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. perniciosum

Common Name

mimosa wilt

Author:Elizabeth Bush, Virginia Tech

Reviewed by: Matthew Kasson, West Virginia University

Pathogen

Fusarium wilt is a common and lethal disease of mimosa (Albizia julibrissin), also commonly known as Persian silktree. Mimosa is native from central China to Iran and was introduced in the early nineteenth century to the United States. Fusarium wilt is caused by a member of the Fusarium oxysporum species complex: Fusarium oxysporum forma specialis perniciosum. Members of the F. oxysporum species complex are cosmopolitan and many cause vascular wilt on many economically significant plants. Albizia spp. are the only known host of F. oxysporum f.sp. perniciosum. Although the pathogen has been suggested as a potential biocontrol for the invasive tree-of-heaven, Ailanthus altissima, (Stipes, 2001) F. oxysporum f. sp. ‘perniciosum’ has not been proven pathogenic to tree of heaven (Kasson et al., 2014). Fusarium oxysporum f. sp. perniciosum colonizes and clogs the tree’s vascular (water-conducting) tissue, which interferes with the movement of water and nutrients. This results in relatively rapid tree death.

Symptoms and Signs

A very early symptom of Fusarium wilt is a brown-streaking in the wood beneath the bark and/or the roots; however, these symptoms typically go unnoticed. The first readily noticeable symptoms are yellowed, stunted, and wilted leaves on one or several branches in early to mid-summer. Later in the summer the affected branches may prematurely defoliate. Yellowed and wilted leaves continue to appear on more branches throughout the summer and fall, although in some cases a tree may die within a month of initial symptom appearance. Streaks of discoloration in the sapwood are very characteristic for the disease and can be observed by removing the bark on diseased branches or trunk or by observation of a discolored ring of sapwood in a cross-section of a diseased branch. As the disease progresses cracks begin to appear in the bark. In some cases, gum or a white, frothy liquid with a fermented odor may exude from cracks. Sometimes a tree survives to the next growing season, but the new leaves will be stunted and yellowed and the tree will continue on a relatively rapid decline toward death. After the aboveground portion of the tree dies roots may still be alive and shoots may continue to sprout from the base of the trunk for some time.

Ecology and Spread

Fusarium oxysporum f. sp. perniciosum is a soilborne fungus. The long-term survival of this fungus occurs by means of chlamydospores in the soil. When roots of host trees grow close to chlamydospores, root exudates are able to stimulate chlamydospores to germinate and produce mycelium. The mycelium then penetrates and colonizes the mimosa roots, moving into the vascular tissue where the fungus begins to produce microcondia. The microcondia move upward with the tree’s sap and become deposited in various locations in the vascular tissue. Thus, a systemic colonization of the plant’s vascular system occurs. This eventually results in a clogged and defective vascular system.

When the tree defoliates prematurely due to this disease, the fungal pathogen grows out from the vascular tissue into the bark and begins production of pink to orange spore masses within sporodochia in lenticels. Macrocondia are produced in these spore masses. The fungus may continue to produce macroconidia for a period of two years after tree death. The macroconidia can be spread to new locations by air, water and insects; however, the primary means of spread occurs through water which washes the macrocondia into the soil where they germinate and produce chlamydospores. The chlamydospores can be spread to new locations through movement of infested soil or on contaminated equipment, tools, and shoes. The fungus may also persist in seed produced by diseased trees.

Geographic Distribution

The first report of Fusarium wilt in the United States was in North Carolina in 1935 and currently its occurrence is reported to be similar to the range mimosa (northeast to Florida and west to Mississippi and California). It has also been reported in Puerto Rico, Argentina, and Greece. How the pathogen was introduced to the U.S. is unknown; however, a similar mimosa wilt disease has been reported in southern Russia and Japan.

Management

Fusarium wilt is a lethal disease for which there are no controls. However, most tree species are not susceptible to Fusarium oxysporum f. sp. perniciosum, so replacing diseased mimosa with another non-susceptible tree species is the best option. Currently, there are only a few mimosa cultivars (‘Charlotte’, ‘Tryon’ and ‘Union’) with resistance to Fusarium wilt. However, these cultivars do not appear to be widely available in the nursery trade and strains of the Fusarium wilt pathogen have been reported to overcome resistance in ‘Charlotte’ and ‘Tryon’. Additionally, resistance to Fusarium wilt has been overcome when root damage from root-knot nematodes (Meloidogyne species) has occurred (Sinclair and Lyon, 2005).

Diagnostic procedures

Examine the trunk or symptomatic branches for sporodochia of the fungus in lenticels. Make microscope mounts from these locations and examine for macroconidia. Macroconidia of F. o. f. sp. perniciosum are hyaline, multi-septate, canoe-shaped and 23-60 µm long and 3-4.5 µm wide. Moist chamber incubation of symptomatic branch or trunk sections may be necessary to induce sporulation. Other Fusarium spp. including F. solani have been reported to sporulate on dead mimosa and these could be mistaken for F. o. f. sp. perniciosum.

Remove bark from symptomatic branches or the trunk and examine the sapwood for streaks of discoloration.F. o. f. sp. perniciosum can be isolated from discolored sapwood on a general media, such as acidified potato dextrose agar (APDA), or a Fusarium-selective media, such as Fusarium selective medium or modified Nash and Snyder medium. Using sterile technique and working in a laminar flow hood, if possible: 1) Disinfest branch samples that exhibit streaking in sapwood by spraying with 70% ethanol. 2) Allow to air-dry. 3) Remove bark and excise chips from the streaked sapwood. 4) Cut excised chips down to a size no larger than 3 mm x 3 mm. 5) Insert chips into culture media. 6) Incubate culture plates in the dark at 28°C. Purification of the culture by single spore isolation is recommended before sub-culturing for examination of morphological features or in preparation for sequencing.

Summerell et al. (2003) provides a very useful, step-by-step protocol for identification of Fusarium spp. Morphological details of F. oxysporum (and other Fusarium spp.) are available in several resources (e.g. The Fusarium Laboratory Manual (Leslie and Summerell, 2006) is an excellent reference for details on key characteristics of F. oxysporum on CLA and PDA). The procedures and morphological characteristics listed below have been summarized from these two publications.

For examination of morphological features sub-culture onto both carnation leaf agar [CLA] and potato dextrose agar (PDA). CLA is recommended for production of morphological structures for identification (i.e. conidiophores, microconidia, macroconidia and chlamydospores). Incubate CLA cultures at room temperature for 7 to 10 days with 12 h light/12 h dark to induce sporulation. Remove the Petri dish lid, place culture under low power objective (e.g. 10X) of compound microscope and examine the culture to observe aerial mycelial morphology (microconidia and chlamydospores). Microconidia of F. oxysporum are abundant and produced in false heads on short monophialides; they are hyaline, usually non-septate and measure 6-12 µm x 2-3.5 µm. Prepare two microscope slide mounts from CLA culture: One from carnation leaf tissue (sporodochia containing macroconidia should be produced on carnation leaf tissue) and one from the agar (microconidia/conidiophores and chlamydospores, if present, can be observed in the agar mount). F. oxysporum typically produce numerous sporodochia in which macroconidia (described above) are produced. Chlamydospores typically take 2 to 4 weeks to be produced and are not formed by all F. oxysporum isolates.

Potato dextrose agar is recommended for observation of culture characteristics of Fusarium spp.; however, features, such as pigmentation, are variable for F. oxysporum. PDA cultures can be incubated similarly to CLA cultures and colony features and growth habit can be observed from the cultures.

Sequencing used in conjunction with morphology is helpful for identifying an isolate to a Fusarium sp. or species complex. The translation elongation factor 1 alpha (Ef-1a) is a genetic locus commonly used for identifying Fusarium spp. However, there are multiple genetic loci that are also used and a multi-loci sequencing approach may be necessary for confident resolution of a species or species complex. Specific details on loci used for identifying Fusarium spp. and a database of known sequence data is available at the Fusarium-ID website (http://isolate.fusariumdb.org)

Resources and References

Fisher N.L., Burgess L.W., Toussoun T.A., Nelson P.E. 1982. Carnation leaves as a substrate and for preserving cultures of Fusarium species. Phytopathology 72: 151-3.
Geiser D.M., Jiménez-Gasco M.D.M., Kang S.C., et al. 2004. FUSARIUM-ID v. 1.0: a DNA sequence database for identifying Fusarium. European Journal of Plant Pathology 110: 473-9.
Kasson, M.T., Short, D.P., O'Neal, E.S., Subbarao, K.V. and David, D.D.2014. Comparative pathogenicity, biocontrol efficacy, and multilocus sequence typing of Verticillium nonalfalfae from the invasive Ailanthus altissima and other hosts. Phytopathology: 104(3): 282-92.
Leslie J.F., Summerell B.A. 2006. The Fusarium laboratory manual. Oxford: Blackwell Publishing.
Nesbitt R.B., Tidwell T.E. Stipes R.J., Griffin G.J. 1999. First report of mimosa wilt disease of silk tree Albizia julibrissin in California caused by Fusarium oxysporum f. sp. perniciosum. Plant Disease 83: 487.
Sinclair, W. A., and Lyon, H. H. 2005. Diseases of Trees and Shrubs, 2nd ed. Cornell University Press, Ithaca, NY.

Skarmoutsou H., Skarmoutsos G. 1999. First report of Fusarium wilt disease of mimosa in Greece. Plant Disease 83: 590.

Stipes R.J., 2001. Fusarium wilt of trees. In Shade Tree Wilt Diseases. Ed. C.L. Ash. St. Paul: American Phytopathological Society. 165-9.
Summerell B.A., Salleh B., Leslie J.F., 2003. A utilitarian approach to Fusarium identification. Plant Disease 87: 117-28.

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)