thumb|Aspergillus flavus in a petri dish
Aspergillus flavus is a saprotrophic and pathogenic fungus with a cosmopolitan distribution. It is best known for its colonization of cereal grains, legumes, and tree nuts. Postharvest rot typically develops during harvest, storage, and/or transit. Its specific name flavus derives from the Latin meaning yellow, a reference to the frequently observed colour of the spores. A. flavus infections can occur while hosts are still in the field (preharvest), but often show no symptoms (dormancy) until postharvest storage or transport. A. flavus is also an opportunistic human and animal pathogen, causing aspergillosis in immunocompromised individuals.
Plant parasitism
Aspergillus flavus is found globally as a saprophyte in soils and causes disease on many important agriculture crops. Common hosts of the pathogen are cereal grains, legumes, and tree nuts. Specifically, A. flavus infection causes ear rot in corn and yellow mold in peanuts either before or after harvest. The pathogen can discolor embryos, damage seedlings, and kill seedlings, which reduces grade and price of the grains. The incidence of A. flavus infection increases in the presence of insects and any type of stress on the host in the field as a result of damage. Stresses include stalk rot, drought, severe leaf damage, and/or less than ideal storage conditions.
Hyphal growth usually occurs by thread-like branching and produces mycelia. Hyphae are septate and hyaline. Once established, the mycelium secretes degradative enzymes or proteins which can break down complex nutrients (food). Individual hyphae strands are not typically seen by the unaided eye; however, conidia producing thick mycelial mats are often seen. The conidiospores are asexual spores produced by A. flavus during reproduction.
The conidiophores of A. flavus are rough and colorless. Phialides are both uniseriate (arranged in one row) and biseriate. Sexual reproduction occurs between sexually compatible strains belonging to different vegetative compatibility groups.
Aspergillus flavus is complex in its morphology and can be classified into two groups based on the size of sclerotia produced. Group I consists of L strains with sclerotia greater than 400 μm in diameter. Group II consists of S strains with sclerotia less than 400 μm in diameter. Both L and S strains can produce the two most common aflatoxins (B1 and B2). Unique to the S strains is the production of aflatoxin G1 and G2 which typically are not produced by A. flavus.
Growth conditions
Aspergillus flavus is unusual in that it is a thermotolerant fungus and can survive and grow at temperatures that other fungi cannot. This allows A. flavus to thrive in hot and humid conditions.
Moisture: A. flavus growth occurs at different moisture levels for different crops. For starchy cereals, growth occurs at 13.0–13.2%. For soybeans, growth occurs at 11.5–11.8%. For other crops, growth occurs at 14%.
Management
Aspergillus flavus infections will not always reduce crop yields alone; however, postharvest disease can reduce the total crop yield by 10 to 30%, and in developing countries that produce perishable crops, total loss can be greater than 30%. In grains and legumes, postharvest disease results in the production of mycotoxins.
Biocontrol
To protect tree nuts and corn plants affected by A. flavus, scientists of the Agricultural Research Service found that treating these plants with the yeast Pichia anomala reduced the growth of A. flavus. The study showed that treating pistachio trees with P. anomala inhibited the growth of A. flavus up to 97% when compared to untreated trees. The yeast successfully competes with A. flavus for space and nutrients, ultimately limiting its growth.
Aspergillus flavus strain AF36 is noncarcinogenic and aflatoxin-free and is used as an active ingredient in pesticides. AF36 is a fungal antagonist and is applied as a commercial biocontrol to cotton and corn to reduce aflatoxin exposure. AF36 was initially isolated in Arizona and has also occurred in Texas. It is grown on sterile seeds which serve as the carrier and a source of nutrients. Following application and colonization and in the presence of high moisture, AF36 growing seeds outcompete aflatoxin-producing strains of A. flavus. Nonaflatoxin spore dispersal is aided by wind and insects.
Disease in animals
Aspergillus flavus is an opportunistic fungal pathogen that is capable of infecting a wide range of animals and causing diseases. Most outcomes are influenced by host immune status and inoculum size. In animal such as mice, rats, and rabbits, A. flavus has been shown to be highly virulent. A. flavus is a recognized pathogen in domestic and farm animals such as chickens, geese, and turkeys. A. flavus is also a recognized pathogen in humans, as it can cause a wide range of diseases.
In animals, diseases associated with A. flavus can be mostly divided into two categories: aspergillosis and mycotoxicosis. Aspergillosis is an active fungal infection that is typically initiated by breathing in airborne conidia, leading to localized or disseminated disease affecting organs such as the lungs, sinuses, or other tissues. Mycotoxicosis results from swallowing aflatoxins, particularly aflatoxin B1. They are very toxic and hepatocarcinogenic compounds that could cause liver damage, immunosuppression, and death in affected animals.
Aspergillosis
After Aspergillus fumigatus, A. flavus is the second-leading cause of aspergillosis. It is a fungal infection caused by A. flavus. Primary infection is caused by the inhalation of spores; spores have a better chance of settling in the upper respiratory tract. It has been hypothesized that the deposition of certain spore sizes could be a leading factor for why A. flavus is a common etiological cause of fungal sinusitis and cutaneous infections and noninvasive fungal pneumonia. Following infection, fungal dissemination usually involves organs such as liver, kidneys, the brain, and the lungs, with the animals infected ending up dying within days to a couple weeks.
The amount of aflatoxins produced by A. flavus is affected by environmental factors. If other competitive fungal organisms are present on host plants, aflatoxin production is low. However, if noncompetitive fungal organisms are present on host plants, aflatoxin production can be quite high. The nature of the host is also an important factor in aflatoxin production. High A. flavus growth on soybean produces very little aflatoxin. High A. flavus growth aided by increased moisture content and warm temperatures on peanut, nutmeg, and peppers produces high concentrations of aflatoxins. A. flavus growth on spices produces low concentrations of aflatoxin as long as the spices remain dry. The deaths of ten conservationists present at the opening of a 15th-century tomb in Kraków, Poland in the 1970s has been attributed to aflatoxins originating from A. flavus present in the tomb.
Possible link to deaths after tomb openings
After the premature death of several Polish scientists following the 1973 opening of the tomb of the 15th century Polish King (and Lithuanian Grand Duke) Casimir IV Jagiellon, microbiologist Bolesław Smyk identified the presence of the A. flavus in samples taken from the tomb, and media reports have suggested that the likely cause of the deaths were the aflatoxins produced by this fungus.