Aspergillus parasiticus
Author: Lanette Sobel, University of Florida
Reviewed by: Jeffrey Rollins, University of Florida
Pathogen
Aspergillus parasiticus is a plant pathogen that produces aflatoxins, a liver carcinogen. This fungus produces aflatoxins B1, B2, G1, G2, and mycotoxins, which are all potent carcinogens. Of these, aflatoxin B1 is considered to be the most potent, naturally occurring carcinogen known. The fungus is commonly found in the soil where it is involved in the decay of plant material. It is also found on stored grains as well as in fields of corn, peanut, and cottonseed. Additionally, it can opportunistically colonize humans and animals (1).
Symptoms and Signs
Symptoms in animals include immunosuppression, liver damage, and death (1, 5). Other symptoms include: decreased feed consumption and feed conversion, hemorrhaging, and neurologic dysfunction. Birds can experience blood clotting, leading to massive bruising and bleeding as well as reduced growth and hatchability of eggs (5).
In humans, especially farmers handling grain, lung tissue damage (extrinsic allergic alveolitis), skin irritation, fever, wheezing, cough, breathlessness, and ulcers have been attributed to this fungus (5).
Aspergillus rot of fig by A. parasiticus results in a dark green to olive color of the flesh. This eventually turns into powdery mass of spores that can infect the entire fruit interior (5). Aspergillus ear rot produces a powdery gray-green to yellow-green or yellow-brown mold growth on and between corn kernels. This disease is caused by the closely related Aspergillus flavus (3).
Ecology and Spread
A. parasiticus is an unspecialized saprophyte, living off dead and decaying matter. It can invade crops either in the field or post-harvest during drying, handling, and storage (2). A. parasiticus is disseminated via wind-borne spores but also has been shown to be transmitted through moist soil when in contact with damaged pecan shells, peanut pods, and kernels (1, 8).
Even though fruits and nuts may be invaded by the fungus to some degree, production of aflatoxins rarely occurs unless fruits have been damaged by insects, drought stress, high temperatures, early frost, windstorms, hail, birds, mites, or generally unfavorable weather (2, 6).
Although sexual reproduction has been observed, Aspergillus parasiticus normally reproduces in the asexual state. A wide range of morphological diversity has been shown. Spores are produced on crops, crop residue on the soil surface, and stored grains. Fungal spores travel to other hosts through the air and can overwinter on plant residue or as sclerotia (i.e., survival structures) in the soil (3).
In the asexual state, conidial heads are dark green and predominately unbranched. Conidia (i.e., spores) are distinctly roughened, globose to subglobose and are borne on stalks, which are commonly covered in small spines (4).
In the sexual state, the fungus is heterothallic (i.e., requiring compatible male and female reproductive structures from different individuals). However, sclerotia are typically formed in individuals of a single mating type and not from sexual mating. The sclerotium is not only an overwintering structure but may also provide genetic recombination by forming ascospores once sexually compatible individuals have mated. Ascospores are sexual spores housed in an ascus (i.e., sac), which is borne by the ascocarp, a fruiting body bearing the ascus. The nonostiolate ascocarp forms within the stroma. Ascospores have fine tuberculation (small, rounded swellings) and an equatorial line. Currently, many details of the sexual life cycle remain unknown (4).
Geographic Distribution
Aspergillus parasiticus is thought to be ubiquitous and can be isolated from agricultural, but rarely indoor environments (4, 7). It is isolated most frequently from plant parts, seeds, and insects (8).
Management
Reduction in the levels of aflatoxins has been obtained by roasting, ammonization, and microbial treatments. Binding agents such as bentonite clays and hydrated sodium calcium aluminosilicate (HSCAS) have been shown to decrease the toxic effects of aflatoxins in swine feed (4).
In Aspergillus rot of fig, proper water management and dust reduction are recommended. No chemical treatments are recommended (6).
In Aspergillus ear rot, damage of corn ears by insects, hail, high winds, or early frost can create an environment conducive for infection by A. parasiticus. The disease is most commonly seen under high temperatures (80° – 100°F), high humidity (85%), and drought conditions during grain fill and pollination (3).
Completely or partially burying residue infected with the fungus as well as practicing overall good sanitation, protecting fruit from external damage, providing adequate fertilization, and planting regionally adapted hybrid plants can reduce disease incidence (3).
The following measures should be taken to minimize infection during harvest and storage: good sanitation, keeping grain insect-free, proper drying, and storage of grain with low moisture and proper temperature (3).
Diagnostic Procedures
This fungus can be cultured on mixed cereal agar, Czapek agar, malt extract agar, malt salt agar, and PDA (potato dextrose agar) medium. After about seven days, the culture has a velvety surface with numerous, dark green conidial heads. Stromata and sclerotia both have similar external appearances, are globose to ellipsoidal in shape, and transform from a white to pink, brown, dark brown and finally black color. The interior is light to dark brown (4).
Malt salt agar has a high osmotic potential, which mimics that of seeds and preferentially selects for fungi in the Aspergillus flavus parasiticus group (9).
Resources and References
1. CAST (Council for Agricultural Science and Technology). 1979. Aflatoxin and other mycotoxins: An agricultural perspective. Ames, IA: Council for Agricultural Science Technical Report.
2. Diener, U.L. 1989. Preharvest Aflatoxin contamination of peanuts, corn and cotton seed: A review. Biodeterioration Research. 2: pp. 217-244.
3. Gupta, R.C. 2012. Aflatoxins, pp.1182. In Coppock, R.W., R.R.G. Christian, B.J. Jacobsen (2nd ed), Veterinary toxicology: basic and clinical principles. Academic press, Waltham, MA.
4. Horn, B.W., J.H. Ramirez-Prado and I. Carbone. 2009. The sexual state of Aspergillus parasiticus. Mycologia. 101: pp. 275 – 280. http://carbonelab.org/system/files/Horn_et_al_2009.pdf
5. Jacobsen, B.J., R.W. Coppock and M. Mostrom. 2007. Aflatoxins and aflatoxicosis. http://wiki.bugwood.org/uploads/AflatoxinsAflatoxicosis-StoredGrain.pdf
6. Michailides and Ferguson, 2009. Fig Aspergillus Rot - Pathogen: Aspergillus flavus, Aspergillus parasiticus, and other Aspergillus species. UC Pest Management Guidelines. http://www.ipm.ucdavis.edu/PMG/r261100711.html
7. Washington State Laboratory of Hygiene. Fungi. Madison, WI. http://www.slh.wisc.edu/wp-content/uploads/2013/11/Fungal-Glossary.pdf
8. Wells, J.M. and J. Payne. 1983. Weather-Related Incidence of Aflatoxin Contamination in Late-Harvested Pecans.
9. Wick, R. and M.A. Hansen. 2010. Malt salt agar. http://wiki.bugwood.org/Malt_salt_agar
