Fusarium oxysporum f. sp. lycopersici (Sacc.) W.C. Snyder and H.N. Hans
Prepared by Mui-Yun Wong
PP728 Soilborne Plant Pathogen Class Project, Spring 2003
Fusarium oxysporum f. sp. lycopersici (Sacc.) W.C. Snyder and H.N. Hans, a soilborne plant pathogen in the class Hyphomycetes, causes Fusarium wilt specifically in tomato. This disease was first described by G.E. Massee in England in 1895. It is of worldwide importance where at least 32 countries had reported the disease, which is particularly severe in countries with warm climate. At one time, the disease nearly destroyed tomato production in parts of Florida and the southeastern states of United States. However, the development and use of resistant cultivars have nearly eliminate the concern over this disease.
Host Range and Distribution
Three physiological races of this pathogen have been reported. Race 1 is the most widely distributed and has been reported from most geographical areas. Although race 2 was first reported in Ohio in 1940, it did not become widespread or of economic concern until its discovery in Florida in 1961. Since then, it was rapidly reported in several of the states and in several other countries, including Australia, Brazil, Great Britain, Israel, Mexico, Morocco, the Netherlands, and Iraq. Race 3 was reported in 1966 in Brazil. Thereafter, it has been found in Australia and in Florida and California.
Isolation from host (Carroll, 2003; refer to link # 6): Cut into the base of the stem of a diseased plant lengthwise to reveal the xylem just below the epidermis. Trim off all the leaves and secondary roots leaving only the main stem and the hypocotyls and main root. Surface sterilize the stem by soaking in 10% bleach solution for 5 minutes. Dry the stem on paper towels. Using sterile technique, cut thin (2-4 mm thick) wedges out of one side of the stem near the root/stem junction making sure to include xylem tissue with each wedge. Place 5-6 wedges on PDA plates. Incubate the plates under fluorescent lights. Once the fungus has grown sufficiently from the pieces, transfer isolates onto fresh PDA plates. Incubate the plates for 10-14 days. Colonies of F. oxysporum are pigmented with a reddish purple color and surmounted by a pinkish white aerial mycelium.
Isolation from soil (Komada’s Medium; refer to Ref. 5): Dilution plate method is used. The soil dilutions are plated on a selective medium. The plates are kept under fluorescent lights for incubation. Colonies of F. oxysporum are pigmented with a reddish purple color and surmounted by a pinkish white aerial mycelium.
The mycelia (Plate 1) of Fusarium oxysporum f. sp. lycopersici (Sacc.) W.C. Snyder and H.N. Hans are delicate white to pink, often with purple tinge, and are sparse to abundant. The fungus produces three types of spores: microconidia (Plate 2), macroconidia (Plate 3), and chlamydospores (Plate 4). Microconidia are borne on simple phialides arising laterally and are abundant, oval-ellipsoid, straight to curved, 5-12 x 2.2-3.5 mm, and nonseptate. Macroconidia, sparse to abundant, are borne on branched conidiophores or on the surface of sporodochia and are thin walled, three- to five-septate, fusoid-subulate and pointed at both ends, have pedicellate base. Three-septate conidia measure 27-46 x 3-5 mm while five-septate conidia measure 35-60 x 3-5 mm. Three-septate spores are more common. Chlamydospores, both smooth and rough walled, are abundant and form terminally or on an intercalary basis. They are generally solitary, but occasionally form in pairs or chains. No perfect stage is known.
Plate 1 Plate 2 Plate 3 Plate 4
Plate 1, Mycelium of Fusarium oxysporum f. sp. lycopersici; Plate 2, Microconidia; Plate 3, Macroconidia;
Plate 4, Chlamydospores. Pictures taken from Toussoun, T.A., and Nelson, P.E. 1976.
The first indication of this disease is a yellowing (Plate 5) and drooping of the lower leaves. This symptom often occurs on one side of the plant or on one shoot. Successive leaves yellow, wilt and die, often before the plant reaches maturity. As the disease progresses, growth is typically stunted, and little or no fruit develops. If the main stem is cut, dark brown streaks may be seen running lengthwise through the stem. This discoloration (Plate 6) often extends far up the stem and is especially noticeable in a petiole scar. The browning of the vascular system is characteristic of the disease and generally can be used for its identification.
|Plate 5. Yellowing and death of leaves on one side of the stem. (image from Miller, Rowe & Riedel, 2003; refer to link #1).||
Plate 6. Dark
brown vascular discoloration. (image from Miller,
Rowe & Riedel, 2003; refer to link #1).
Fusarium wilt is a warm-weather disease, most prevalent on acid, sandy soils. The pathogen is soilborne and remains in infested soils for up to ten years. Soil and air temperatures of 28°C are optimum for disease. Too warm (34°C) or too cool (17-20°C) soils retard wilt development. If soil temperatures are optimum but air temperatures below optimum, the pathogen will extend into the lower parts of the stem, but the plants will not exhibit external symptoms. In general, factors favoring wilt development are: soil and air temperatures of 28°C, soil moisture optimum for plant growth, plants preconditioned with low nitrogen and phosphorus and high potassium, low soil pH, short day length, and low light intensity. Virulence of the pathogen is enhanced by micronutrients, phosphorus, and ammonium nitrogen and decreased by nitrate nitrogen. The pathogen enters the plant through the roots and is then spread throughout the plant by the vascular system.
Dissemination of the pathogen is via seed, tomato stakes, soil, and infected transplants or infested soil adhering to transplants. The pathogen could be disseminated long distance through seed and transplants. Local dissemination is by transplants, tomato stakes, windborne and waterborne infested soil, and farm machinery.
Control measure is mainly through the use of resistant cultivars. The control of races 1 and 2 utilizes both polygenic and monogenic resistance while monogenic resistance to race 3 has been developed. Pasteurization of infested soil with steam or fumigants, raise the soil pH to 6.5-7.0, and usage of nitrate nitrogen rather than ammonium nitrogen help to reduce the incidence of wilted plants and greatly increases marketable and total yields.
1. Jones, J.P., and Woltz, S.S. 1981. Fusarium-incited diseases of tomato and potato and their control. Pages 157-168 in Fusarium: Diseases, Biology, and Taxonomy. P.E. Nelson, T.A. Toussoun, and R.J. Cook, eds. Pennsylvania State University Press, University Park.
2. Jones, J.P. Page 15 in Compendium of Tomato Diseases. J.B. Jones, J.P. Jones, R.E. Stall, and T.A. Zitter, eds. APS Press.
3. Toussoun, T.A., and Nelson, P.E. 1968. A Pictorial Guide to the Identification of Fusarium Species. Pennsylvania State University Press, University Park.
4. Toussoun, T.A., and Nelson, P.E. 1976. A Pictorial Guide to the Identification of Fusarium Species, Second Edition. Pennsylvania State University Press, University Park.
5. Pierre Davet, and Francis Rouxel. 2000. Detection and Isolation of Soil Fungi. Science Publishers, Inc., Enfield, NH, USA.