Taxonomy

Domain	Eukarya
Kingdom	Fungi
Phylum	Ascomycota
Subphylum	Pezizomycotina
Class	Leotiomycetes
Subclass	Leotiomycetidae
Order	Helotiales
Family	Sclerotiniaceae
Genus	Sclerotinia

Scientific Name

Sclerotinia sclerotiorum

Scientific Name Synonyms

Sclerotium varium

Sclerotinia libertiana

Sclerotium opuntiarum

Sclerotinia sclerotiorus

Common Name

Sclerotinia timber rot

Author: Sanju Kunwar, University of Florida

Reviewed by: Jeffrey Rollins, University of Florida

Pathogen

Sclerotinia sclerotiorum (Lib.) de Bary is a devastating necrotrophic fungal plant pathogen that infects more than 400 plant species worldwide (2, 3). The pathogen is the causal agent of white mold (4). Host species include lettuce, cabbage, sunflower, canola, and bean (2, 3).

Symptoms and Signs

Depending on the host, S. sclerotiorum produces either water-soaked lesions or dry lesions on stems, leaves, fruits, or petioles. When humidity is high, the pathogen produces white fluffy hyphae on the host surface (4). Multi-hyphal black resting structures called sclerotia (2) are produced during infection. Sclerotia are a means of longterm survival to the pathogen. White mold and sclerotia are obvious signs of the S. sclerotiorum infection.

Sclerotinia sclerotiorum infection of celery stalk produces pinkinsh watery rot symptom often called “pink rot”. In beans, infection is seen in pods both before and after harvest (5). In cabbage, the infection of the lower leaves produces leaf drop symptoms, which looks similar to that produced by S. minor (1). In lettuce, initial outer infection followed by inward movement of the fungus causes wilting and falling of the leaves from the head in succession. Wilting symptoms are observed in sunflower around the time of flowering. The spread of infection forms basal cankers in sunflowers that cause the pith becomes shredded and bleached. The formation of the cankers decrease the turgor pressure in plant, thereby resulting wilting symptom (5).

Ecology and Spread

Sclerotinia sclerotiorum infects the lower leaves of host, but can also infect upper leaves by producing aerial spores. The aerial spores usually go through a saprophytic growth stage on damaged or senescent plant tissue before further infection. At late stages of disease, white fluffy mycelia and sclerotia are produced surrounding the lesions. The sclerotium enable the pathogen to survive up to 8 years (1). Depending on environment, sclerotia germinate to give either mycelia or apothecia. Apothecia produce ascospores, which are the main inoculum source of primary infection in many hosts (2). Contaminated soil, infected seedlings, and contaminated farm equipment and footware also spread sclerotia or mycelium from the infected to the healthy fields.

Geographic Distribution

Originally believed to be a pathogen of cool, moist areas, the organism is cosmopolitan and now is found in both cold and dry regions of the world.

Management

Minimize the humidity in the foliage by providing adequate spacing during planting.
Prevent foliage drop by wire trellis support.
Avoid wet soil. Keep the bed surface dry by providing proper and careful irrigation.
Prevent water-logging in the field by preventing uneven surfaces.
Remove and properly dispose of the infected plants to reduce the inoculum in the field.
Apply fungicides appropriately, starting at the early rosette stage (during disease conducive conditions) to minimize the disease.
Consult your local extension specialist for legal and efficacious fungicide products available in your state. Remember, the label is the law and the product applicator is responsible for reading and following all chemical labeling.

Diagnostic Procedures

White fluffy mycelium
Hyphae- hyaline, branched and multinucleate with septations
Sclerotia- (0.25–0.50) inch, large and irregular, dark-brown to black in color
Apothecioid ascoma yellow-brown to tan
Apothecia arise from a sclerotium (not formed on stroma) and produce asci that stains blue with iodine.
A semiselective media (PDA with pentachloronitrobenzene, bromophenol blue, streptomycin, and penicillin) can be used for isolating fungus. Oxalic acid produced by the fungus changes the color of the media from blue to yellow (6).
Molecular methods are also available for detection (7).

Resources and References

1. Adams, P. B., and W.A. Ayers. 1979. “Ecology of Sclerotinia species.” Phytopathology 69(8):896-899.

2. Bolton D. M., Thomma H. J. B., and Nelson D. B., 2006. Sclerotinia sclerotiorum (Lib.) de Bary: biology and molecular traits of a cosmopolitan pathogen. Molecular Plant Pathology 7(1):1–16.

3. Cubeta, M. A., Cody, B. R., Kohli, Y., and Kohn, L. M. 1997. Clonality in Sclerotinia sclerotiorum on infected cabbage in eastern North Carolina. Phytopathology 87:1000-1004.

4. Hanlin R. T., 1998. Combined Keys to Illustrated Genera of Ascomycetes volumes I & II. APS Press. St. Paul, MN.

5. Heffer Link, V., and K. B. Johnson. 2007. White Mold. The Plant Health Instructor.

6. Steadman, J.R., J. Marcinkowska, and S. Rutledge. 1994. A semi-selective medium for isolation of Sclerotinia sclerotiorum. Canadian Journal of Plant Pathology 16:68-70.

7. Yanni, Y., Xin, L., Zhiqi, S., Zhonghua, M. 2010. A multiplex allele-specific PCR method for the detection of carbendazim-resistant Sclerotinia sclerotiorum. Pesticide Biochemistry and Physiology 97(1):36-42.

Acknowledgements

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)