Taxonomy

Domain	Eukarya
Kingdom	Fungi
Phylum	Ascomycota
Subphylum	Pezizomycotina
Class	Sordariomycetes
Subclass	Hypocreomycetidae
Order	Glomerellales
Family	Glomerellaceae
Genus	Colletotrichum

Scientific Name

Colletotrichum circinans

Scientific Name Synonyms

Colletotrichum dematium f. circinans

Vermicularia circinans

Common Name

onion smudge

Author: Maxwell Reitman, University of Florida

Reviewed by: Jeffrey Rollins, University of Florida

Pathogen

Colletotrichum circinans is an ascomycete parasite of the Allium family, which includes leeks, onions, garlic, and shallots among its hosts. It has large, dark setae along its acervuli 80-315µm thick and bears cream-colored spores, 14-30µm long and 3-6µm wide.

Symptoms and Signs

The disease can cause damping off and seedling blight in young plants. On leaves, it creates necrotic lesions. Deep tissue penetration can cause minute, sunken, yellowish spots. In colored hosts, the pigment vanishes forming an uncolored halo. However, most distinctive is the black-green stroma that develops sub-cuticularly and in spots or concentric rings 1-2 cm across. These stromata produce conidiomata, which bear masses of cream spores and black setae.

Ecology and Spread

The pathogen can persist in the soil on colonized onion debris for years after an outbreak. During spring and summer months when conditions become warm and wet, it will colonize outer scales with water-dispersed conidia. It is possible for C. circinans to complete its life cycle in less than a week. Its sexual stage is rarely seen in the wild.

Geographic Distribution

Colletotrichum circinans has been found ubiquitously anywhere onions are grown.

Management

Rotate crops on a 2-3 year basis as a solution for white cultivars.
Maintain good drainage.
Do not harvest during wet weather.
Remove affected bulbs from group when found.
Keep stored bulbs dry.
Use only healthy transplants.
Use colored varieties of onion.
Cultivate in soil free of disease for 2-3 years if possible.
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

If disease is found on onions, the following signs indicate C. circinans:

smooth, curved hyaline conidia borne on branching conidiophores;
black, largely subdermal stroma with bullseye appearance; and
black setae detectable with hand lens.

In culture the organism does not produce aerial mycelium and tends to remain flat and entire. It gradually turns from brown tones to salmon, orange, or white as conidia accumulate.

On SNA medium, the appressoria are solitary and variable in appearance all being pale or medium brown.

Colletotrichum circinans can be grown on general media such as PDA.

Resources and References

1. “Colletotrichum circinans” Mycobank. http://www.mycobank.org/MB/260903

2. “Colletotrichum circinans” Q-bank. http://www.q-bank.eu/Fungi/BioloMICS.aspx?Table=Colletotrichum%20species%20ONLINE&Rec=3&Fields=All

3. J.C. Walker. “Onion Smudge.” Journal of Agricultural Research Vol. XX, No. 9 Feb 1, 1921. http://naldc.nal.usda.gov/download/IND43966345/PDF

4. “Onion Smudge (Colletotrichum circinans)” Plantwise Knowledge Bank. http://www.plantwise.org/KnowledgeBank/Datasheet.aspx?dsid=14897

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)