history of previous disease incidence. Once a palm is known to be affected by Fusarium, it should be cut down and burned or otherwise removed. As long as it remains in place, it provides a potential continuing source of infection for the surrounding landscape (1).

Diagnostic Procedures

For field diagnositic: See Symptoms and Signs section.

For lab diagnostic: Isolate spores from leaf tissue. Incubate leaf tissue to allow production of spores. (Single spore technique can be found on Bugwoodwiki.)

Diagnostic features: Three types of spores are produced: macroconidia, microconidia, and chlamydospores.On potato dextrose agar (PDA), F.o. palmarum colonies have pale pinkish or purple color on the top surface in general. Macroconidia are formed in orange sporodochia, they are three to four septate with curved apical cell and a foot-shaped basal cell. Microconidia are unicellular, oval, elloptical, or reinform in shape. Within 4 weeks after isolation, chlamydospores are produced (1,4). Molecular tests are required to confirm the pathogenic isolate because morphology of pathogenic and nonpathogenic isolates are indistinguishable in culture. Molecular diagnostic methods include PCR.

Culture media: Low nutrient general growth media such as WA, ½ PDA are used in initial isolation. Fusarium selective media such as Komada’s media is selective for F. oxysporum isolation. However, the disadvange is that it can be only good to species level (5). (PDA recipe: distilled water 1L, potato:200g, dextrose: 20g, agar: 20g)

Resources and References

1. Elliott, M.L., 2010. Fusarium wilt of queen palm and Mexican fan palm. EDIS. Publication #PP278

2. Elliott, M.L., E.A. Des Jardin, K. O'Donnell, D.M. Geiser, N.A. Harrion and T.K. Broschat. 2010. Fusarium oxysporum f.sp. palmarum, a novel forma specialis causing a lethal disease of Syagrus romanzoffiana and Washingtonia subusta in Florida. Plant Dis. 94: 31-38.

3. Elliott, M.L. 2011. First Report of Fusarium Wilt Caused by Fusarium oxysporum f. sp. palmarum on Canary Island Date Palm in Florida. Amer. Phytopath. Soc. Pp. 356.

4. Elliott, M.L., T.K. Broschat, J.Y. Uchida, G.W. Simone. 2004. Compendium of ornamental palm disease and disorders. Amer. Phytopath. Soc. Press, St. Paul, MN.

5. Windels, C.E. Fusarium. L.L. Singleton, J.D. Mihail, and C.M.Rush. Methods for research on soilborne phytopathogenic fungi. Pp. 122.

Acknowledgments

• National Institute of Food and Agriculture, U.S. Department of Agriculture, under Agreement No. 2011-41530-30708 as part of "Diagnostic Image Series Development for Supporting IPM in the Southern Region" (USDA-NIFA-RIPM-003351)