Taxonomy

Domain	Eukarya
Kingdom	Fungi
Phylum	Ascomycota
Subphylum	Pezizomycotina
Class	Eurotiomycetes
Subclass	Eurotiomycetidae
Order	Eurotiales
Family	Trichocomaceae
Genus	Aspergillus

Scientific Name

Aspergillus niger

Scientific Name Synonyms

Sterigmatocystis nigra

Common Name

black mold

Author: Lanette Sobel, University of Florida

Reviewed by: Jeffrey Rollins, University of Florida

Pathogen

Aspergillus niger is not only a species of plant pathogen, but a group in the genus Aspergillus that is made up of 15 varieties, all with black conidia (2).

The host range includes 37 genera of fruits and vegetables such as tomatoes, peanuts, grapes, onions, and mangoes. The fungus is commonly found in the soil, living as a saprophyte on decaying vegetation and leaves, compost piles, and stored grain. It is also found in indoor environments and as a contaminant of food. As the fungus is opportunistic, humans, birds, and animals can also be hosts, although this is extremely rare (3).

Aspergillus niger is generally recognized as safe by the USDA. In fact, many strains are used in commercial food production, including in fermentation processes and the production of citric and gluconic acid (3).

Symptoms and Signs

Aspergillus niger causes several diseases including black mold (or black rot) of onions and garlic, crown rot of peanuts, and vine canker of table grapes. In black rot of onions and garlic, A. niger develops between the outer (dead, flaky) skin and the first fleshy scales of the bulb which become water soaked. If conditions are dry, the area dries and shrivels, revealing black spore masses between outer scales. Secondary invaders are common and transform the bulb into a soft, watery mass (4).

Crown rot of peanuts is characterized by root curling and deformation of the upper parts of the plant (3).

In vine canker of table grapes, preliminary symptoms include pinhead drops of reddish sap at the infection site. As the disease progresses, the trunk of the vine becomes girdled, spongy, and slightly larger. The vine displays fall colors prematurely and the inside of the canker contains powdery black spores. The cankers average about 8 cm and are commonly calloused (3).

Other diseases attributed to A. niger include: tuber rot of yams, stem rot of Dracaena, black mold rot of cherry, kernel rot of maize, fruit rot of grapes, fruit rot of banana, rot of tomatoes, boll rot of cotton, root stalk rot of Sansevieria, and mango rotting (3).

Post-harvest decay results in discoloration, quality deterioration, and reduction in the commercial value of various crops (3).

In humans, rare cases have linked A. niger to aspergillosis, a group of diseases that relate to the growth of and allergic responses to the fungus (3).

Ecology and Spread

Aspergillus niger typically reproduces in the asexual state, although sexual reproduction has been observed. In the typical asexual state, conidia (i.e, spores) are released and disseminated via wind. Conidia germinate into hyphae. Following hyphal colonization of the substrate, aerial hyphae will emerge, producing conidiophores (i.e., stalks) and conidial heads that form conidia (3).

This fungus is spread via the air, soil, and water. It is generally a saprophyte, living off dead and decaying matter. Therefore, it is commonly seen as a post-harvest disease. In the case of humans and animals, a compromised immune system is typically present when the disease manifests. In plants, irrigation practices such as drip irrigation lines buried in soil and hot, humid growth conditions are conducive to disease development (3).

Geographic Distribution

Aspergillus niger is ubiquitous and commonly found in soils, seeds, plant litter, plant rhizospheres, dried fruit, and nuts. It is one of the most common fungi found on foods (1).

Management

This fungus can be managed with cultural controls such as incorporation of green manures (e.g., alfafa) into the soil and the use of suppressive soils (i.e., soils with high microbial populations which outcompete A. niger). Planting marigolds (Tagetes spp.) is also recommended, as they release chemicals (i.e., terthienyls) which are toxic to A. niger. Antibiotics and blends of macro- and micro-nutrients with beneficial microorganisms are also used for control a wide variety of fungus (3).

Chemical controls are generally not recommended to control diseases (4). However, mycelial growth may be reduced by some fungicides and antibiotics have been shown to control certain diseases (3).

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

This fungus is typically cultured on PDA (potato dextrose agar) medium. After 3-4 days, a compact layer of yellow or white mycelia is covered by a thick layer of large (up to 3 µm x 15 -20 µm), dark-brown to black, globose, conidial heads. The conidial heads radiate, tending to split up into loose columns as the fungus ages, and are arranged in two rows. Conidia are dark-brown to black, globose to sub-glubose and rough-walled. These conidia are connected to phialiades (i.e., non-spore structures), which are attached to often septate, brown metulae (i.e., another non-spore structure). The heads are attached to conidiophores (i.e., stalks) which are smooth walled and hyaline (i.e., clear), which are darker the closer to conidial head it is (3).

Resources and References

1. Klich, M.A., J.I. Pitt. 1988. A laboratory guide to common Aspergillus species and their teleomorphs. Commonwealth Scientific and Industrial Research Organisation, Division of Food Processing, Australia.

2. Raper, K.B. and D.I. Fennell. 1965. The genus Aspergillus. Williams and Wilkins Company, Baltimore, MD.

3. Sharma, R. 2012. Pathogenicity of Aspergillus niger in plants. Cibtech Journal of Microbiology. http://www.cibtech.org/J%20Microbiology/PUBLICATIONS/2012/Vol-1-No-1/08-010...Ruby...Pathogenecity...Plants.pdf

4. UC Pest Management Guidelines, Onion and Garlic Black Mold, Pathogen: Aspergillus niger. http://www.ipm.ucdavis.edu/PMG/r584100411.html?printpage

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)