Prepared by Elizabeth J. Fichtner
Sclerotium rolfsii Sacc. : ‘Kudzu of the Fungal World’
Agonomycetes
teleomorph Athelia rolfsii (Curzi) Tu & Kimbrough


Host Range 

Sclerotium rolfsii, an omnivorous, soilborne fungal pathogen, causes disease on a wide range of agricultural and horticultural crops. Although no worldwide compilation of host genera has been published, over 270 host genera have been reported in the United States alone. Susceptible agricultural hosts include sweet potato (Ipomea batatas), pumpkin (Cucurbita pepo L.), corn (Zea mays), wheat (Triticum vulgare) and peanut (Arachis hypogea). Horticultural crops affected by the fungus are included in the genera Narcissus, Iris, Lilium, Zinnia, and Chrysanthemum. (4)

Distribution and Economic Importance

Although S. rolfsii is thought to have caused serious crop losses over many centuries, the first unmistakable report of the fungus dates back to 1892 with Peter Henry Rolfs’ discovery of the organism in association with tomato blight in Florida (1). Since Rolfs’ report in the late 19th century, the over 2,000 publications on the pathogen support evidence of its worldwide distribution, particularly in tropical and subtropical regions (1, 10).

The wide host range, prolific growth, and ability to produce persistant sclerotia contribute to the large economic losses associated with the pathogen. From a global perspective, and local perspective for North Carolina, peanut crops sustain higher losses than any other agricultural crop (1). In 1959, the United States Department of Agriculture estimated losses from $10 million to $20 million associated with S. rolfsii in the southern peanut-growing region (5), with yield depletions ranging from 1-60% in fields in the NC coastal plains region (1).

Morphological and Diagnostic Features
Photo: H.D. Shew
Colonies of S. rolfsii are readily distinguished on plant material or artificial media by gross morphological characteristics. Rapidly growing, silky-white hyphae tend to aggregate into rhizomorphic cords (1). In culture, the whole area of a petri plate is rapidly covered with mycelium, including aerial hyphae which may cover the lid of the plate. In fact, the fungus’ growth is so fast, Rolfs mentioned that "if the temperature is 80-90 F, in 48 hours you will have a growth that will in appearance rival swan down (1)." Both in culture and in plant tissue, a fan-shaped mycelial expanse may be observed growing outward and branching acutely (8).

At least two types of hyphae are produced (1). Coarse, straight, large cells (2-9um x 150-250um) have two clamp connections at each septation, but may exhibit branching in place of one of the clamps. Branching is common in the slender hyphae (1.5-2.5um in diameter) which tend to grow irregularly and lack clamp connections. Slender hyphae are often observed penetrating the substrate (1).

Sclerotia (0.5-2.0mm diameter) begin to develop after 4-7 days of mycelial growth (2,10). Initially a felty white appearance, sclerotia quickly melanize to a dark brown coloration (1). Townsend and Willetts (9) recognize four zones in the mature sclerotium: i) thick skin, ii) rind of thickened cells, iii) cortex of thin walled cells, and iv) medulla containing filamentous hyphae. Sclerotia forming on a host tend to have a smooth texture, whereas those produced in culture may be pitted or folded (6). Serving as a protective structure, sclerotia contain viable hyphae and serve as primary inoculum for disease development.

Signs and Symptoms

Although symptoms vary with the host affected, infection is usually restricted to plant parts in contact with the soil. S. rolfsii attacks stems, roots, leaves, and fruit (4). Disease on cucurbits, known as Southern Blight, results from infection of fruit and runners at the soil surface (11). The watery, soft, decayed tissue tends to remain on the ground when an infected fruit is picked up (11). In peanuts, early symptoms of infection are manifest in yellowing and wilting of the lateral branches, main stem, or the entire plant (2). Peanuts also sustain brown lesions at the stem base and developing pegs (2). Infected pods rot, becoming wet, tan, and spongy (2).
    Diagnostic signs of the fungus include characteristic white mycelial fans and brown sclerotia extending from infected tissues. In sweet potato beds, white mycelium is reported to cover the soil surface and grow up and over sprouts (3).

Epidemiology

Sclerotia serve as the principle overwintering structures and primary inoculum for disease (1). Persisting near the soil surface, sclerotia may exist free in the soil or in association with plant debris (2). Those buried deep in the soil may survive for a year or less, whereas those at the surface remain viable and may germinate in response to alcohols and other volatiles released from decomposing plant material (2,3). Thus, deep plowing serves as a cultural control tactic by burying sclerotia deep in the soil. High temperatures and moist conditions are associated with germination of sclerotia (7). High soil moisture, dense planting, and frequent irrigation promote infection (1,3,6).

Since S. rolfsii does not produce spores, dissemination depends on movement of infested soil and infected plant material. Use of contaminated equipment and machinery may spread sclerotia to uninfested fields.

Literature Cited
1. Aycock, R. 1966. Stem rot and other diseases caused by Sclerotium rolfsii. N.C. Agr. Expt. St. Tech. Bul. No. 174.

2. Backman, P.A., and T.B. Brenneman, 1984. Compendium of Peanut Diseases. Amer. Phytopath. Soc., St. Paul, Minnesota.

3. Clark, C.A., and J.W. Moyer. 1988. Compendium of Sweet Potato Diseases. Amer. Phytopath. Soc., St. Paul, Minnesota.

4. Farr, D.F., G.F. Bills, G.P. Chamuris, and A.Y. Rossman. 1989. Fungi on Plants and Plant Products in the United States. Amer. Phytopath. Soc., St. Paul, Minnesota.

5. Garren, K.H. 1959. The stem rot of peanuts and its control. Virginia Agr. Exp. Sta. Bull. 144.

6. Paolo, M.A. 1933. A sclerotium seed rot and seedling stem rot of mango. Philippine Journal of Science. 52:237-261.

7. Punja, Z.K. 1985. The biology, ecology, and control of Sclerotium rolfsii. Annual Review of Phytopathology. 23:97-127.

8. Takahashi, T. 1927. A sclerotium disease of larkspur. Phytopathology. 17:239-245.

9. Townsend, B.B., and H.J. Willetts. 1954. The development of sclerotia of certain fungi. Ann. Bot. 21:153-166.

10. Weber, G.F. 1931. Blight of carrots caused by Sclerotium rolfsii, with geographic distribution and host range of the fungus. Phytopathology 21:1129-1140.

11. Zitter, T.A., D.L. Hopkins, and C.E. Thomas. 1966. Compendium of cucurbit diseases. Amer. Phytopath. Soc., St. Paul, Minnesota.


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Other related links
N.C. State University Department of Plant Pathology
University of California Statewide Integrated Pest Management Project on  S. rolfsii
Alabama Agricultural Experiment Station--Chemical Control of S. rolfsii
American Phytopathological Society